Project1.11

Preface

This presentation is submitted as the summer dissertation project as the major

prerequisite for the Master of Science degree in Chemical Engineering at the

University of Nottingham, United Kingdom. Both laboratory experiments and thesis

construction were conducted under the full supervision of Dr Buddhi

Hewakandamby in the Department of Chemical and Environmental Engineering,

University of Nottingham between June 2010 and September 2010. I hereby declare

that this thesis is a work of my own and has not been previously submitted by me at

another institution for any degree. I also cede copyright of the thesis in favour of

the University of Nottingham, United Kingdom. This piece of work has not been

shared or used by any other persons other than me and my project supervisor, Dr

Buddhi Hewakandamby. Where I have used and quoted the work of other people,

they have been acknowledged and detailed explicitly throughout the entire

presentation.

____________________

Chan Yung Khiong

17th September, 2010

1

Abstract

The drive for the study of droplet coalescence stretches across numerous liquid-

liquid emulsion engineering applications including food, lubricants, pharmaceutical

and oil industry. Such research is also very beneficial for the environmental sectors.

However, liquid-liquid emulsion is complex in nature and its full comprehension is

yet to be achieved. To help break this barrier a step further, emulsion was studied

in this experiment on a smaller scale before escalating to more complicated

conditions. The experiment was composed of the investigation of binary

coalescence dynamics of a pair of kerosene droplets forming at low inlet flowrate

through capillaries in reverse osmosis (RO) water. Throughout the whole

experiment, the evolution of the kerosene droplet coalescence process was

recorded using a high speed video camera and the mean binary coalescence times

were calculated for different conditions. Various concentrations of glycerol were

added into the system to alter the interfacial tension and hence surface free energy

between the kerosene droplets and RO water. Increasing the glycerol concentration

decreased the interfacial tension and hence, surface free energy. Two methods

were employed during the experiment: (i) Non-flow induced and (ii) Flow induced.

For non-flow induced method, the mean binary coalescence time increased with

decreasing surface free energy which was in good agreement with most of the

literature review while the opposite result was found for the induced flow method.

Without the presence of glycerol, the mean coalescence time was higher for

induced flow method than non-induced flow method which drew a conclusion that

an application of force pressing the droplets together yielded higher stability.

However, the addition of glycerol reverses this effect and as a consequence, with

increasing glycerol concentration, the induced flow method reduces the stability of

the droplets and thus coalescence time was lower in comparison with those

obtained from the non-induced flow method. The nature of glycerol on kerosene

droplets when force is not injected is to be investigated further to gain more insight

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in coalescence aspect. Sceptical results were found on the non-induced flow method

which could be due to errors during experimentation. Further repetitions on the

non-flow induced method are to be performed.

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Acknowledgement

I would like to take this opportunity to first thank Dr Buddhi Hewakandamby for his

utmost kind assistance and care in facilitating the progress of the experiment and

my thesis. Along the way, he told various motivational short stories of his own life

and shared his wisdom which filled the atmosphere with more than just pure

academics. He was attentive to the various problems encountered for the past 3

months and making matters unworkable seem hopeful by nourishing me with

logical ideas.

I would also like to thank my parents who have been giving me the essential

support and motivation to reach the few final steps of this project with a positive

academic attitude. Without their support, such completion might not have been

successful.

A word of thanks also goes out to Katerina Loizou who shared the same experiment

throughout to cut time the entire summer instead of alternating turns. Together we

worked as a team and she made the long and tedious laboratory hours a bit more

enjoyable through exchanging personal life experiences and cultures. I would like to

thank the laboratory technicians, Phil, Mick, Terry, Fred, Marion and Vikki for getting

the materials I need without any difficulties and hesitations. Appreciation also goes

out to Dr. David Hann and Andy Matthews who made it possible for me to use of the

high speed camera which is the one of the primary elements of the experiment. I

would also like to express my gratitude to Aime for lending me the wonderful

creation of his thermocouple. To Natalia and Anna, thanks for lending the pump.

Also thanks to Nazrul for sharing the bits of information.

Chan Yung Khiong

17th September 2010

Contents

4

Preface.........................................................................................................................1

Abstract........................................................................................................................2

Acknowledgement........................................................................................................4

List of tables and figures..............................................................................................5

1 Introduction...........................................................................................................5

1.1 Quality energy source demand...........................................................................................................................5

1.2 Environmental issues..............................................................................................................................................5

1.3 Lifestyle.........................................................................................................................................................................5

1.4 Relationship between this study and the motivations.............................................................................5

2 Literature review...................................................................................................5

2.1 Mechanisms for two phase separation............................................................................................................5

2.2 Interfacial dynamics of droplet coalescence.................................................................................................5

2.2.1 Interfacial rigidity...................................................................................5

2.2.2 Interfacial mobility.................................................................................5

2.3 Role of surfactants and other surface active agents..................................................................................5

2.3.1 Emulsifiers.............................................................................................5

2.3.2 Demulsifiers...........................................................................................5

2.3.3 Other surface active agents...................................................................5

2.4 Effect of interfacial tension on coalescence...................................................................................................5

2.4.1 The Marangoni Effect.............................................................................5

2.5 Role of interfacial repulsive and attractive forces......................................................................................5

2.6 Effects of hydrodynamics on the coalescence..............................................................................................5

2.7 Drop size on coalescence.......................................................................................................................................5

2.8 Thermodynamics......................................................................................................................................................5

3 Aims and objectives...............................................................................................5

3.1 Aims................................................................................................................................................................................ 5

3.2 Objectives..................................................................................................................................................................... 5

4 Methodology..........................................................................................................5

4.1 Materials....................................................................................................................................................................... 5

4.1.1 Rig.........................................................................................................5

4.1.2 Rig core..................................................................................................5

4.1.3 Attachments..........................................................................................5

5

4.1.4 Delivery..................................................................................................5

4.1.5 Image capturing and video recording....................................................5

4.1.6 Lighting system......................................................................................5

4.1.7 Temperature measurement...................................................................5

4.1.8 Chemicals..............................................................................................5

4.1.9 Others materials....................................................................................5

4.2 Experimental procedures......................................................................................................................................5

4.2.1 Cleaning.................................................................................................5

4.2.2 Set up of experiment.............................................................................5

4.2.3 Method of data acquisition.....................................................................5

4.2.4 Interfacial tension measurements – Pendant drop method....................5

4.2.4.1 Equations....................................................................................................5

5 Results and Discussion..........................................................................................5

5.1 Time evolution of the coalescence process....................................................................................................5

5.2 Effect of induced flow and glycerol concentration.....................................................................................5

6 Conclusion.............................................................................................................5

7 Future work...........................................................................................................5

7.1 Experiment inefficiency and improvements.................................................................................................5

7.1.1 Rig body.................................................................................................5

7.1.2 Rig core..................................................................................................5

7.1.3 Pumps....................................................................................................5

7.1.4 Thermocouple........................................................................................5

7.1.5 Lens.......................................................................................................5

7.2 Environment............................................................................................................................................................... 5

7.2.1 Vibrations...............................................................................................5

7.2.2 Temperature..........................................................................................5

7.3 Data collection............................................................................................................................................................5

7.4 Image processing.......................................................................................................................................................5

7.5 Batch saturated..........................................................................................................................................................5

7.6 Overnight......................................................................................................................................................................5

8 Bibliography..........................................................................................................5

6

List of tables and figures

Table 1. Calculation of kerosene drop dimensions.....................................................5

7

Figure 1. Water in oil emulsion under macroscopic inspection (Mullin, 2006)............5

Figure 2. Oil spillage in the sea (Solutions, 2002)......................................................5

Figure 3. Emulsion used in manufacturing drugs (Pharma, 2009)..............................5

Figure 4. Sequential mechanisms governing phase separation. Adapted from

Paunov (2008)...........................................................................................................5

Figure 5. Ostwald Ripening involving droplet’s molecules (Bfigura, 2007).................5

Figure 6. Effect of adding surfactants on interfacial tension......................................5

Figure 7. Picture showning Marangoni stress in a wine glass.....................................5

Figure 8. Successive stages of the Marangoni flow in a wine glass............................5

Figure 9. Total free energy required for coalescence (Paunov, 2008)........................5

Figure 10. Components of the resultant net energy..................................................5

Figure 11. Rig body with cover, protrusion and walls.................................................5

Figure 12. Laboratory experiment set up...................................................................5

Figure 13. Dissembled rig core with cover.................................................................5

Figure 14. Rig core with needles. Figure 15.Top view of rig core.................5

Figure 16. Rig core immobilized by Perspex Glass.....................................................5

Figure 17. Rig core in rig body. Figure 18. Capillary attachments................5

Figure 19. Pump 1. Figure 20. Pump 2...................................5

Figure 21. Pump 1 calibration graph..........................................................................5

Figure 22. Pump 2 calibration graph..........................................................................5

Figure 23. High speed camera Phantom v12.1..........................................................5

Figure 24. High speed camera bird eyes view...........................................................5

Figure 25. Dedocool lighting system..........................................................................5

Figure 26. Thermocouple...........................................................................................5

Figure 27. LabVIEW temperature measurement........................................................5

Figure 28. Thermocouple simulation..........................................................................5

Figure 29. Temperature calibration equation.............................................................5

Figure 30. Schematic diagram of rig..........................................................................5

Figure 31. Image of the kerosene droplet required for the calculation of the

interfacial tension......................................................................................................5

Figure 32. Dimensions of the kerosene drop needed to be determined for interfacial

tension determination................................................................................................5

Figure 33. Droplets evolution at 0s............................................................................5

Figure 34. Droplet evolution at 0.256s.......................................................................5





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Figure 50. Droplet evolution at 1.758s......................................................................5



Figure 53. Interfacial tension per area against % concentration v/v glycerol.............5

Figure 54. Induced flow cumulative percentage against coalescence time graph for

0% glycerol................................................................................................................5

Figure 55. Induced flow individual percentage against coalescence time graph for

0% glycerol................................................................................................................5


0.5% glycerol.............................................................................................................5


0.5% glycerol.............................................................................................................5


1.0% glycerol.............................................................................................................5


1.0% glycerol.............................................................................................................5


5.0% glycerol.............................................................................................................5


5.0% glycerol.............................................................................................................5


10.0% glycerol...........................................................................................................5


10.0% glycerol...........................................................................................................5

Figure 64. Induced flow mean coalescence time against glycerol concentration.......5

Figure 65. Non-induced flow cumulative percentage against coalescence time graph

for 0% glycerol...........................................................................................................5

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Figure 66. Non-induced flow individual percentage against coalescence time graph

for 0% glycerol...........................................................................................................5


for 0.5% glycerol........................................................................................................5


for 0.5% glycerol........................................................................................................5


for 1.0% glycerol........................................................................................................5


for 1.0% glycerol........................................................................................................5


for 5.0% glycerol........................................................................................................5


for 5.0% glycerol........................................................................................................5

Figure 73. Non-induced flow mean coalescence time against glycerol concentration.

..................................................................................................................................5

1 Introduction

Emulsions are crudely dispersions of fluids in another immiscible fluid phase. In the

context of this project, only liquid-liquid emulsions are considered. Such typical

occurrences are complex in nature and they appear in various aspects of

engineering and science applications. For this reason, emulsion remains worthy to

be understood through research to yield benefits in the world of engineering and

science. In the next few sub-chapters, several major emulsions related industries

are elaborated.

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1.1 Quality energy source demand

To date, exploitation of crude oil to meet the insatiable global energy demand in the

petroleum industry has advanced to the point where the era of large fields with both

high quantity and satisfactory quality crude oil is at the brink of exhaustion. The

most crucial issue when focusing on existing large oilfields is to increase the

recovery rate. This can however be connected with typical flow assurance problems

which is often related to the multiphase transportation issues of the crude oil. At

many oil fields, the co-production of water from wells is in most cases substantial

which can be up to an extent of 50% to 70% during extraction (Ali & Alqam, 2000).

It is also known that in the petroleum industry, the issue of more than 95% of the

crude oil emulsions which are in the form of stable water-in-crude oil emulsion

exists as shown in Figure 1 (Xia et al., 2004) (Gaaseidnes & Turbeville, 1999).

Through the extensive experiences in the petroleum industry, water-in-crude oil

emulsions has nevertheless been seen to be encountered at many stages during

drilling, producing, transporting and processing of crude oils. Such emulsions are

extremely stable and viscous materials that increase pumping and transportation

expenses, cause corrosion of pipes, pumps, production equipment and distillation

columns and poison downstream refinery catalysts (McLean et al., 1998) which adds

to the already costly extraction process due to rise in demand (EIA, 2010) and oil

resource exhaustion (Petroleum, 2007). Recent studies also have shown that a

significant portion of the operating cost associated with the daily oil production is

spent on the oil-water separation (Li & Gu, 2005) (Crossdale et al., 1999). Moreover,

the performance and properties of crude oils are to a great degree determined by

the presence of water (Poznyshev, 1982) (Likhterova et al., 2003). Minimizing the

water levels in the crude oils by oil dehydration method can reduce pipeline

corrosion dramatically, improve crude oil quality and maximize pipeline usage for

transportation (Xia et al., 2004). Crude oil free from water is mandatory for pipeline

flow and refinery operations. The emulsion ageing tends to increase its stability; the

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breaking must be carried out as soon as possible in the production facility close to

the well (Kilpatrick & Spiecker, 2001).

