EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY FACULTY OF ENGINEERING DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY MSc in OIL AND GAS TECHNOLOGY MASTER THESIS EFFECT OF NANOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF WATER ELISAVET MICHAILIDI B.Sc. Petroleum Engineer SUPERVISOR:PROF. ATHANASIOS MITROPOULOS KAVALA2016
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EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY
FACULTY OF ENGINEERING
DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
MSc in OIL AND GAS TECHNOLOGY
MASTER THESIS
EFFECT OF NANOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF WATER
ELISAVET MICHAILIDI B.Sc. Petroleum Engineer
SUPERVISOR:PROF. ATHANASIOS MITROPOULOS
KAVALA2016
EFFECT OF NANONOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF
WATER
by
Elisavet D. Michailidi
Submitted to the Department of Petroleum and Natural Gas Technology,
Faculty of Engineering
in Partial Fulfillment of the Requirements for the Degree of
Masters of Sciences in the Oil and Gas Technology
at the
Eastern Macedonia and Thrace Institute of Technology
APPROVED BY:
Thesis Supervisor: Athanasios Mitropoulos
Committee member:
Committee member:
Date defended: xx.xx.2016
EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY
FACULTY OF ENGINEERING
DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
MSc in OIL AND GAS TECHNOLOGY
MASTER THESIS
EFFECT ON NANOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF WATER
As stated in Chapter 1, this study aimed at giving evidence for the existence of
nanobubbles as well as studying their effect on the properties of water. As part of the
research, a number of experiments were conducted; the experimental procedures are
extensively described at Chapter 3. The purpose of this chapter is to summarize the
collected data, present them in a comprehensive way. Finally, the results are
extensively discussed and associated the extracted results with the theory.
4.2 TYNDALL EFFECT
The first experimental evidence of the existence of micro-nanobubbles in water was the
observation of the Tyndall effect in MNB-enriched water.
The sample was hit with monochromatic green light beam, and the Tyndall effect was
observed as it can be seen in Figure 4.1 and Figure 4.2.
Figure 4.1 Presence of Tyndall scattering in a sample containing micro-nanobubbles
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 52 - 2016
Figure 4.2 Tyndall scattering is observed in the first two samples which contain nanobubbles. The
phenomenon cannot be observed to the last sample (right) which is simple water
The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a
colloid or else particles in a very fine suspension (Figure 4.3). In this case, the Tyndall
effect indicates the presence of gaseous phase in the form of nanobubbles in the water.
Due to the particles (in this case bubbles), which absorb light energy and then emit it
the beam can be seen in the sample.
Figure 4.3 Tyndall effect. in colloidal solution light beam is visible. This is due to the particles (in
this case bubbles) absorb light energy and then emit it
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 53 - 2016
4.3 SIZE DISTRIBUTION
The size distribution of O2nanobubbles in water was determined by Dynamic Light
Scattering. The results were extracted using the ZetaSizer software and processed
using Microsoft Excel.
4.3.1 SIZE OF MNB AS A FUNCTION OF TIME
The distribution of size of micro-nanobubbles, is varied as a function of time. The
samples were measured for 7 weeks after their production, and the results are
presented in the following diagrams.
Figure 4.4 Size - Time Diagram for Porous Plug 10 min Sample
Obviously, during the first day after production, microbubbles still occur in the water and
the average size of the MNB is 1071 nm. In the second day after production, the mean
size drops to 649 nm. This fact indicates that bigger bubbles had burst while smaller
size bubbles still occur. However, three days after production the mean size rises again
up to 865 nm and reaches 991 nm at day 8. This result, indicates that some of the
bubbles a coalescing and form bigger bubbles, following the Ostwald rippening
phenomenon.
1071
649,9
865,1 859
974,3 991,7
0
200
400
600
800
1000
1200
Size
(n
m)
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 54 - 2016
Figure 4.5 Size - Time Diagram for Porous Plug 20 min Sample
The same phenomenon was also observed for the 20 min porous plug sample. Here,
the decrease of size is much more steep, dropping from 2370 nm the first day, to 769
nm two days after production and then remaining quite stable.