Figure 1. Water in oil emulsion under macroscopic inspection (Mullin, 2006).

1.2 Environmental issues

In both offshore and onshore oil processing, one of the most challenging

environmental problems today is the effective removal of stable oil deposits from

water sources such as reservoirs, oceans and seas which is essentially necessary for

industrial and domestic consumption. These are known as oil-in-water emulsions

stabilised by naturally occurring surfactants. Offshore oil spillage as shown in Figure

2, runoff oil released during oil well extraction and oily wastewater from other

industries such the recycling industry and water desalination are undesirable events

contribute to this concern. The oil content which has low biodegradability must be

separated as much and quick as possible from the aqueous phase to prevent mass

ecological problems.

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Figure 2. Oil spillage in the sea (Solutions, 2002).

1.3 Lifestyle

Daily products in the olden days have shifted from simple to sophisticated genres

with variety if improvements in the new millennium. As such, the demand for such

astonishing materials has increased tremendously. One of the sources for such

complexity often can be subjected to the incorporation of numerous components

into a product so that the benefits of each constituent may be garnered

simultaneously for product enhancement. This is often desirable due to the

resultant change in tactile properties, for instance in mayonnaise and other foods

which are oil-in-water emulsions.

Alternatively, there may be a more sophisticated purpose such as when an oil-

soluble material must be delivered to an aqueous environment. In this case, the

material may first be dissolved in an organic liquid which in turn is dispersed in an

aqueous medium to create the emulsion. Such schemes find application in drug

delivery as shown in Figure 3. To create dispersion they must inherently be

insoluble in one another and so a third component usually must be added to

prevent them from phase separating. Other products include, lubricants, cosmetics

and paints (Venugopal & Wasan, 1983). Such technology is often the correct entry

for emulsion applications. A difficulty that arises in such applications however is the

incompatibility of the materials making up the emulsion.

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Figure 3. Emulsion used in manufacturing drugs (Pharma, 2009).

1.4 Relationship between this study and the motivations

In a nutshell, there is an endless list of activities that make use of the emulsion

technology to function in the designated way. While effective separation of

unwanted oil dispersions from water and water from crude oil has long been a

challenging technical task in the petroleum, oil spills and water industries, such

emulsions or dispersions are useful in the food and pharmaceutical industries.

At present, there exist several oil-water separation methods which are the current

state of the art technologies. However, most of them have rather limited

applications in separating oil form the produced fluids due to their large capital and

or operating costs, low separation efficiency and or slow separation process – long

residence time. Therefore, some significant improvements in oil water separation

techniques are still required in practice. This applies to the food and pharmaceutical

and other industries as well to improve the quality of the products.

During these industrials operations, the emulsion drops are subjected to flow fields

which may cause them to break or coalesce. The simultaneous occurrence of the

two phenomena controls the drop size distribution, which in turn has significant

effect on the processing conditions and characteristics of the product. The addition

of components such as surfactants can cause the inversion of different phases.

Moreover, the lifetime of emulsions may vary from seconds to minutes to hours to

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days to weeks to months and to years depending the nature of the surfactants,

nature of both fluids and their volume ratio. Despite the large amount of work

devoted to this issue, predicting the emulsions lifetime still remains a challenge.

It is necessary to go back to the science of the process to understand the emulsion

stability mechanisms from both macroscopic and microscopic points of view.

Understanding the underlying principle mechanisms of one type of emulsion with

regards to its stabilization strength and formation can improve the design of the

phase separator equipments and demulsifying/ emulsifying agents or techniques so

that effective separation of the dispersed phase or emulsification can be made

successful. Generally, an improved understanding of coalescence behaviour is a

prerequisite for better engineering and thus would potentially allow one to improve

formulations, quality, stability, process yields and stability during product shelf-life,

through engineering of interfaces, bulk rheology, and process.

In particular, it is the interfacial dynamics between the droplets that is of great

interest in this project. The interfacial dynamics involve the mechanisms that

governed the probability of coalescence between the droplets and are the very first

several things to look at, in the beginning of an emulsion research.

2 Literature review

In the field of liquid-liquid emulsion, the behaviour of coalescence in many forms

such as between a pair of drops, a pair of bulk phases, many drops, drop and bulk

phase have been increasingly popular and continuously studied theoretically and

experimentally in detail for more than 50 years now. Investigators have conducted

15

experiments in many possible ways with either pure chemicals or in the presence of

added components that affect the interfacial properties. The effects of varying

interfacial tension, hydrodynamics and other conditions such as temperature and

pH levels were investigated to confirm the proposed coalescence mechanisms and

to characterize the coalescence behaviour of various liquids systems.

2.1 Mechanisms for two phase separation

From the visible perspective regardless of the presence of surfactants which is

discussed later in Section 2.2, an emulsion will eventually phase separate given

sufficient time without introducing external forces or energy. Figure 4 shows the

generally accepted time evolution of an emulsion breaking occurrence (phase

separation). The four mechanisms responsible in destabilizing the emulsion:

Flocculation – The droplets form aggregates of two or more drops. The inter-

particular distance between the droplets is strongly diminished due to a net

attraction between the droplets.

Creaming – Due to the difference in the density between the dispersed phase

and continuous phase, this results in the formation of a dispersed phase

concentration gradient in the mixture.

Coalescence – This significant occurrence involves specifically the amalgamation

of two drops.

Ostwald ripening – In more microscopic terms as shown in Figure 5, the diffusion

of droplet’s molecules through the medium causes small droplets to decrease in

size and disappear while large drops grow. It was found that this process

actually takes place prior to coalescence (Schmitt & Leal-Calderon, 2004).

16

Figure 4. Sequential mechanisms governing phase separation. Adapted from

Paunov (2008).

Figure 5. Ostwald Ripening involving droplet’s molecules (Bfigura, 2007).

However with respect to the field of this research project, only the coalescence

stage is of concern. Therefore, the interaction between the two droplets at very

close proximity to each other especially at dynamics at the interface is reviewed in

this section.

2.2 Interfacial dynamics of droplet coalescence

According to an early theory developed by Smoluchowski (1917), two drops merge

and coalesce immediately upon collision straightforwardly entailing that there is no

resistance to coalescence imparted by the thin liquid film trapped between the

drops (Mitra & Ghosh, 2007). However, in practice, this theory is only applicable

only when the continuous phase has very low viscosity and there is no surface

active species present in the medium that can stabilize the drops. Also, the

aforementioned theoretical work by Smoluchowski is only applicable to non-

deformable particles (Danov et al., 1993). However, many studies now also include

deformable droplets where the viscosity of the dispersed phase or droplets is low.

17

It is generally accepted that the coalescence behaviour of approaching emulsion

droplets is said to be controlled by the dynamics of the film between their surfaces.

Zapryanov et al (1983) reported that for both types of oil/water emulsion,

coalescence between two droplets occurs in three specific steps:

(1) Approach of the droplets through the continuous phase where the droplets

eventually come into contact driven by the applied forces of the flow field or

induced flow;

(2) Deformation of the droplets by flattening of their contact surfaces to form a thin

film between them which starts to drain through a process commonly known as film

drainage;

(3) Thinning of this film to a critical thickness below which the droplets coalesce due

to intermolecular forces like van der Waals forces become dominant and cause the

film to rupture followed by the formation of the bridging or merging of the two

droplets.

Danov et al (1993) and Aarts and Lekkerkerker (2008) supported the sequential

mechanisms listed above and investigated the dependence of external forces on

the interaction of the droplets such as van der Waals, electrostatic, hydrodynamics

and also the energy for the deformation of the droplets which are also studied by

many researchers which are discussed later. Auflem (2002) also reported that the

main mechanisms that influence coalescence are film drainage and film rupture

which are considered acceptable mechanisms in the vast literature. Despite such

logical hypothesized mechanisms, the coalescence probability depends only on the

details on the drainage of the film between the droplets.

However, with regards to the film thinning occurrence, many have thought that this

film should be nevertheless ‘invisible’ to the naked eye until Pu and Chen (2001)

investigated on the surprising phenomenon of ‘jumping’ coalescence of two largely

separated water droplets of unequal size under microgravitation where essentially

18

no external force acts which defies largely against the film thinning theory. In their

work, coalescence was successful even when the continuous phase film was seen to

be very obviously larger than even the size of the drops. It was concluded that the

coalescence driving force within the liquid drops under microgravitation to be

responsible for the jumping phenomenon but no further investigation was

attempted at such unusual phenomenon under microgravitation. However, quite

recently, results obtained from a study by Kim and Longmire (2009) might be able

to lead clues to the coalescence under microgravitation. They concluded the vortex

rings within colliding drops must be oriented such that they induce streaming flow

with a strong component toward the centerplane and the drops must collide with

sufficient inertia such that they deform significantly, increasing the velocity

magnitude in the streaming flow and also if the large velocities in the streaming

flow approach the center plane, then they can induce a faster outflow in the thin

film between the drops.

2.2.1 Interfacial rigidity

Often in many emulsion studies, surface rigidity plays as one of the most important

dynamics that governs the coalescence probability (Groeneweg et al., 1993).

Surface rigidity reflects the tendency of the interface to deform as the two drops

approach each other. In the aspect of forces in the vicinity of the interacting drops,

it is obvious that the separation distance between the two facing surfaces at which

the deformation takes place, increases and decreases with attraction and repulsion

respectively i.e. the higher the attractive force between the droplets, the larger the

distance at which deformation starts to occur. Perhaps, the drainage of the thin film

may be analyzed by means of a force balance comprising the force exerted on the

droplets by the flow field and the resistance to drainage due to the viscous flow in

the film.

Ivanov et al (1985) observed that often in the beginning of the deformation the

drop caps acquire a bell-shaped form called a dimple. Ivanov and Dimitrov (1988)

19

added that this film thins with time and at a certain critical thickness ruptures as

observed by Zapryanov et al (1983). Chesters (1991) concluded that the rigidity of

the colliding interfaces of the drops governs the thinning rate of the liquid film to its

critical thickness. Specifically, the interfacial rigidity determines how much the

colliding interfaces will flatten and is influenced by drop size and interfacial tension.

If the film can be thermodynamically stable, the thinning stops at the equilibrium

thickness where the disjoining pressure in the film equilibrates the capillary

pressure in the drops. In the case of thermodynamically unstable films which always

rupture: then the film lifetime depends mainly on the rate of thinning and the

critical thickness. If the interface is undeformable, which means that the pressure in

the liquid film is lower than the Laplace pressure inside the drops, the film can

easily be drained.

2.2.2 Interfacial mobility

The interfacial rigidity is often related to the interfacial mobility. The flow in the film

is coupled to the flow inside the approaching particles via the mobility of the

interfaces, which is especially relevant for pure fluids. Abid and Chesters (1994)

observed that the interfacial mobility is governed by the tangential stresses exerted

on the film by the drops. Saboni et al (1995) found that this tangential stress

depends on the viscosity ratio of the dispersed and continuous phases. There are

three types of interfacial behaviour during coalescence identified based on the

dispersed/continuous phase viscosity ratio: (1) immobile interfaces, when the

dispersed phase viscosity is much higher than the continuous phase one, or when

surfactants are present that retard the drainage of the liquid film; (2) fully mobile

interfaces, when the continuous phase viscosity is very large compared to that of

the dispersed phase; (3) the most commonly encountered partially mobile

interfaces, when the viscosity ratio is moderate (Chesters, 1991).

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2.3 Role of surfactants and other surface active agents

Previously the information garnered only indicated the interfacial dynamics without

specifying whether the dynamics of the coalescence behaviour are the

consequences of the addition of surfactants or otherwise. The role and types of the

surfactants should be clearly understood beforehand.

According to Bancroft (1913) rule, a liquid containing the surfactant becomes the

continuous phase. In other words, when dispersing equal volumes of liquids for

instance oil and water, the emulsion obtained is oil-in-water if the surfactant is more

soluble in water and vice versa.

Surfactants are substances that alter the interfacial tension between the two or

more phases of liquids and by altering the interfacial tension; this can influence the

coalescence behaviour very much which is discussed in Section 2.4. However, it is

important to distinguish that there are two types of surfactants – emulsifier and

demulsifier. Many articles have confused readers where the authors did not

specifically draw the line between emulsifiers and surfactants; they indicate that

surfactants are emulsifiers and the other way around. Perhaps it is crucial to review

the simple definitions of the types of surfactants available in the chemical industry.