Figure 4.6 Size - Time Diagram for Porous Plug 30 min Sample
However, as the production time is increased to 30 mins, the size, one day after
production is significantly lower (796.8 nm). This means that the generator produces
less microbubbles.
2370,00
769,60 756,90 794,70 776,30 777,90
0,00
500,00
1000,00
1500,00
2000,00
2500,00
1st day 2nd day 3rd day 4th day 5th day 8th day
Size
(n
m)
796,8
847,9 860,9
764,7
722,4
650
700
750
800
850
900
1st day 2nd day 3rd day 4th day 8th day
Size
(n
m)
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Elisavet D. Michailidi - 55 - 2016
Figure 4.7 Size - Time Diagram for Porous Plug 40 min Sample
The 40 min sample seems to have a great stability over time. It can be observed that for
a period of 7 weeks, the mean size of bubbles dropped from 578 nm the first day after
production to 516 nm the last day. Due to the long production time, no micro-bubbles
occur.
Figure 4.8 Size- Time Diagram for Vibrating Generator Samples
The samples generated from the vibrating device were measured every 24 hours for
seven days. Here, it is also observed that the NB size tends to increase over time, while
the production time has an important effect on the size. The 40 min sample, appears to
578,8 545,4 534,3
610,6 597,8
546,7 563,1
588,5 554,3
588,8
435,2
516
0
100
200
300
400
500
600
700
1st day 3rd day 4th day 5th day 8th day 9th day 10th day 11th day 15th day 17th day 24th day 7 weeks
Size
(n
m)
0
200
400
600
800
1000
1200
1400
1600
1800
1st day 7th day
10 min
20 min
30 min
40 min
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 56 - 2016
not only have smaller sized bubbles but also is more stable. As the time passes, the
water is re-circulates into the tank, so bigger bubbles burst driving to the formation of
smaller size bubbles.
The 40 min sample was measured again after seven weeks. The average size is at this
time 200 nm. The hypothesis is again that bigger bubbles had burst but the
concentration is much lower.
Figure 4.6 depicts the size - intensity distribution for porous plug generator and vibration
generator for the 40 min sample. Also, the Auto Correlation Function diagram is cited.
Comparing the results of DLS for both methods (i.e. porous plug and nozzle) it can be
seen that the size of NB from porous plug generator is smaller than those produced by
the nozzle (824 nm), fact also shown from auto-correlation function (ACF) decay time,
as it can be seen that the curve for the porous plug is much steeper. However, the
nozzle performs more uniform distribution compared to the porous plug; where two
peaks are observed at 580 nm and 120 nm. Figure 4.6 shows the correlation curve of
the typical size of particles. Since the Brownian motion of large particles is slow and the
fluctuation of scattering light intensity changes slowly, the correlation will persist for a
long period of time. Moreover, since the Brownian motion of small particles is fast and
the fluctuation of scattering light intensity changes quickly, the correlation will reduce for
a short period of time. The diffusion coefficient of particles
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 57 - 2016
Figure 4.9 Size distribution of NB produced by porous head (blue) and nozzle (green) generators.
At the left: The auto-correlation coefficient (ACF) diagram in the same colors.
Figure 4.10 Size-Production Time Diagram for Vibrating and Porous Plug Generator
y = -9,2173x + 979,14
y = -16,813x + 1427,4
300
500
700
900
1100
1300
1500
0 5 10 15 20 25 30 35 40 45
Size
(n
m)
Production Time (min)
Porous Generator Vibrating Generator Linear (Vibrating Generator)
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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Figure 4.7, presents the effect of production time to the average size of the MNB for
both generators. It is observed that for both samples the tendency is negative, and the
average size decreases as a function of time. This can be explained by the fact that as
the process continues, the O2 saturation of the water increases as it re-circulates into
the tank, leading to the formation of smaller bubbles. In any case, the porous plug
generator produces smaller sized bubbles than the vibrating generator.
4.3.2 SIZE OF NB AS A FUNCTION OF TEMPERATURE
Temperature is an important factor affecting the size distribution of nanobubbles. The
samples were measured at different temperatures and the results are shown below.