It remains very important to identify the surfactant type because different

categories have different effects (Pichot et al., 2010).

Surfactants commonly are divided into two families – (i) emulsifiers and (ii)

demulsifiers. An emulsifier when added into a mixture of oil and water stabilizes the

emulsion. Hence, emulsions are dispersions of two immiscible liquids, kinetically

stabilized by the action of a surfactant or specifically an emulsifier. The function of

the demulsifier can only work to break the emulsion which is already stabilized. In

other words, demulsifier can only be added to a mixture which is already emulsified

or stabilized by natural occurring or induced emulsifiers. Hence demulsifiers are

emulsion breakers that separate for instance, crude oil emulsion into distinct oil and

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water phases (Mikula & Munoz, 2000). However, demulsifiers can also be added to

emulsion free of emulsifiers to facilitate phase separation.

2.3.1 Emulsifiers

In the absence of emulsifiers, drops aggregate rapidly as a consequence of the van

der Waals force. In their presence, the emulsifier particles adsorb to the interface of

the drops creating a barrier that decelerate aggregation and Ostwald ripening

preventing the higher probability of drops coalescing. Hence, they favour the

occurrence of immobile interfaces, delaying the drainage of the intervening film

between flocculated drops. Depending on their interfacial properties, these films

can drain and rupture after a period of time significantly longer than that of without

the presence of emulsifiers or, remain stable for long periods of time (Vrij, 1964)

(Vrij & Overbeek, 1968). Conversely, emulsifiers lower the interfacial tension of

these films, favouring the appearance of surface oscillations and holes which

happens to reduce the drainage rate (Ivanoc et al., 1999). Hodgson and woods

(Hodgson & Woods, 1969) investigated the effect of additions of SDS on the

coalescence of oil toluene drops in water. Most of the surfactants used by the

aforementioned researchers previously are regular emulsifiers which can be found

in a handbook by Mukerjee and Mysels (1971). Chen and Pu (2001) investigated on

the addition of an emulsifier concentration and found out that the binary

coalescence time was increased under microgravitation. They concluded that a film

is apparent when sodium sulphate dodecyl (SDS) was present at a higher level

concentration. This film prevents the coalescence of the droplets. There are other

works that used regular emulsifiers to investigate on the coalescence behaviour of

various liquid systems and yielded similar outcome (Bazhlekov et al., 2000)

(Chesters & Bazhlekov, 2000) (Binks, 2002) (Yeo et al., 2003) (Chevaillier et al.,

2006) (Giribabu & Ghosh, 2007) (Sacanna et al., 2007).

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2.3.2 Demulsifiers

Based on the amount of work done on liquid coalescence using surfactants,

compared with emulsifiers, the work done using demulsifiers was not as extensive.

Few researchers developed similar scientific hypothesis that supported the

replacement of the emulsifiers by demulsifiers at the interfaces and the

consequence of this is the increased facilitation of coalescence. Demulsifiers are

surfactants found to develop high surface pressure at the crude oil-water interface

and promote phase separation (Bhardwaj & Hartland, 1994). It was known that

during the demulsification process, the effective demulsifiers replace the emulsifiers

adsorbed on the oil water interface (Kang et al., 2004). Deng et al (2005) added

that the presence of demulsifiers results in replacement of rigid film of natural

crude oil emulsifiers by a film which is conducive to coalescence of water droplets.

When two drops approach each other due to external forces, the thickness of the

intervening continuous phase film decreases. The shear stress associated with

drainage tends to concentrate the emulsifier molecules outside the film and their

concentration inside the film is lowered. Thus an interfacial tension gradient is set

up with high interfacial tension inside the film and low tension outside the film. Such

phenomenon is discussed in more detail in Section 2.4.1. The demulsifier molecules

migrate from the surface adjacent to the interface where the demulsifier molecules

are adsorbed in the spaces left by the emulsifier molecules in the film so the

interfacial tension falls quickly. Once the surface layer is depleted, demulsifier

molecules have to diffuse from the bulk which is a slow process and therefore the

interfacial tension falls more slowly. Ultimately the interface becomes saturated and

the equilibrium interfacial tension is reached. As indicated by the variation in

interfacial tension with time, the rate of adsorption of the demulsifier at the crude

oil water interface is much faster than that of emulsifiers in the crude oil. Adsorption

of the demulsifier reverses the interfacial tension gradient and enhances film

drainage. Ultimately, a stage is reached when the film becomes very thin and due

23

to the proximity of the dispersed phase the van der Waals forces of attraction

dominate and the droplets coalesce. More presently, Abdurahman and Yunus (2009)

confirmed with higher concentration of the demulsifier used, the droplet size was

larger than that of with lower concentration. It was also observed that increase in

demulsifier will accelerate the coalescence of droplet faster. But regardless of

concentration levels, the presence of a demulsifier is characterized by very short

initial coalescence time. Demulsification was also studied by Borges et al (2009),

Rondon et al (2006) (Rondon et al., 2008) (Hung et al., 2007) (Bhardwaj & Hartland,

1994) (Miller & Bohm, 1993)

Figure 6. Effect of adding surfactants on interfacial tension.

2.3.3 Other surface active agents

Other surface active agents have been used as well. In crude oil emulsions, it has

been known that they are stabilized by naturally present substances such as

asphaltenes and resins which behave like emulsifiers (Makhonin et al., 1979).

Authors McLean et al (1998) were reported that, asphaltenes and resins adsorb at

the water-oil interfaces and form interfacial films that confer stability against phase

separation or in other words these surfactants provide steric hindrance to droplet-

droplet coalescence which are similar to functions of the regular surfactants or

specifically emulsifiers. The stability of water in oil emulsions in petroleum systems

24

using the asphaltenes were also investigated by other researchers (Kumar et al.,

2001) (Sjoblom et al., 2003) (Sullivan et al., 2007). Such mechanism was also

similarly described by Sundararaj (1995) with polymeric materials (Borrell & Leal,

2008) which also act as surfactants and they are called compatilizers. Electrolytes

(Li & Slattery, 1988), salts like sodium chloride NaCl (Koh et al., 2000) (Mitra &

Ghosh, 2007) and even gelatine (Lobo, 2002) were also used as surfactants to

investigate coalescence behaviour or droplets and yielded the same stabilization

effects as emulsifiers.

The paradigm that indicates that the water-in-oil emulsions are stabilized by

asphaltenes often seems to be a gross oversimplification without any specific

details regarding the true observable contribution of asphaltenes. Because of this,

Czarnecki and Moran (Czarnecki & Moran, 2005) created a model to explain the

mechanism of water-in-oil emulsion stabilization in petroleum systems in order to

provoke further potential discussions. Their model suggested that only a small sub

fraction of asphaltenes and not all is being used as the stabilization. There appears

to be another chemical responsible for the stabilization which is a low molecular

weight surfactant material. The competition for the oil/water interface between

these two substances is based on the difference of their adsorption kinetics where

the asphaltenic material adsorbs slowly and irreversibly and forms rigid skins while

the other material adsorbs faster. Abdurahman and Yunus (2009) also investigated

the dependency of co-surfactants. For instance, by increasing the ratio of resin

concentration to asphaltene concentration effectively increases the water

separation from crude oil emulsion while an increase in the asphaltene

concentration in the crude oil decreases water separation rate. The effect of resin is

that it solubilises the asphaltenes into the oil phase minimizing asphaltene

interaction with the water droplets and hence more coalescence.

25

2.4 Effect of interfacial tension on coalescence

Previously, we have seen only the decrease of magnitude of the interfacial tension

of fluid systems by addition of surface active or surfactants. In this section, the

effect of altering the interfacial tension on the coalescence behaviour is elaborated

in more detail.

Hartland and Wood (1973) concluded that with decreasing interfacial tension, the

film drainage rate will be reduced, which leads to longer coalescence times. They

also found that during the coalescence of a liquid drop with a flat interface the

drainage rate decreased with a decrease in interfacial tension and applied force or

with an increase of the drop volume. Li and Slattery (1988) confirmed that the

coalescence time of nitrogen bubbles increased when the surface tension was

decreased by changing the concentration of sodium chloride in the aqueous

continuous phase. So far, many researchers have only found the effect of

decreasing the interfacial tension of binary liquid/liquid systems and gas/liquid

systems by addition of emulsifiers on the coalescence frequency (Nielsen et al.,

1958) (Charles & Mason, 1960) (Charles & Mason, 1960) (Mackay & Mason, 1963)

(Mackay & Mason, 1963) (Vrij & Overbeek, 1968) (Hodgson & Lee, 1969) (Hodgson

& Woods, 1969) (Burrill & Woods, 1973) (Chen et al., 1998) (Dreher et al., 1999)

(Chen & Pu, 2001) (Ghosh, 2004).

On the other hand, Abdurahman et al (2007) found that by using many demulsifiers,

which did not only decrease the interfacial tension, but increased the interfacial

tension and promoted separation of oil and water phases. Surprisingly, Wang et al

(2009) found that by decreasing the interfacial tension, the coalescence time

decreases which is a contrast to the present literature. Such unusual findings are

not very common in the literature.

26

2.4.1 The Marangoni Effect

Several researchers investigated into more detail with regards to the mechanisms

responsible for the low probability of coalescence in the presence of certain

emulsifiers. The most commonly accepted idea is that the flow out of the thin film

produces a gradient in the surfactant concentration, with the lowest concentration

at the centre of the film and the highest ones near the edge of the film (Yoon et al.,

2007). This yields the Marangoni stresses that are hypothesized to immobilize the

interfaces within the film, and thus slow the drainage process (Aversana et al.,

1995) under micrograviation. This was also previously observed in a recent

dissertation by Yoon (2006). Other previous researchers also yield such

phenomenon (Mackay & Mason, 1963) (Allan & Mason, 1961). With surfactant

concentrations being the highest at the edge this yield the surface tension at the

edge of the film to be the lower while the surface tension at the centre of the film is

highest. According to the Marangoni effect which was first observed by James

Thomson (1855), and later in more detail by Marangoni (1865) suggests that the

film moves from region of low interfacial tension (emulsifiers concentration is high)

to high interfacial tension (emulsifiers concentration is low) causing a film at the

centre of the point of contact between the drops.

However, Dai , Graham and Leal (2008) showed that during coalescence, the

interface in the thin gap actually exhibits a significant degree of mobility which

explains that at least to a certain extent, the fact that the assumption of a

completely immobilized interface is not in agreement with their experimental

conclusions. Hence the unexpected new result is that the role of Marangoni effect

on the coalescence process does not occur via immobilization of the interface within

the thin gap region, but rather is due to its effects on the hydrodynamics outside

the thin film. In particular, Marangoni stresses immobilize the drop interface outside

the thin film, and this increases the total external hydrodynamic force that pushes

the drops toward each other. This, in turn increases the degree of flattening and

27

dimpling of the thin film, and it is primarily this change that slows the film drainage

process and thus increases the required drainage time prior to coalescence.

To help understand the Marangoni mechanisms better, it is best to demonstrate the

appearance of the wine tears.

Figure 7. Picture showning Marangoni stress in a wine glass.

Stage 1 – Alcohol is first spread across a curved dish.

Stage 2 – As evaporation occurs everywhere along the free surface, a film can be

seen around the outer edges of the dish.

Stage 3 – The film seems to spread out further. The alcohol concentration in the thin

layer is thus reduced relative to that in the bulk owing to the enhanced surface area

to volume ratio. As surface tension decreases with alcohol concentration, the

surface tension is higher in the thin film than the bulk. The associated Marangoni

stress drives up flow throughout the thin film

Stage 4 – The alcohol thinly spread across the curved dish climbing up to reach the

top of the dish where it accumulates in a band of fluid that thickens.

28

Stage 5 – Eventually the thick band becomes gravitationally unstable and releases

the tears of wine. Marangoni flows are created successfully as tensions draw the

liquid towards the centre of the dish.

Stage 1 Stage 2

Stage 3 Stage 4

Stage 5

29

Figure 8. Successive stages of the Marangoni flow in a wine glass.

However, it seems that the Marangoni effect is only applicable to emulsifiers which

happen to cause the increased stability of drops. With respect to demulsifiers, no

such effect was reviewed. Perhaps, with demulsifiers, the reason for the promotion

of coalescence frequency can be due to their natural structure which is beyond the

scope of this project. Previously, it was mentioned that the most demulsifiers

decrease the interfacial tension of liquid systems which is in common with

emulsifiers. With this, if the Marangoni stresses hold true, the demulsifiers replace

the spaces left by emulsifiers as the thin film is being squeezed out so quick that

the concentration of demulsifiers at the centre of the film is very large compared to

that at the edge of the film and hence the reverse of interfacial gradient which

promotes coalescence. However, this does not apply to demulsifiers which increase

the interfacial tension which was observed by Abdurahman et al (2007) because it

would defy the coalescence role of demulsifiers.