Table 4.1 NB size as a function of temperature
Day 1
p40 size (nm)
37 469,1
25 529,9
5 640
Day 1
v40 Size (nm)
37 538,9
25 404,2
5 664,5
Figure 4.11 Nanobubble size as a function of temperature for the porous plug (P40) and nozzle
(V40) generators after 40 minuntes of operations
The samples were measured at 37 oC, 25 oC and 5 oC. Interestingly, the mean size
slightly decreases as the temperature is increased. This could be caused by the fact
0
100
200
300
400
500
600
700
800
900
1000
37 25 5
P40
V40
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 59 - 2016
that in lower temperatures the hydrogen bonds are stronger, then preventing the gas
escaping the bubble and expanding.
4.4 ZETA POTENTIAL
As mentioned earlier, zeta potential is indicative of the stability of nanobubbles. The
measurements were run along with DLS measurements and the results are shown
below.
Figure 4.12 Zeta potential as a function of time for the porous plug generator samples
The absolute value of zeta potential increases to a maximum of -18.3 mV for the 20 min
sample and then constantly decreases. However, seven days after the production, zeta
potential increases as the production time increases. In conjunction with the DLS
results, absolute zeta potential value increases as the bubble size decreases. This is
explained by the fact that smaller bubbles offer more surface for ions to be absorbed
onto the surface, thus repelling the bubbles.
The high value of ζ-potential can be related to the stability of bubbles, explained by the
repulsion forces generated by the electrically charged surfaces of bubbles, which avoid
the bubble coalescence.
The negative value is explained by Kelsall et al. (1996) as attributed to the
predominance of hydroxide ions in the first molecular layers of water at the gas-liquid
interface. It is also described by Najafi et al. (2007) that the negative charge on the
bubble surface is believed to be due to preferential adsorption of hydroxyl ions (OH- ). It
is also described that as the enthalpy of hydration of hydrogen ion (H+ ) and OH- is -
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
1st day 7th day
10 min
20 min
30 min
40 min
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 60 - 2016
1104 and -446.8 kJ·mol-1, respectively, H+ preferentially remain in the bulk aqueous
phase, leaving space at the gas-water interface for OH- . Similar understanding is that
an increase in OH concentration near the bubble surface suppresses the dissolution of
gas from bubbles into water and serves as “shells” for the bubbles, thus improving
stability (Takahashi, 2005). Apart from the zeta potential, an explanation for the NBs
stability is reported as the interface of NBs consists of hard hydrogen bonds that are
similar to the hydrogen bonds found in ice and gas hydrates (Ohgaki et al., 2010).
At a high absolute zeta potential, the electrical charged particles tend to repel each
other, avoiding aggregation of particles in a colloidal dispersion. In the case of NB
dispersion, the high absolute values of zeta potential could create repulsion forces that
would avoid the coalescence of NBs and contribute to the stabilization of the NBs.
In conclusion, the higher initial concentration of dissolved gas in water could explain the
extension of the NB stability because a higher dissolved gas concentration is expected
to suppress the dissolution of gas from NB into water.
However, for the vibrating generator, the results are quite different.
Figure 4.13 Zeta potential as a function of time for the vibrating generator samples
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
1st day 7th day
10 min
20 min
30 min
40 min
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 61 - 2016
The combined results for the average zeta potential for both generators, is presented
below.
Figure 4.14 Effect of production time on zeta potential for the porous plug (blue) and nozzle
generator (red)
4.5 OPTICAL & CONFOCAL MICROSCOPY
Images of nanobubbles were recorded using laser scanning confocal microscope. The
sample was dyed with fluorescein. Fluorescein is a synthetic organic compound
available as a dark orange/red powder slightly soluble in water and alcohol. It is widely
used as a fluorescent tracer. In low concentrations, the color in aqueous solutions is
green. The images are a strong evidence of the existence of nanobubbles in aqueous
solutions. The sample used was produced from the low-pressure cavitation nanobubble
generator. It is observed that the size of bubbles is approximately 1000 nm.
For the optical microscopy images, the sample used was produced from the nozzle (left)
and porous plug generator (right), after 40 minutes of operation. It can be optically
observed that the porous plug generator, gives a larger concentration of nanobubbles
compared to the nozzle generator.
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
0 5 10 15 20 25 30 35 40 45
zeta
po
ten
tian
(m
V)
Production time (min) Porous Plug Vibrating
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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More specifically, it was calculated that the NB concentration for the porous plug
generator is approximately 750×103 NB/cm2 or 750×106 NB/ml. On the other hand, the
nozzle generator only produces 125×103 NB/cm2 or 125×106 NB/ml.
Figure 4.15 Optical microscopy images for the nozzle (left) and porous plug generators (right).
Both of the samples were taken after 40 mins of operation.
Furthermore, it can be seen that in the nozzle sample the bubbles are aggregated. The
aggregation happens due to the fact the size of the bubbles is bigger compared to the
porous plug generator. As a result, the bubbles are less stable and this leads to form
aggregates. According to Hunter, this happens due to the fact that bigger bubbles have
low potential and fluctuate[30].
Figure 4.16, is a photograph taken with confocal microscope. The nanobubbles, colored
with fluorescein can be clearly seen as bright green spots. Indicatively, some of them
are marked with red arrows. The mean size is 1000 nm.
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 63 - 2016
Figure 4.16 Confocal Microscopy Image of a Nanobubble Sample, with fluoresceine
4.6 POROUS PLUG CHARACTERIZATION
The porous plug was characterized using a Scanning Electron Microscope, and the
following images were captured.
The mean size of the sinterened metal spheres is about 150 μm. If it is assumed that
the angle formed between three tangents of the circles is 60o then the radius of the
inscribed circle is:
r=75(1-cos30o)/cos30o=11.5μm.
The porous plug structure plays a critical role to the size of the produced nanobubbles
as the mixture of water and gas is forced through the plug under great pressure and
nanobubbles are formed during this procedure. Thus, the smaller size of the metal
spheres leads to smaller bubbles. Moreover, the NB size distribution is depended on the
size distribution of the spheres; a more uniform sintered spheres distribution leads to
more uniform NB size distribution. Hence, the porous plug is an expedient to control the
properties of the produced samples.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 64 - 2016
Figure 4.17 Scanning Electron Microscope images from the sintered porous plug at X140 (up) and
X40 (down)
4.7 VAPOR PRESSURE MEASUREMENTS
The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to
the tendency of particles to escape from the liquid (or a solid). A substance with a high
vapor pressure at normal temperatures is often referred to as volatile. The pressure
exhibited by vapor present above a liquid surface is known as vapor pressure. As the
temperature of a liquid increases, the kinetic energy of its molecules also increases. As
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 65 - 2016
the kinetic energy of the molecules increases, the number of molecules transitioning
into a vapor also increases, thereby increasing the vapor pressure.
Figure 4.18 Vapour Pressure of NB samples, produced from the porous plug generator at different
temperatures; 20 oC (blue), 30
oC (red) and 40
oC (green)
It is observed that VP value for the NB45 sample shows an increase of approx.116%. It
is well known that increased vapor pressure indicates weaker intermolecular forces.
This is related to the surface tension of water, changes with the introduction of
nanobubbles according to many researchers. Ohgaki et al.[36] suggested that this is
strongly related to hydrogen bonding at water–gas interface. They reported that the
surface of the nanobubble contains hard hydrogen bonds. More recently, Wang, Liu,
and Dong[37] (2013) reported that the surface of a nanobubble is kinetically stable and
the water–gas interface is gas impermeable. A reduction in surface tension is observed.
Tolman and others predict a decrease of the surface tension for large curvature on
small scales[38-41]. Specifically, Tolman calculated theoretically that the surface tension
in drops should decrease significantly at small sizes.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 66 - 2016
4.8 CONDUCTIVITY MEASUREMENTS
Figure 4.19 Electrical Conductivity as a function of production time
Figure 4.16 presents the effect of production time on the electrical conductivity of the
samples as a function of time. It is evident that electrical conductivity highly increases
with the introduction of NB in water and is strongly depended on production time.