2.5 Role of interfacial repulsive and attractive forces

The coalescence behaviour of emulsion droplets upon collision depends on the

interplay between these two types of forces. If the total interaction is very repulsive,

the droplets can rebound; if the intermolecular repulsive forces can hold the film at

an equilibrium separation, the droplets may flocculate; and if the total interaction is

attractive, the film will become thinner and finally rupture, i.e. the droplets

coalesce. Coalescence requires rupture of a thin liquid film between the drops. In

most real-life applications there are effects associated with the thinning of the liquid

film and interfacial repulsion originated from Derjaguin Landau Verwey Overbeek

(DLVO) and non-DLVO forces. Such DLVO forces are the van der Waals and

electrical double layer forces which have received much studies (Chen, 1985)

whereas non-DLVO forces are attractive depletion steric and repulsive structural

forces (Kumar et al., 2001) which on the other hand did not receive much attention.

30

The condition for rupture is the critical film thickness such that the attractive force

which is the van der Waals forces must be larger than the electrostatic repulsive

force and the Marangoni effects as discussed in Section 2.4.

This is illustrated in Figure 9 where the total free energy of the particles to be at

very minimal distance must be at most at zero Joules. k is the Boltzmann constant

and T is the temperature. D is the distance between the two surfaces of the drops

facing each other and U is the free energy of the particles.

Figure 9. Total free energy required for coalescence (Paunov, 2008).

Figure 10. Components of the resultant net energy.

31

The red line in Figure 9 shows the net energy which is composed of the double layer

repulsive force and the van der Waals attractive force as shown in Figure 10. Hence

in order for coalescence to commence, the net energy curve must be below zero

value at the vertical axis. Many researchers failed to take into account their

significance when characterising the model for coalescence behaviour. Because

other relevant forces play an important role in determining the stability of emulsion

droplets, neglecting them (van der Waals) will unavoidably lead to deviation from

the real situation (Greene et al., 1994). Chen (1985) confirmed that the van der

Waals disjoining pressure destabilized the film, whereas the electric double layer

stabilized it using a model that describes the film profile evolution between two

equal sized drops and predicts the film stability, time scale and film thickness given

only the radius of the drops and the required physical properties of the fluids and

surfaces.

2.6 Effects of hydrodynamics on the coalescence

While some researchers focussed on coalescence under stagnant flow, several other

researchers investigated droplet coalescence under flowing conditions. For

instance, Hartland and Wood (1973) found that during the coalescence of a liquid

drop with a flat interface the drainage rate decreased with applied force. Many

research groups have studied theoretically and numerically the film drainage

process under constant approaching velocity or constant driving force (Stergios et

al., 1991) (Chesters, 1991) (Jeelani & Hartland, 1993) (Saboni et al., 1995) (Rother

et al., 1997) (Nemer et al., 2004). Their results indicate that the film drainage rate is

mainly controlled by the interfacial tension, the viscosity ratio between the

dispersed and suspending phases and the external force. These factors have

profound effect on the drainage rate, which often depends on the deformability and

tangential mobility of the droplet surface as discussed previously. Wang et al (2009)

criticized that most of the models assumed simple boundary conditions, such as

32

constant interaction force or constant approach velocity. In reality both will vary

during the collision of a drop with a flat interface or with another drop.

Al-Mulla and Gupta (2000) concluded that using a Coutte device at lower shearing

rate favoured coalescence. Similarly, Nandi et al (2001) indicated that their

surfactant stabilized emulsions were prepared in a stirred tank and low shear rates

prevented drop breakup and at high shear rates, the emulsions were stabilized

further. However, Nandi et al (2005)extended their shearing experiments with

surfactant-less liquid mixtures and found that without surfactants, higher shearing

rates promoted coalesce. Chen and Tao (2005) also found that by increasing the

stirring intensity it increases the oil in water emulsion stability. Previously, Leal

(2004) added the film drainage is slowed down upon addition of emulsifiers in a

four-roll mill where the binary coalescence of a pair of droplets were exposed to

external shearing flows. Dai and Leal (2008) validated the film drainage and rupture

using polymeric materials in which the experiment was based on the coalescence of

a pair of equal size drops undergoing head-on collision in a biaxial linear flow under

constant linear velocity. They also investigated that there is an equally important

effect which is due to the increasing hydrodynamic force pushing the drops together

causing the film to be more strongly deformed into a dimpled configuration which

further slows the film drainage process. They also confirmed the validity of the

Marangoni effects such that the immobilization of the interface within the thin film

as expected and the force pushing the drops together is increased by the

Marangoni immobilization of the interface outside the thin film. This caused the thin

film to be much more dimpled and deformed than it is in the absence of surfactant

at the same capillary number and the more dimpled film shapes slow the rate of

film drainage. An observation holds true in this respect in the opposite manner in

which, Bremond and Bibette (2008) investigated the high affinity of a pair of

emulsion drops to favour coalescence under decompression or rather, separation.

Briefly, the experiment was undergoing first expansion and then decompression of

33

a flow of trains of emulsion droplet pairs passing through a coalescence chamber.

No coalescence was observed until the decompression of the downstream droplet of

the pair. It was also observed that at this instant, both droplets form a pair of facing

nipples in the contact area prior to coalescence. The separation term is used

because the action of decompressing the downstream droplet leads to acceleration,

causing it to move away faster from the upstream droplet. Gary leal (Baldessari et

al., 2007) (Borrell & Leal, 2008) (Yang et al., 2002) studied the stability of film

drainage with respect to flow induced coalescence. They investigated on the growth

of disturbance in relation with the film drainage phenomenon.

Contrarily in the same perspective, the demulsifying experiment conducted by

Abdurahman and Yunus (2009) on different stirring rates concluded that with higher

stirring rates and hence, larger turbulence and high shear rates, the average

droplet size was larger. This relatively means that shear induces higher drainage

rates. However, despite the obvious introduction of shear stress into the medium,

no significant observation was made on the flow patterns.

The capillary number, which is the ratio between viscous and interfacial forces,

characterizes the droplet deformation and plays an important role in determining

whether two droplets coalesce upon their collision. It was found that the

coalescence behaviour of two droplets in a simple shear flow strongly depends on

the capillary number. Coalescence only occurs at a capillary number that is lower

than a critical value. Also in a simple shear flow, Loewenberg and Hinch

(Loewenberg & Hinch, 1997) numerically studied the collision between two

deformable droplets. They found that the collision behaviour is dictated by capillary

number and predicted that, for flow with capillary number very much less than 1,

coalescence tends to occur when the droplets are pressed together, whereas for

capillary number ~ 0 the tendency for coalescence reaches its maximum when the

droplets begin to separate in the extensional quadrant. The latter behaviour was

experimentally verified by Guido and Simeone (1998). In a different flow fashion

34

where a pair of equal size drops was on a head-on collision, the findings were very

similar (Yoon et al., 2007). Leal (Leal, 2004) investigated on the effect of capillary

numbers on the coalescence of the droplets and added that coalescence requires

very ‘gentle’ collisions, i.e., collisions at low capillary numbers.

A simplistic view of the coalescence process begins with the observation that the

thin layer of fluid that separates the two drops once they collide must become thin

enough for van der Waals attraction to destabilize the film to produce film rupture.

During the whole collision process, the net force on each drop is actually zero

assuming that they are neutrally buoyant. However, when they are in close

proximity, it is convenient to think of the net force as being the sum of two equal

magnitude but oppositely directed forces; one the hydrodynamic force due to the

external flow which tends to push the drops together when they first come into

apparent contact, and the other the lubrication force due to the extra pressure in

the thin film. From this point of view, the question of whether a pair of drops can

coalesce is then the question of whether the film between them thins sufficiently

before the drops rotate to the orientation where the external force changes from

pushing the drops together to pulling them apart. Leal (Leal, 2004) investigated

such phenomenon in a study of the effect of the capillary number on droplet

coalescence in four roll mill device which generates flow to the surrounding fluid.

The device is advantageous because there is a stagnation point at the geometric

centre where coalescence and drop breakup experiments can be conveniently

carried out and monitored. It was found that at a capillary number larger than the

critical capillary number, the pair of droplets was first in a horizontal orientation.

Then the orientation varied such that there was an increase in inclination of the

droplets and the droplets were getting closer to each other. However, at an angle of

45 degrees, the droplets move away from each other due to the influence of the

external forces. In contrast, at a capillary number lower than the critical capillary

number, the droplets coalesced at an angle of inclination of only 10 degrees. It is

35

also important to note that at horizontal level, after the collision, the droplets

rotated afterwards. Such are glancing collision, where the two drops rotate in the

flow from the point of initial contact to the point where they hydrodynamic force

along the lone of centres changes sign and the drops begin to separate. The most

important finding here was that the influence of the capillary number. Hu et al

(2002) studied the collision between two droplets of equal size in a simple shear

flow. They showed that, for a fixed initial position and viscosity ratio, the minimum

separation between two droplets is solely determined by the capillary number.

2.7 Drop size on coalescence

It is commonly accepted that with large drops the large contact area reduces the

film drainage rate resulting in larger coalescence time (also called rest-time)

compared to small drops. Results produced by Dreher et al (1999) show that the

coalescence time depends almost linearly on drop size. Moreover, Mackay and

Mason (1963) found that for easily deformable drops, coalescence time and drop

stability increase with drop size and was later validated by Chen el al (1998).

Hartland and Wood (Hartland & Wood, 1973) also found that during the coalescence

of a liquid drop with a flat interface the drainage rate decreased with an increase of

the drop volume.

2.8 Thermodynamics

Surface free energy is another term in the aspects of thermodynamics for interfacial

tension. There information regarding the interfacial mechanisms which can still be

considered to be under proposition or being hypothesized because there is no

concrete or credible proofs that can defend its existence. However, the only basis

that we can still rely on is the change of the surface free energy which changes with

the curvature of the droplets. The curvature of the droplets is related to the

interfacial tension and this is not only available in the literature but also logical and

visible to the eye. By incorporating thermodynamics, one can visualize the

36

coalescence behaviour in terms of the period of coalescence by changing the

interfacial tension. With this and the established numerical equations of

thermodynamics through Gibbs free energy, which can link the surface curvature to

the concentration of surfactants, then there is solid verification that the presence of

surfactants can increase or decrease the coalescence time.

Following the conclusions obtained for published articles, the behavior of a system

under the influence of a surfactant is to be observed. The surfactant is expected to

lower the interfacial tension of the system, keeping in mind that the molecular

weight of the surfactant to be used must be low, so that coalescence is enhanced.

Thermodynamics can be utilised to speculate the behavior as shown below:

Helmholtz equation:

F=U-TS (1)

Differential of Helmholtz equation at constant temperatures :

dF=dU-TdS (2)

Internal energy:

dU=TdS+μdn+g dA (3)

Combining equations (2) and (3) :

dF=μdn+g dA.

(4)

where, F= Helmholtz’s Free energy, U= internal energy T= temperature, S=

entropy, μ= chemical potential, n= number of species,γ=interfacial tension and A=

surface area.

37

As a conclusion, lowering the interfacial tension and keeping the system constant,

will result to the decrease of dF, thus the energy required to create a system after

the spontaneous energy transfer from the environment has taken place, is lowered

and coalescences is favored.

3 Aims and objectives

3.1 Aims

The aim of this project is to investigate the coalescence behaviour of oil-in-water

emulsion under the influence of surfactants and the presence of stagnation in the

vicinity of the region of coalescence. It is crucial to identify that the coalescence

process be it successful or unsuccessful, to be free of external forces other than the

generation/induction of flow to allow the growth of the droplets. This will involve the

study of the interactions between the close proximity facing surfaces of a pair of oil

droplets suspending in water undergoing coalescence or otherwise through a high

speed camera. Observations of the physical changes of the interface between the

pair of oil droplets are to be validated with the available literature to further support

or criticize the suggested mechanisms and wherever possible, draw new

conclusions.

With these contributions, more understanding of the coalescence behaviour can be

gained. This is beneficial to both the science and engineering fields in the context of

better design for emulsion breaker devices or techniques and the synthesis of

better and more efficient demulsifying chemicals.

38

3.2 Objectives

The behaviour of these coalescences is variable due to the presence of factors such

as external hydrodynamic forces and surfactants. The specific contributions made

by each factor with regards to the coalescence behaviour are unpredictable. Hence

there are many potential investigations available to investigate the dependence of

each factor on each other. The objectives of this project are to:

Investigate the effect of different surfactants on the coalescence frequency

Investigate on the effect of surfactant concentration on the interfacial tension of

the system

Investigate on the relationship between interfacial tension and coalescence

frequency

Investigate the effect of induced and non induced flow on the coalescence

frequency

39

4 Methodology

This section brings about the details in which how the experiment was carried out,

what materials were used and their sources and what were measured. The

experiments of binary drop coalescence were carried out using the experimental set

up as shown in Figure 1. Figure 2 shows the whole experimental set up. For step by

step procedures, this is available in the Appendix.