The results are in agreement with the theory, which suggests the existence of –OH and
–H ions. . It is described by Najafi et al[34]. (2007) that the negative charge on the bubble
surface is believed to be due to preferential adsorption of hydroxyl ions (OH- ). It is also
described that as the enthalpy of hydration of hydrogen ion (H+ ) and OH- is -1104 and
-446.8 kJ·mol-1, respectively, H+ preferentially remain in the bulk aqueous phase,
leaving space at the gas-water interface for OH-. Therefore, hydrogen ions (H+) are
remaining in the bulk and their presence increases the electrical conductivity.
0
5
10
15
20
25
30
35
-10 10 30 50 70 90 110 130 150
Co
nd
uct
ivit
y (μ
S)
Production Time (min)
Conductivity-Production Time
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 67 - 2016
Figure 4.20 Distribution of ions at and near the gas-water interface in an aqueous solution of
electrolyte. The electrolyte ions are attracted to the interface and create the electrical double
layer.
4.9 EFFECT ON BIOLOGICAL MATTER; THE CASE OF PLANTS
The application of MNB technology in biological processes has been examined. Water
that contains MNBs has been reported to accelerate the growth of plants. Experiments
were conducted using oxygen and atmospheric air nanobubbles on soya and oat plants.
Micro-nanobubble enriched water was used on the aforementioned plants and their
growth rate was examined in comparison with normal water. All the plants were
exposed to the same environmental conditions and watered with the same volume.
In Figure 4.21, the results for oat plants are pictured after 8 days, while figure 4.22
shows the soya plants growth.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 68 - 2016
Figure 4.21 Left: Oat seeds watered with oxygen nanobubbles; Middle: Oat seeds watered with
atmospheric air nanobubbles; Right: Oat seeds watered with normal water
Figure 4.22 Left: Soya seeds watered with oxygen nanobubbles; Middle: Soya seeds watered with
atmospheric air nanobubbles; Right: Soya seeds watered with normal water
More experiments were conducted on wheat; and total plant weight was measured.
CHAPTER 4: RESULTS AND DISCUSSION
Elisavet D. Michailidi - 69 - 2016
Figure 4.23 Wheat plant dry weight as a function of time.
These results suggested that NBs in water could influence its physical properties, which
provides an explanation for the effect of NB promotion on the physiological activity of
living organisms. Negatively charged NBs may influence the bioelectric field of plants,
which is strongly related to their elongation growth. Previous studies[81] have
demonstrated that hyperoxia promotes the growth of plants; air and oxygen-
nanobubbles may affect the growth of life by changing oxygen condition. Furthermore, it
is speculated that larger specific surface area of the microbubbles as well as negative
electronic charges on their surface may promote the growth of plants because
microbubbles can attract positively charged ions that are dissolved in the nutrient
solution.
It is suggested that hyperoxia may induce hypermetabolic state to maintain higher rate
of food digestion and absorption. These reports are in accordance with the results of our
study, suggesting that air and oxygen-nanobubble water solution may contribute to
elevated metabolism and promoted growth.
0
50
100
150
200
250
1 2 3 4 5
We
igh
t (g
r)
Days
NANOBUBUBBLES Ο2
Normal Water
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
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As we have not used other gas-nonobubble water, whether promoting effect on growth
is due to nanobubbles themselves or elevated oxygen concentration in the water is still
unresolved. Further examination using other gas nanobubbles is required to determine
the effect of nanobubbles themselves on growth of lives. Although there are several
limitations, air and oxygen-nanobubble water significantly promoted the growth of plants
After completely understanding NBs' ability to promote plant growth is achieved, the
manipulation of NBs will provide an efficient and cost-effective approach for the
cultivation of hydroponic vegetables and allow the development of a new technology in
agriculture applications.
CHAPTER 5: CONCLUSIONS
Elisavet D. Michailidi - 71 - 2016
5. CHAPTER 5
CONCLUSIONS
5.1 CONCLUSIONS
The results of this study are strong experimental evidences for the existence of
nanububbles in the bulk as well as their effect on important water properties such as
vapor pressure and conductivity.