4.1 Materials

4.1.1 Rig

The coalescence cell or the rig shown in Figure 11 is a vertical rectangular acrylic

cell with a square cross section of side length 26 cm and height 25 cm. The rig is

made out of square and rectangular pieces of Perspex glass glued together using

thermal glue and let dried. The bottom centre of the rig has a protrusion made out

of Perspex glass which is also glued to the bottom. The protrusion is made such that

the rig core can be inserted without any mobilisation. The rig has a cover to prevent

the entry of dust particles during experiment.

40

Figure 11. Rig body with cover, protrusion and walls.

41

42

Figure 12. Laboratory experiment set up

4.1.2 Rig core

The rig core consists of several components combined together including a 4

screws, cover, needles and connectors. The rig core was a cube of sides 7cm with a

vertical cylindrical hollow centre which was designed to fit in through the protrusion

in the rig as shown in Figure 13. Two Perspex glasses of dimensions are cut and

inserted into the bottom of the rig to prevent any movement of the rig core during

experiment as shown in Figure 14.The rig core also had four V-shaped grooves to fit

in the plastic needles such that the rig core cover is pressed firmly with screws on

top of the rig core body to lock in the plastic needles as shown in Figure 14 and 15.

By adjusting the tightness of the screws, this adjusts the levels of the tip of the pair

of needles. The needles were bent at 90o in the mechanical workshop as accurate

as possible. The outer diameter of the needles was 0.966mm. The tip of the vertical

needles should be aligned with each other horizontally and in line with each other

so that droplets can be produced equally with sides touching each other.

Figure 13. Dissembled rig core with cover.

Figure 14. Rig core with needles. Figure 15.Top view of rig core.

43

Figure 16. Rig core immobilized by Perspex Glass.

4.1.3 Attachments

Drops were formed through two polyethylene capillaries with outer diameter 0.7mm

attached to the needle on one end and the other to the syringe connector as shown

in Figure 16. Each capillary was connected to the inlet of the needle at one end and

the other end to the syringe connectors shown in Figure 17. Hence when the rig

core with its attachments was placed within the rig, care was taken not to damage

or apply pressure on the vulnerable capillaries. The cover was slid open and a thin

film of cling paper is used to cover the opened part of the cover to prevent entry of

dust particles as shown in Figure 18.

Figure 17. Rig core in rig body. Figure 18. Capillary attachments.

44

4.1.4 Delivery

Two different infusion pumps were used due to lack of resources: (i) Pump 1 and (ii)

Pump 2 as shown in Figures 19 and 20 respectively. Due to this reason and the

unequal inefficiencies of both pumps, pump calibration was necessary. The pump

calibration was carried out such that the true mean volumetric flowrate was

calculated for a particular pump setting flowrate. The methodology of the pump

calibration can be viewed as the volume of liquid discharged per unit time for a set

of pump setting flowrates (0.1ml/hr – 1.0ml/hr). The liquid used for pump calibration

was RO water. 5mL syringes were inserted securely into the infusion pumps and

connected to the syringe connectors as shown in Figures 18 and 19. The desired

volumetric flowrate of the pump is checked through the pump calibration curve

obtained from pump calibration. For instance, if 0.5ml/hr was needed to induce flow,

then the corresponding pump setting flowrate at the x-axis of the pump calibration

graph with error bars as shown in Figures 21 and 22 was used.

Figure 19. Pump 1. Figure 20. Pump 2.

This configuration allows equal and steady delivery in each syringe of low inlet

flowrates required in this work. Throughout the experiment, a mean setting flowrate

of 0.5ml/hr was used corresponding to setting flowrates of 0.63ml/hr for Pump 1 and

0.62ml/hr for Pump 2.

45

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.0

0.2

0.4

0.6

0.8

1.0f(x) = 0.535393994884959 x² + 0.471063185898291 x

Mean real setting flowrate (ml/hr) against Pump 1 setting flowrate (ml/hr)

Pump 1 setting flowrate (ml/hr)

Mea

n re

al se

ttin

g flo

wra

te (m

l/hr

)

Figure 21. Pump 1 calibration graph.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

f(x) = 0.0678819933664271 x² + 0.729006575325994 x

Mean setting flowrate (ml/hr) against Pump 2 setting flowrate (ml/hr)

Pump 2 setting flowrate (ml/hr)

Mea

n se

ttin

g flo

wra

te (m

l/hr

)

Figure 22. Pump 2 calibration graph.

46

Syringes were purchased from the Medical School University of Nottingham.

4.1.5 Image capturing and video recording

The coalescence was captured with a high speed video camera (Phantom v12.1)

placed in front of the rig, as shown in Figures 23 and 24 which has a CMOS sensor

with a maximum resolution of 1280 × 800 pixels. The average recording rate used

throughout the coalescence process was 1000 pps, while even higher recording

rates up to maximum 6200 pps at maximum resolution (1280 × 800 pixels) were

necessary to capture the formation of the liquid bridge between the two drops as

soon as coalescence started.

Figure 23. High speed camera Phantom v12.1.

Figure 24. High speed camera bird eyes view.

4.1.6 Lighting system

Back lighting for the camera was provided by Dedocool lighting system which was

composed of a spotlight and a motor as shown in Figure 25. The spotlight was

47

placed behind of the rig to allow light to pass through the camera where the camera

was focussing. Dedocool lighting system was able to avoid generating temperature

gradients within the rig. The spotlight was clamped with its position adjusted to

obtain the best image and videos possible in terms of contrast, colour and

brightness. Baking paper was clamped when necessary to provide a translucent

environment of the lighting to capture clear images and videos when the frame rate

used was at maximum. This was necessary because at maximum frame rate, the

exposure to light was very low and so the spotlight is switched on at its brightest

level. To balance the brightness, the baking paper was used.

Figure 25. Dedocool lighting system.

4.1.7 Temperature measurement

The temperature measurement was done using a thermocouple designed,

programmed and fabricated by Aime as shown in Figure 26. The temperature

detector is a metal which is attached at the side of the rig submerged in the

mixture. Calibration of the thermocouple was necessary for accurate temperature

measurements. Calibration was performed to obtain an equation shown in Figure 29

which relates the voltage and the temperature by measuring different temperatures

(0oC – 100oC) of mixtures of the ice cubes and boiling water.

48

Figure 26. Thermocouple.

Figure 27. LabVIEW temperature measurement.

A voltmeter was used to measure the temperature in terms of voltage. The

thermocouple is then inserted into the rig attached to the walls of the rig and

49

submerged in the liquid mixture. The temperature was read off on the computer

screen using LabVIEW as shown in Figure 26. The program was composed by Aime

as shown in Figure 27.

Figure 28. Thermocouple simulation.

This equation is to be inserted into LabVIEW. LabVIEW was used as the program to

measure the temperature during the experiment.

0.9 1 1.1 1.2 1.3 1.40

20

40

60

80

100

120

f(x) = 258.155577299412 x − 255.439359099804

Temperature (oC) against Voltage (V)

Voltage (V)

Te

mp

era

ture

(o

C)

Figure 29. Temperature calibration equation.

50

4.1.8 Chemicals

Kerosene was sourced from Sigma Aldrich and used as the dispersed phase in all

the experiments. The continuous phase was either pure RO water or RO water with

different surfactants glycerol or sodium dodecyl sulphate (SDS) concentrations.

Glycerol was purchased as 200mL bottles from Boots while SDS powder was

obtained from Sigma Aldrich.

4.1.9 Others materials

25L PVC container

Glassware beakers (200mL, 500mL and 5L)

Measuring cylinder

Pipette and pipette filler

Propanol (cleaning solvent)

Methanol (cleaning solvent)

Latex gloves

4.2 Experimental procedures

This section will give specific treatment of major experimental procedures so that in

future if this work is to be repeated, results can be compared to verify the validity of

the results obtained in this current experiment.

4.2.1 Cleaning

The investigation of surface science requires very clean and pure system without

any contamination. Any contamination will affect results and disallow comparison

between repeated results which leads to waste of time. Hence extreme care was

taken when preparing prior to setting up of the experiment. Below is a list cleaning

procedures that was performed and is required to be followed for future

experiments:-

51

Latex sterilised gloves were worn throughout the whole experiment. They are

rinsed with methanol and RO water prior to making any contact with the

experimental materials.

Before the start of each experiment the rig was strongly sprayed and rinsed

several times with tap water from the laboratory room. Care was taken to

remove any oily stains or dust or solid particles in the rig by spraying strongly

the tap water. The rig is then filled with tap water and left overnight to pull out

the contamination particles remained as residues in the rig.

The rig core body, cover and the Perspex glass pieces were also washed and

sprayed with tap water and dipped into the rig of tap water to allow any residual

oily stains to float and rise to the surface.

All connectors, needles and screws were dipped in methanol in a beaker under

sonification thermal bath at least twice for 10 minutes before rinsing them with

RO water and leaving them to dry in the fume cupboard. The methanol dissolved

any residual stains attached to these smaller components and the RO water

washes the dissolved particles away.

The capillaries were filled and washed with methanol, RO water and air in that

order for several times using new clean syringes. The capillaries were then

curled into a small beaker where they were washed with methanol and rinsed

with RO water.

Stirrer is to be rubbed and rinsed with methanol, RO water and dry in fume

cupboard.

All glass beakers were strongly sprayed with tap water to remove oily stains and

then with methanol and tap water and rinse with RO water and left dry in the

fume cupboard.

Pipette is washed with RO water several times.

52

4.2.2 Set up of experiment

The setting up of the experiments was started off with the cleaning procedures

mentioned in the previous Section 4.2.1. Once the equipments were cleaned, RO

water was brought to mix and saturate with 100mL of kerosene in two 5L glass

beakers. This was done so that there is no diffusion within the liquids during

experiments. The beakers were left for at 12 hours sealed with cling film paper to

prevent entry contamination in the fume cupboard.

The rig was to be emptied of the overnight tap water and rinsed with RO water. The

rig core was set up with the needles, connectors and tubes. Care was taken when

handling the needles without touching the tips. The distance separating the needles

was taken to be around 2.8mm, measured using a vernier calliper. It was very

important that the needles are secured tightly with the tips aligned horizontally in

line with each other as shown schematically in Figure 30. The needles were made

also vertically parallel to each other.

The excess kerosene on floating on top of the saturated RO water in the 5L beaker

was removed out carefully using a pipette and placed in a beaker. The syringes

were then filled with the saturated kerosene. The syringes were also rinsed with the

relevant aqueous phase before the experiment. Once the rig core and the

connectors are inserted into the rig as shown in Figure 17, the syringe connectors

are attached to the syringes located at the infusion pumps. Care was taken not to

touch the capillaries inside the cell after the cleaning. The saturated RO water was

then fed into the rig slowly without splashing on the rig core while saturated

kerosene was delivered to both capillaries by the syringes through pumping. Air

within the syringes was discharged by continuous pumping of the saturated

kerosene.

53

Figure 30. Schematic diagram of rig.

4.2.3 Method of data acquisition

Pairs of kerosene drops were formed at the tips of the needles through the

capillaries and settled on top of the saturated RO water in the cell. At the low flow

rates used the drops formed in each capillary were well separated from subsequent

ones and did not interfere with each other and with the coalescence process. It was

made sure that minute organic phase was generated so that it does not affect the

coalescence process for instance 20% of the rig is covered with kerosene.

The coalescence process in the rig was carried out using two methods:-

54

i. Non-induced flow

ii. Induced flow

For both methods, the binary coalescence time was recorded using the camera

software Phantom Showcase. The kerosene inlet flowrate used throughout the

whole experiment was 0.5mL/hr (mean pump flowrate – refer to Figures 20 and 21)

According to Ban et al. (2000), the coalescence time is defined as the difference

between the time the two drops come into contact and the time the two drops

‘begin to coalesce’. ‘Begin to coalesce’ can also refer to the instant at which the

bridge between the two drops is formed. It is important to define illustratively the

point at which the two drops come into contact and also the point at which the two

drops ‘begin to coalesce’. This is because later in the experiments, there was

difficulty in judging the point where the two drops start come into contact.

The difference between the two methods was that for non-induced flow, the flow of

kerosene in the syringe was such that it was stopped at the instant just before the

two drops come into contact for the first time. The binary coalescence time for this

method was measured at this instant until the formation of the bridge was first

witnessed clearly on the camera software depicted on the laptop screen.

As for the induced flow method, the flow of kerosene was maintained at 0.5ml/hr

until the end of the coalescence process or the drops leave the needles due to

unsuccessful coalescence. Hence, if coalescence occurred, the binary coalescence

time was calculated from the point at where the drops first contact until the

formation of the bridge occurs.

For the two methods, glycerol was added at 4 different concentrations v/v of 0.5%,

1.0%, 5.0% and 10.0% to compare the system when glycerol concentration was

zero.

For each glycerol concentration, 10-15 pairs of drops were formed. Between the

generations of each pair, 5 minutes were given to allow the flow patterns to

55

disperse so that during the coalescence process, the environment was maintained

as a stagnant one so that there is no potential shear or disturbance that can affect

the coalescence of the pair of droplets. 5mL of kerosene in a syringe was more than

sufficient to generate data for all concentration. It was also noted that the change in

the liquid level height in the rig was insignificant and did not affect the coalescence

process. At the end of the experiment, the rig was emptied and cleaned to repeat

the same batch of experiments. This procedure allowed consistency in the system

for comparison of results.

The coalescence between two drops can be affected by factors such as external

vibrations and temperature gradients (Charles & Mason, 1960) (Davies, 1992). Due

to the limitations of the experiment, it was unable to avoid any such effects; no

thermostatic environment was available and also vibrations were inevitable. Despite

the thermocouple was available to monitor the temperature change of the system,

if a large deviation from the constant temperature of the system occurs, there was

nothing that could be done to remove this occurrence. There were also insufficient

benches to place equipments that produce vibrations. However, despite such

disturbances, they were attempted to be minimized to as low as possible.

4.2.4 Interfacial tension measurements – Pendant drop method

One of the ways to measure the surface tension is by using the pendant drop

method which was used in this project. The pendant drop method can be used to

determine the static surface and interfacial tensions of liquids. is probably the most

convenient, versatile and popular method to measure interfacial tension between

emulsion. It involves the determination of the profile of a drop of one liquid

suspended in another liquid at mechanical equilibrium. It was important to eliminate

any vibration that can disrupt the still.

The measurement of the interfacial tension requires certain dimensions of the

kerosene droplet in the medium. It was relatively difficult to measure the

56

dimensions of the oil droplet because there was no close access to it where the

droplet can be measured off directly using ruler. Hence, measurements of the oil

droplet are to be made through the use of the camera and an image processing

software called GNU Image Manipulation Program (GIMP) which is freely distributed

over the internet (GIMP, 2001-2010). The image of the drop in the medium was as

shown in Figure 31 was taken by Phantom Camera Control software while GIMP

calculates the dimensions necessary for the interfacial tension to be measured by

converting pixels to distances in SI units. For each glycerol concentration, this was

done several times to account for the error in consistency. Hence the average

interfacial tension was used. A droplet was generated each time slowly at 0.5ml/hr

and the volume of the droplet at which the droplet is detached from the tip of the

needle is recorded. The droplet was regenerated again at about 95% of the

recorded volume. The flow was then stopped and the droplet was monitored for at

least minute before the image of the oil droplet in the aqueous phase was taken.

Figure 31. Image of the kerosene droplet required for the calculation of the

interfacial tension.

57

4.2.4.1 Equations

There are various methods of calculating the boundary tensions from the pendant

drop profiles, but the method of Andreas, Hauser and Tucker (Andreas et al., 1938)

is the most commonly used. With this method, the equatorial diameter de and the

diameter ds in a selected plane, which is located by measuring vertically from the

vortex a distance equal to de are measured as shown in Figure 6. The ratio of the

two diameters ds/de is designated as S, the drop shape; the quantity 1/H is a

function of S. The interfacial tension Υ is calculated by the equation (1):

Υ=g ρ de

2

H (1)

where g is the acceleration due to gravity and ρ is the difference in density of the

two phases.

The relationship between S and 1/H is relatively important to characterize all the

profiles of the oil droplet. The range of calculated values of S for 1/H has been

extended (Fordham, 1948) (Mills, 1953) (Stauffer, 1965). Misak (Misak, 1968) stated

that there are 5 equations that can relate 1/H with S for 5 corresponding ranges of S

that are sufficiently accurate to be used in the most exacting interfacial tension

calculations :

For S = 0.401 to S = 0.46

1H

=( 0.32720S2.56651 )−0.97553S2+0.84059S−0.18069 (2)

For S> 0.46 to S = 0.59

1H

=( 0.31968S2.59725 )−0.46898S2+0.50059S−0.13261 (3)

For S> 0.59 to S=0.68

58

1H

=( 0.31522S2.62435 )−0.11714 S2+0.15756S−0.05285 (4)

For S> 0.68 to S=0.90

1H

=( 0.31345S2.64267 )−0.09155S2+0.14701S−0.05877 (5)

For S> 0.90 to S=1.00

1H

=( 0.30715S2.84636 )−0.69116 S3+1.08341 S2−0.18341S−0.20970 (6)

The dimensions were calculated and shown in Table 1. The density of glycerol was

800kg/m3.

Figure 32. Dimensions of the kerosene drop needed to be determined for

interfacial tension determination.

59

Table 1. Calculation of kerosene drop dimensions.

Conc. v/v glycerol de ds S=(ds/de) H ρDispersed VROwater VGlycerol ρContinuous ρ γ

% mm mm kg/m3 m3 m3 kg/m3 kg/m3 kg/m2s2

04.52

3 2.711 0.599381 0.818622 800 8000 0.0 998.2 198.2 0.04859

0.54.51

2 2.715 0.601729 0.827047 800 8000 40.0 999.5 199.5 0.04818

1.04.50

1 2.718 0.603866 0.834761 800 8000 80.8 1000.8 200.8 0.04781

5.04.51

1 2.788 0.618045 0.887051 800 8000 421.0 1011.3 211.3 0.04755

10.04.46

8 2.810 0.628917 0.92846 800 8000 888.9 1024.4 224.4 0.04733

5 Results and Discussion

5.1 Time evolution of the coalescence process

The time evolution of the coalescence process of two kerosene drops is depicted

from Figures 33-52 at the highest frame rate of 6200pps at maximum resolution.

From Figure 33, it can be seen that there is a visible thin gap between the two

drops and the point of contact is defined in Figure 35. Although in other perception,

the ‘dark bridge’ that occurred at Figure 35 seemed relatively like a shadow, but

generally, this point is the start of the binary coalescence time. The bridging of the

drops starts at Figure 37 and this represents the point where the binary

coalescence time stops. The evolution of this coalescence process corresponded to

a binary coalescence time of about one second which happened very rarely

throughout the period of the experiment. All other figures show the aftermath of the

bridging process which leads to coalescence of the two droplets. The bridge gets

larger in size until it becomes the same size as the two droplets. The formation of

nipples can be seen on the two opposing sides of the two droplets. They deformed

such that the nipples disappear and reappear; the frequency of the sudden

expansion and contraction of the coalesced droplets gets smaller with time until it

settles on top of the needles as shown as Figure 50.

60

5.2 Effect of induced flow and glycerol concentration

Figures 53-63 and 64-71 illustrate the cumulative coalescence time distributions

and also the individual coalescence time distributions during binary kerosene drop

coalescence at the same centre-centre capillary separation distance of 2.8mm for

the two methods – (i) Induced flow and (ii) Non-induced flow respectively. In a

general perspective, the coalescence times for all experiments were not as

randomly distributed as shown in the figures. The interfacial tension per unit area

was found to decrease with increasing concentration of glycerol as shown in Table 1

and Figure 53 which was also the same result as obtained by Wang et al (2009).

From Figure 53, the trend decreased sharply at lower concentration. At higher

concentrations of glycerol, the decrease in interfacial tension per unit area was

reduced. Glycerol was used in this experiment to confirm the work of Wang et al

(2009) because it was surprising to notice that with glycerol, the interfacial tension

decreased but the coalescence time decreased which was a contrast to the current

literature. However, this work proved that their findings were nevertheless

substantial and I further extended their work without inducing flow to monitor the

coalescence trend.

From the results obtained, it can be seen that for induced flow, the coalescence

time decreases with increasing concentration of glycerol which is in agreement with

the work done by Wang et al. (2009). The only difference between this work and

theirs was that they had oil as the continuous phase and deionised water as the

dispersed phase while in this work, it was the opposite. Despite this phase inversion

issue, the results remained comparable with that of Wang et al’s because both

experiments investigated the same interface between water and oil under the

influence of glycerol. However, the result was a contrast for non-induced flow. For

non-induced flow, increasing the concentration of glycerol led to stabilization of the

drops and is in very good agreement with the vast literature. At 10% concentration

v/v glycerol, the non-induced method had 0% coalescence and this was the reason

61

for the missing graphs for the non-induced method for 10% concentration v/v

glycerol.

But however, comparing the coalescence time between between the two methods

at 0% glycerol, the non-induced flow actually promotes coalescence which is in

good agreement with the literature review where some authors mentioned that with

lower force, coalescence frequency was enhanced. But as the concentration of

glycerol is increased, such effect was reversed. One logic reason could be the

inaccuracy of the measurement of the binary coalescence time for the non-induced

flow. As it was difficult to judge the first point of contact between the two drops,

most of the readings in the data collection could be wrong. This can be possible

because it may have been overlooked that they made their first contact but in

reality they did not and this could be the reason for the increased stability of the

drops. Neglecting this error by assuming that the drops did made their first contact;

another error that arises could be the contamination of the liquid system by solid

particles that may have entered the liquid system unnoticeably during the

experiment. Putting aside the human error aspects for such trends, in scientific

terms and together with equipment deficiencies, such trend can be possible, given

that glycerol behaves like regular surfactants which happened to have been

stabilizing emulsion and drops in liquids. When the pumps were switched off, due to

the relaxation of the syringes, the flow might still be induced at very low flowrate.

This will keep pushing the drops together, sweeping away glycerol particles away

from the centre of the gap between the drops. When this happens, the interfacial

tension is highest at this centre and Marangoni effects are induced thus increasing

the drainage time. However, this is a defying statement for the induced flow

method where the drainage time was decreased. So far, glycerol has not been

identified as a general emulsifier or demulsifier in the literature. But however,

Griffin (1954) did found that the HLB (hydrophilic-lipophilic balance) value of

glycerol is 11.09. This indicated that specifically glycerol is an oil in water

62

emulsifier. Perhaps with this statement, the results obtained from this experiment

can be held true coupled with several other explanations such as the flows within

the drops.

Simply, with glycerol as an oil-in-water emulsifier, for non-induced flow, with

increasing concentration of glycerol, this would lead to stabilization. For 0% glycerol

concentration, the result obtained from induced flow method is in good agreement

with other researchers (Mackay & Mason, 1961) (Chen et al., 1998) (Dreher et al.,

1999) as discussed in the literature review. Inducing flow produced force which

presses the drops together and causes the continuous film to drain which depends

on the capillary pressure and on van der Waals forces given by Δp=( 2ΥR )+(AH6 π h3

)

(Marrucci, 1969) where AH is the Hamaker constant. This was also observed by

Wang et al. (2009). On the aspects of flowrates, the experiments performed only

used non-induced flow and induced flow at 0.5ml/hr. It was found that with an

increase in the drop diameter and hence radius which was caused by induction of

flow, this decreases the overall pressure and lead to larger coalescence times.

Borreal and Leal (2004) (2008) also indicated that as the inlet flow rate increases,

the effect of drop size on coalescence time diminishes. It can be concluded that an

increase in the inlet flow rate for the same capillary separation distance leads to an

increase in the coalescence time. Furthermore, Borreal and Leal (2008) also

indicated that with increasing inlet flowrate, Q, the flat film area a between the two

drops increases as suggested by the equation 1 for coaxial drop coalescence where

ao is the initial value of the flat film area which is equal to 0 for drop-drop

coalescence. The increase in area will therefore increase the time for the film to

drain to its critical value thus increasing the coalescence time

a=√ tQ2 πR+ao2 (1)

63

Another factor that affects the coalescence behaviour in this experiment was the

solubility of a component into the opposite phase which allows mass transfer to

occur. Nielsen et al (1958) found that coalescence taking place in mutually

saturated phases always results in longer coalescence times compared to the

unsaturated ones. Charles and Mason (1960) also had similar findings on the same

aspect with different liquids. Wang et al (2009) found that coalescence times

increased obviously in the saturated mixtures which were in agreement with Nielsen

et al. However, in the current experiment, the saturation method was different from

Wang et al’s. Their saturation method included glycerol within the mixture of the oil

and aqueous phases whereas the current experiment had the saturation of water

with oil only. Wang et al used glycerol in the saturation process and this halted all

possible mass transfer. However, in this experiment, when glycerol is transferring to

the oil phase due to unsaturation with glycerol in the first place, then a

concentration gradient of the glycerol within the drop can appear. The film region

between the pair of drops is easily saturated as the area involved is very small

compared to the rest of the continuous water phase. As a result there would be a

high glycerol concentration on outer edge of the drops that could give rise to

Marangoni flows that will help film drainage (Davies, 1992). In the current

experiment, such explanation could render validity for the decreased coalescence

time when the mixture is not saturated with glycerol and this allowed mass transfer.

Wang et al (2009) however, found that this was not the case. This suggests that

mass transfer alone cannot fully account for the increased coalescence efficiency in

the presence of glycerol in water. The addition of glycerol tends to make the

interface to deform easier and the non uniform distribution of surfactants on the

interface has been shown to cause fusion of drops because of fluctuations

generated at the interfaces (Evans & Wennerstrom, 1999). This was also observed

by Dickinson (1992) where thermal fluctuations were known to cause coalescence

of drops. Finally, it was noted that the hydrophobic part of glycerol within the oil

64

phase is not long enough to generate steric interactions that would prevent drops

from approaching very closely and would delay coalescence (Tadros, 1996).

Figure 33. Droplets evolution at 0s.

Figure 34. Droplet evolution at 0.256s.







65

Figure 41. Droplet evolution at 1.735s. Figure 42. Droplet evolution at 1.736s.








66


Figure 51. Droplet evolution at 1.797s. Figure 52. Droplet evolution at 1.820s.

67

Figure 53. Interfacial tension per area against % concentration v/v glycerol.

68

0 1 2 3 4 5 6 7 8 9 100.04650

0.04700

0.04750

0.04800

0.04850

0.04900

Interfacial tension per area (kg/m2s2) γagainst % concentration v/v glycerol

Percentage concentration % v/v glycerol

Inte

rfa

cia

l te

nsi

on

pe

r a

rea

(k

g/

m2

s2)

Induced method

69

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 4200

20

40

60

80

100

Cumulative percentage (%) against Coalescence time (s) for 0% concentration

v/v glycerol

Coalescence time (s)

Cu

mu

lati

ve

pe

rce

nta

ge

%

Figure 54. Induced flow cumulative percentage against coalescence time graph for 0% glycerol.

0 5 10 15 20 25 30 35 40 45 50 60 70 80 90 100 200 300 4000

5

10

15

20

25

30

Individual percentage of coalesced drops (%) against Co-alescence time (s) for 0% concentration v/v glycerol


Ind

ivid

ual

per

cen

tage

of c

oale

sced

dro

ps

(%)

Figure 55. Induced flow individual percentage against coalescence time graph for 0% glycerol.

70

71

0 20 40 60 80 100 1200

20

40

60

80

100

Cumulative percentage (%) against Coalescence time (s) for 0.5% concen-

tration v/v glycerol


Cu

mu

lati

ve

pe

rce

nta

ge

(%

)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 320

20

40

60

80

100

Cumulative percentage (%) against Co-alescence time (s) for 1.0% concentration

v/v glycerol

Series2


Cu

mu

lati

ve

pe

rce

nta

ge

(%

)

0 10 20 30 40 50 60 70 80 90100

11005

10152025303540

Individual percentage of coalesced drops (%) against Coalescence time (s) for 0.5%

concentration v/v glycerol

Series1


Ind

ivid

ua

l p

erc

en

tag

e o

f co

ale

sce

d d

rop

s (%

)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10010511005

10152025303540




Ind

ivid

ua

l p

erc

en

tag

e o

f co

ale

sce

d d

rop

s (%

)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 320

20

40

60

80

100

120




Cu

mu

lati

ve

pe

rce

nta

ge

(%

)

Figure 56. Induced flow cumulative percentage against coalescence time graph for 0.5% glycerol.

Figure 57. Induced flow individual percentage against coalescence time graph for 0.5% glycerol.


0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300

5

10

15

20

25

30




Ind

ivid

ua

l p

erc

en

tag

e o

f co

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sce

d d

rop

s (%

)


72

73

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 210

20

40

60

80

100

120




Cu

mu

lati

ve

pe

rce

nta

ge

(%

)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200

5

10

15

20

25

30




Ind

ivid

ua

l p

erc

en

tag

e o

f co

ale

sce

d d

rop

s (%

)

0 5 10 15 20 25 300

20

40

60

80

100

120

Cumulative percentage (%) against Coalescence time for 10.0% concentration

v/v glycerol

Series2


Cu

mu

lati

ve

pe

rce

nta

ge

(%

)

0 1 2 3 4 5 6 7 8 9 100

102030405060708090

Mean coalescence time (s) against Percentage concentration (%) v/v

glycerol

Percentage concentraion v/v glycerol (%)

Me

an

co

ale

sce

nce

tim

e (

s)




0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260

4

8

12

16

20



Series1


Ind

ivid

ua

l p

erc

en

tag

e o

f co

ale

sce

d d

rop

s (%

)


Figure 64. Induced flow mean coalescence time against glycerol concentration.

Non-Induced flow method

740 5 10 15 20 25 30 35 40 450

20

40

60

80

100

120

Cumulative percentage (%) against Coalescence time (s) for 0 % concentration

v/v glycerol


Cu

mu

lati

ve

pe

rce

nta

ge

(%

)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 400

5

10

15

20

25

Individual percentage of coalesced drops (%) against Coalescence time (s) for 0 %



Ind

ivid

ua

l p

erc

en

tag

e o

f co

ale

sce

d d

rop

s (%

)

0 20 40 60 80 100 120 140 160 1800

20

40

60

80

100

120




Cu

mu

lati

ve

pe

rce

nta

ge

(%

)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 16005

10152025303540



Coalescence time (s)Ind

ivid

ua

l p

erc

en

tag

e o

f co

ale

sce

d d

rop

s (%

)

0 20 40 60 80 100 120 1400

20

40

60

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120




Cu

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nta

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(%

)

0 10 20 30 40 50 60 70 80 90 100 110 1200

5

10

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20

25



Coalescence time (s)Ind

ivid

ua

l p

erc

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tag

e o

f co

ale

sce

d d

rop

s (%

)

0 20 40 60 80 100 120 140020406080100120

Cumulative percentage (%) against Co-alescence time (s) for 5.0% concentration v/v glycerol

Coalescence time (s)Cumulative perce

ntage (%)

0 10 20 30 40 50 60 70 80 90 100 110 120 13005101520253035

Individual percentage of coalesced drops (%) against Coalescence time (s) for 5% concentration v/v glycerol

Coalescence time (s)Individual percen

tage of coalesced d

rops (%)

Figure 65. Non-induced flow cumulative percentage against coalescence time graph for 0% glycerol.

Figure 66. Non-induced flow individual percentage against coalescence time graph for 0% glycerol.

Figure 67. Non-induced flow cumulative percentage against coalescence time graph for 0.5% glycerol.

Figure 68. Non-induced flow individual percentage against coalescence time graph for 0.5% glycerol.

Figure 69. Non-induced flow cumulative percentage against coalescence time graph for 1.0% glycerol.Figure 70. Non-induced flow individual percentage against coalescence time graph for 1.0% glycerol.Figure 71. Non-induced flow cumulative percentage against coalescence time graph for 5.0% glycerol.Figure 72. Non-induced flow individual percentage against coalescence time graph for 5.0% glycerol.

75

0 1 2 3 4 5 60

10

20

30

40

50

60

Mean coalescence time (s) against Per-centage concentration (%) v/v glycerol

Percentage concentration (%) v/v glycerol

Me

an

co

ale

sce

nce

tim

e (

s)

Figure 73. Non-induced flow mean coalescence time against glycerol concentration.

6 Conclusion

In this work, the work of Wang et al (2009) was extended such that non-induced

flow coalescence method was introduced to compare the results obtained with

induced flow. The induced flow method yielded very similar trends with the work of

Wang et al. It was found that with increasing concentration of glycerol, the

interfacial tension decreased and hence the surface free energy. Decreasing the

interfacial tension by increasing the glycerol concentration on both methods

showed opposite results. For non-induced method, the binary coalescence time

increased and at 10% glycerol concentration v/v, zero coalescence was observed.

For induced method, the binary coalescence time decreased favouring coalescence

with increasing glycerol concentration. In another perspective, at 0% glycerol

concentration, the non-induced flow resulted in lower coalescence time than that of

induced flow. This result was in good agreement with the vast literature such that

and introduction of force or shear stress increases the coalescence time enhancing

stability. It can be concluded generally that the glycerol acts as a demulsifier for

induced flow method while an emulsifier when the flow was not induced. However,

the results yielded by the non-induced flow remain sceptical due to human and

experimental errors. Due to time limitation, further investigation prohibited from

investigating repeatedly on the non-induced flow method. Hence, with this result,

the work of Wang et al can remain substantially concrete while further repetitions

should done on the non-flow induced method.

76

7 Future work

7.1 Experiment inefficiency and improvements

The design of the rig is a simple one but with various small degrees of inadequate

design approaches. The surroundings also contribute to large variations of results

obtained from the experiments. This section will detail the necessary improvements

needed to be applied onto the rig and procedures if a more accurate reproducible

work should be done to compare future results based on the similar basis.

7.1.1 Rig body

7.1.1.1 Thermal glue

The rig body is made up of polyethylene plastic which is very inert and does not

contribute any effects to the liquid systems. However, the rig is made by attaching

the edges of the pieces of polyethylene plastics together using thermal glue. During

the fabrication of the rig, the excess thermal glue was removed carefully without

leaving any residual attached to the internal parts of the rig body. However, this

was not achievable and there remained a small amount of excess dried thermal

glue at the joined edges of the plastic pieces which are significantly visible to the

naked eye. They appear to be flaky and light. This could be the reason for the

appearance of very tiny solid particles suspended in the liquid mixture. As seen

before in the literature review, solid particles do appear to stabilize the emulsion

droplets preventing them from coalescing. The thermal glue is shown in Figure.

7.1.1.2 Acetone and methanol contamination

Apart from the thermal glue issue, there were visible stains from acetone and

methanol contamination which were mistakenly used for cleaning the rig previously.

However, it was not advisable as acetone left several white translucent patches of

stains in the rig. It might have damaged the rig as the white patches were not

removable. Methanol is not advisable because of the chemical reactions with the

77

polyethylene plastic pieces. These unknown stains may not be inert and could have

potentially affected the coalescence experiment in many ways.

7.1.1.3 Protrusion cork

The protrusion which is made of polyethylene plastic to partly immobilise the rig

core is inserted and attached securely through a hole drilled at the centre bottom of

the rig body using thermal glue. The protrusion has a hollow space running vertical

through its centre. Hence the protrusion can be viewed as a solid annular cylinder

attached to the bottom of the rig body at its bottom. Obviously the hollow space will

drain the liquid mixture away and hence the cork was inserted at the bottom of the

protrusion to cease any flow of liquid out of the rig. An inert rubber cork is shaped

and inserted very tightly at the bottom of the hollow space in the protrusion.

Despite this, it may have accumulated very minute amount of kerosene or glycerol.

It was found that overnight kerosene will lead to higher stability of the kerosene

droplets. The leftover glycerol or kerosene may deposit and being trapped between

the tiny spaces between the cork and internal sides of the cylindrical protrusion.

They may have altered in their physical properties overnight. Moreover, such

trapped kerosene liquid is extremely difficult to remove once inserted tightly and

securely into the bottom of the hollow space. During washing the hollow space with

the inserted cork were strongly sprayed with RO water and the cork was not

removed for a more considerable clean. The issue of thermal glue is also similar to

that of Section 5.1.1.1.

An improvement for this is a solid cylinder instead of the annular one. The solid

cylinder will cease all potential leakage. However, the attachment of the solid

cylinder requires thermal glue.

7.1.1.4 Slanting of rig core

This is a visual problem for the camera to focus. The rig core is a cubical solid made

out of polyethylene plastic with a vertical cylindrical hollow space at the centre for

78

insertion of the protrusion. When fitted through the protrusion, the cube should lie

flat horizontally on the bottom of the rig body. However this was not the case. It

appeared that the rig core is tilted an angle less than 10o on the right. This problem

was thought to be the cause of the unequal size fitting between the cylindrical

hollow space of the rig core and the protrusion. More specifically, the diameters of

the protrusion and the cylindrical hollow space of the rig core at near bottom are

not similar. Obviously for the rig core to fit through the protrusion, the diameter of

the protrusion should be less than 1% larger than the cylindrical hollow space of the

rig core. However, it appeared that near bottom, the right side of the protrusion at

near bottom was not fabricated at the diameter as top part; like an appearance of

very small outgrowth that is invisible to the eye as shown in Figure.

The problem that this discrepancy in design can cause is the inaccuracy of image

processing of the droplets to obtain the correct interfacial tension. Due to time

limitation, there were no amendments made neither the protrusion nor the

cylindrical hollow space of the rig core. Instead, when taking images or recording

videos of the coalescence process, the camera position was adjusted to match the

inclination of the rig core, and hence the inclination of the needles, so that the tip

needles are correctly aligned horizontally and the height vertically. However, in the

videos and images, the droplets might look a bit less than 90o vertically due to

gravity and buoyancy effects.

Improvement is to eradicate the unequal shape of the protrusion with respect to the

cylindrical hollow space of the rig core.

7.1.1.5 Rig cover and entry of experimental objects

The purpose of the rig cover is to prevent the entry of unwanted solid particles

during the experiment. However, throughout the whole experiment, the cover was

79

left opened as shown in Figure. The reason for this is to allow the capillaries or tube

and the metal temperature detector to pass into the rig body. However, the gap left

by the position of the rig cover may introduce dust particles into the rig and

contaminate the liquid mixture despite the fact that these dust particles may be

light and only affects the top section of the liquid mixture. Larger solid or liquid

particles may gain entry by accident such as coughing and sneezing.

An improvement is to close the exposed top of the rig body with the rig cover

completely. Three small holes can be drilled on top of the cover for the passage of

the capillaries and metal temperature detector into the system. The size of the

holes should be small enough to just fit these objects to reduce the chance of

particles entering and contaminating the system. Sealing these holes can be

difficult because these objects are washed often.

7.1.1.6 Mobility of capillaries and thermocouple

The capillaries and thermocouple were both partly submerged in the liquid mixture.

During experiment it is essential that these objects remain stagnant to go accordant

with the basis of the experiment – no external force to disrupt the coalescence

process. From Figure, it can be seen that the capillaries and thermocouple were

stable clinging onto the top edges of the rig body. However, from experience, these

objects moved significantly due to relaxation, in this case the capillaries. The

thermocouple is attached to a BLUTACK on the top wall of the rig body and due to

the inelasticity of the BLUTACK, the thermocouple fell and moved several times in

the run. The BLUTACK contaminated the system nevertheless and the experiment

has to be redone.

To avoid such experimental slip, it is recommended that a special locking system

with screws and locks that can be custom built on the rig body to secure these

objects to cease any chance of mobility that can disturb the system.

80

7.1.1.7 Plates

The plates were cut to prevent further mobilisation of the rig core. However, more

precision and accuracy of the dimension were needed to immobilise the rig core in a

correct form and not by tilting as shown in Figure.

7.1.1.8 Mixing

Surfactants are added in by removing the rig body cover and slowly pouring the

surfactants while stirring. It is not advisable to pour at a large rate; very small rate

is needed to sufficiently mix well. The stirrer which is a metal spatula is washed and

rubbed with methanol to remove stains and rinse with RO water. Removing the rig

body cover introduces particles into the system.

7.1.2 Rig core

7.1.2.1 Placement of needles

The rig core comes with two parts – the rig core body and the rig core head as

shown in Figure. On the top sides of the cubical rig core body, V-shaped or

triangular grooves were cut off from the rig core body. The function of the grooves

is to allow the cap needles to sit on them while the rig core head presses on them

and screwed to fix and hold the positions of the needles securely. The grooves were

cut accurately to say the least as shown in Figure. However, the problem arises

when adjusting the needles.

The pair of needles was initially straight as shown in Figure and they were bent

using hands on a strongly held vertical metal pole. Such method was very crude but

due to limitations of the experimental sources, it was the only possible way to get

as much similarly bent as possible. Due to limited amount of such needles, only the

best few were chosen for the experiment. As shown in Figure, needles were bent

differently at least. The diameters of the needles were checked using a vernier

calliper instrument.

81

Because the needles are not generally similar in terms of the bent areas,

they were difficult to place in line with each other. The needles should be

vertical parallel to each other and their tips should be horizontally in line

with each other. Nevertheless such difficult task was always completed

through different levels of screw tightness. Despite this, to the naked eye

this may seemed to be aligned but if they were to be accurate measured

their relative positions, they might be not. Such accurate position was to

allow accurate determination of the coalescence time of the two droplets. In

front of the camera view, apart from the needles being vertically parallel and

the tips horizontally aligned, another criterion for accurate ‘measurement’ of

the coalescence behaviour is that when the droplets come in contact, the

droplets should be seen pressed against each other side by side without one

side behind of the other. Their circumference at the point of touching should

be like Figure and not in Figure.

There were experimental sessions where the droplets were generated as shown in

Figure which is not desirable. A major improvement for this is to replace or replicate

a similar needle of the same shape and size.

Another factor is the reproducible positions for every repetition of experiment.

Because every component has to be dissembled for thorough cleaning, upon

reassembly of the components, the general position which reflects the angle of

inclination, distance between the two needles and height will be different at least to

the one tenth of a centimetre.

82

7.1.2.2 Connectors

The connectors from the rig core come from the needles. As shown in Figure, the

connectors were not as tight as it seemed. Because the experiment requires

periodical stirring when adding surfactants, to mix well, care has to be taken to

avoid contacting the connectors. The connector to the needle is not of screw type;

slide, slot, twist and tighten type as shown in Figure and hence they are very

susceptible to the slightest disturbance introduce such as the spatula slightly

contacted it and displaced its initial position which can cause potential leakage of

the oil phase when loosen. It is rather difficult to tighten the loosen connector

without introducing contaminants into the system. When such event occurred, the

whole experiment is to be redone.

The solution to such issue is to use a screw type connector which can be very

secure.

7.1.2.3 Overall design of rig core

The rig core design was simplified one but however, the fact that it has a many

components linked to it made it more exposed to accidental errors. Perhaps to

repeat the experiment using the same methodology, the design of the rig core has

to be significantly improved. For instance, instead of locking the positions of the

needles by pressing and screwing the rig core head, a rig core can be designed

such that the needles can be inserted through a fix hole within the rig core without

any difficulty.

7.1.2.4 Capillaries

The capillaries used are made of polyethylene and they are fragile and susceptible

to sharp bents. Hence pressuring the capillaries should be avoided. One advantage

of this tube is its inert material and its internal section is visible enough to detect

undesirable solid particles from entering the system. The capillaries were more than

83

70 cm which is unnecessarily too lengthy and difficulty arises during cleaning

because even after properly cleaned, the skin of the capillaries are easily

contaminated upon contacting surfaces like the bench or laboratory coat. Hence

more caution has to be taken when handling the capillaries because part of them

was submerged into the liquid mixture. Any contamination from other particles such

as sand or oil particles can ruin the consistency of the system.

7.1.3 Pumps

Due to lack of pump resources, two different infusion pumps were used instead of

the same ones. From the pump calibration figures, it can be seen that at the same

rate, the two different pumps generated two different pump rates. Calibration was

necessary to find the true volumetric flowrate due to inefficiencies of the

instruments. It is suggested that two new similar pumps to be purchased and the

calibration from the two pumps should give equivalent true volumetric flowrate at a

given setting.

Often the pumps give obstacles for a clear coalescence process. This happened

because upon the release of the coalesced droplets or unwanted droplet from each

needle due to unsuccessful coalescence, the pumps were simultaneously stopped to

prevent the growth of droplet, but this did not happen. Even when the pumps had

stopped, the growth of the droplets from the two needles remained continuous

despite the growth rate was slow enough to allow the flow patterns to disperse

away under the basis of time 5 minutes before making first contact. However, in the

middle of the allowance time, say 2.5 minutes, the size of the droplets could be

large enough to cause force of attraction and repulsion between them. However,

despite the efforts in making changes to the pumps, such occurrence was inevitable

throughout the whole experiment.

84

7.1.3.1 Controlling the growth rate

The volumetric flowrate of the two pumps were set to the true volumetric flowrate

of 0.5ml/hr and this corresponds to 0.63ml/hr and 0.68ml/hr for red pump and white

pump respectively look at Figures. When generating the droplets, the size or

specifically the volume of the droplets has to be equal for consistent measurement.

However, difficulty arises when using the experimental pumps. Efforts were made to

generate the droplets volume before contacting each other. If the one droplet is

seen to be outgrowing the other, the flowrate generating the larger droplet is

stopped and then switched on when the smaller droplet comes to the same size as

the larger droplet. The method used in this is very inaccurate as it seemed to be.

The droplets size were always monitored regularly throughout the experiment by

observing the growth rate and the size of the droplets by placing a ruler horizontally

on the computer screen which shows the live magnification of the droplets in the

rig, and then move the horizontal ruler vertically and stop when the tip of one

droplet is found. If the tip of the droplet is not aligned horizontally with this tip, then

the flowrates are adjusted such that the tips are aligned horizontally as shown in

Figure. Again this method show very little accuracy in determining the equality of

both volumes. The volumetric flowrates must be equal to one another.

Fix volumes of droplets can be generated but such function is only available in the

red pump but not on the white one. Solution is to get the same pumps preferably

new.

Overall the pumps should be control by a controller such that one switch will trigger

the motors of the pumps and will induce equivalent flow at low flowrates. Previously

a three way valve was used but had been a failure due to the unequal flowrates

from two sides as shown in Figure.

85

7.1.4 Thermocouple

The fabrication of the thermocouple is complex and was completed successfully by

Aime. The components of the thermocouple were of detectors as shown in Figure

which are programmed to function as temperature measurer. Calibration was

necessary to obtain correct temperature measurements using the instrument.

However, throughout the experiment, the sensitivity of the temperature was very

high as shown in Figure. The x- axis can be taken as per seconds while the y-axis is

the temperature. The range of temperature variation was large from 19oC- 28oC.

And it was impossible for temperature to rise in an instant given that the room

temperature did not vary as such. The sensitivity of the thermocouple was too

large.

7.1.5 Lens

A more desirable lens is such that it can focus clearly on the interface with higher

resolution.

7.2 Environment

7.2.1 Vibrations

The vibrations were caused by 4 objects, the 2 pumps, cold light and fume

cupboard. The vibration from the pump is caused by the clicking of the motor that

turns the screws which pushes the syringe to induce flow. The motor of the cold

light also produces vibration. The fan in the fume cupboard produces the vibration.

Due to the lack of benches, the pumps and cold light were put on the same bench

as the rig. The fume cupboard is turned on continuously. These vibrations will

nevertheless affect the surroundings of the droplets.

An improvement for this would be to place the pumps and light on different and

separate benches. The fan from the fume cupboard should be switched off when not

using.

86

7.2.2 Temperature

The coalescence process is influenced by the change in temperature. Slight

temperature change can change the interfacial tension and cause inconsistency

with the coalescence behaviour. The only source of temperature gradient is the cold

light. A transparent plastic glass should be placed in front of it to absorb the heat

radiated from the light.

7.3 Data collection

To collect the binary coalescence time, for the first experiment where only touching

was allowed and no flow induced. Such method was tedious because it is very

difficult to define the point at which the surfaces touch and then wait for the

coalescence to occur without inducing flow as shown in Figure. This method of data

collection is very inaccurate. Moreover, in Section, the pumps had relaxation and

often generated slower growth rates even after the pumps were switched off. In

Figure, the dark shadow which forms at this point is taken as the point of contact.

When this occurs to the naked eye, the stopwatch begins to run until the bridging

appears. Due to the limitation of the camera software, the camera was unable to

record very high frame rates for a larger period of an event. For instance, at frame

rate 6200pps, it can only record 3.589 seconds with maximum resolution. Maximum

resolution and frame rate are desirable to obtain the point where the contact and

bridging occur. However, throughout the experiment, the videos and images were

recorded using 1000pps which allowed 22 seconds of recording.

However for the second part of the experiment, this was simpler because the flow is

induced. When the shadow starts to appear, the recording is started and the

stopwatch starts until the droplets coalesced. It was not necessary for the exact

time at which the shadow appears because the difference between the exact time

at such event occurs and the point at which the observer notice its occurrence and

simultaneously starts the stopwatch should be a fraction of a second.

87

This issue is the same as the bridging and the full coalescence of the droplets. We

humans can see Figure as a whole and stopped the time when this occurs but not

Figure (bridging). The time difference between the two should also be a fraction of a

second. Hence the accuracy in such cases is not necessary with respect to the

coalescence time recorded.

It is important that only the middle side of the surface touch of the droplets touch

and not other parts.

7.4 Image processing

The quality of the image and videos are very important especially with regards to

the formation of the bridge

7.5 Batch saturated

When carrying out the experiment for one day, it is recommended that it is

important to use just one syringe batch enough for the experiment. 5mL was

sufficiently enough to produce up to 15 droplets for 0, 0.5, 1.0, 5.0 and 10.0 %.

7.6 Overnight

It is recommended to carry out one whole batch of experiments before proceeding

to repeat the second batch to perform reproducible data or result. The saturated RO

water is prepared 12 hours before the experiment starts.

88

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noninduced flow method

mean coalescence time

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presence of glycerol

addition of glycerol