As it is is mentioned, two types of nanobubble generators were designed and
manufactured; the “porous plug generator” and the “nozzle generator”. The nozzle
generator is based on the Venturi effect. In the Venturi-type generator system, both gas
and liquid are passed simultaneously via the Venturi tube to generate the bubble. When
pressurized fluid is introduced in the tubular part, the liquid flow velocity in the cylindrical
throat becomes higher whereas pressure becomes lower compared to the inlet section,
thus resulting in cavitation. According to the literature, similar generators already exist
and are studied by many researchers. However, the porous plug generator is an
innovative device which was designed in EMaTTech and is under EPO patent. Hence,
it was of vital importance to thoroughly examine both generators and compare their
performance.
The first experimental evidence of the existence of micro-nanobubbles in water was the
observation of the Tyndall effect in MNB-enriched water. In this case, the Tyndall effect
indicates the presence of gaseous phase in the form of nanobubbles in the water. Due
to bubbles, which absorb light energy and then emit it, the beam can be seen in the
sample.
As it derives from Dynamic Light Scattering and zeta potential measurements,
nanobubbles produced from the porous plug generator are smaller (≃580 nm) and more
stable ( -20 mV for 40 mins of operation) compared to those produced from the nozzle
generator, the mean size of which is ≃580 nm and their zeta potential is -6 mV. This
fact is also shown from auto-correlation function (ACF) decay time, as it can be seen
that the curve for the porous plug is much steeper. However, the nozzle performs more
uniform distribution compared to the porous plug; where two peaks are observed at 580
nm and 120 nm.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 72 - 2016
The high value of ζ-potential can be related to the stability of bubbles, explained by the
repulsion forces generated by the electrically charged surfaces of bubbles, which avoid
the bubble coalescence. The negative value is explained by the predominance of
hydroxide ions in the first molecular layers of water at the gas-liquid interface.
The zeta-potential measurement shows that the nanobubbles are negatively charged
with an electric double layer, presumably due to adsorption of negative OH- ions at the
gas/water interface. It is this double layer that plays a critical dual role in the formation
of stable nanobubbles in aqueous solutions. It not only provides a repulsive force to
prevent interbubble aggregation and coalescence but also reduces the surface tension
at the gas/water interface to decrease the internal pressure inside each bubble.
Another important factor to examine was the concentration of nanobubbles in the liquid.
The concentration was calculated based on microscopy images. It seems that the
porous plug generator produces ≃750×103 NB/cm2 or 750×106 NB/ml. On the other
hand, the nozzle generator only produces ≃125×103 NB/cm2 or 125×106 NB/ml.
It was observed that production time is an important factor for both generators; large
production time leads to the formation of smaller and more stable bubbles. . It is
observed that for both samples the tendency is negative, and the average size
decreases as a function of time. This can be explained by the fact that as the process
continues, the O2 saturation of the water increases as it re-circulates into the tank,
leading to the formation of smaller bubbles.
All of the samples were examined for several weeks. According to Dynamic Light
Scattering Measurements, the average size of the bubbles tends to decrease a few
days after production due to the fact that larger bubbles burst and smaller ones remain.
However, as times passes, the mean size tends to increase again. This can be
explained by the phenomenon of Ostwald ripening.
Again, porous plug nanobubbles seem to have an excellent stability over time. After 8
weeks their size dropped from 578 nm to 516 nm.
Temperature also has an effect on size, which decreases with reduction of temperature.
Electrical conductivity and vapor pressure which are some of the most important
properties of water were studied. It turns out that both of them were affected from the
introduction of nanobubbles. Nanobubble-enriched water has a significantly higher
electrical conductivity than normal water, due to the excess of free ions in the bulk.
CHAPTER 5: CONCLUSIONS
Elisavet D. Michailidi - 73 - 2016
Moreover, the vapor pressure is higher due to the fact of weaker bonds between the
molecules.
It is observed that vapor pressure value shows an increase of approx.116%. It is well
known that increased vapor pressure indicates weaker intermolecular forces. This is
related to the surface tension of water, changes with the introduction of nanobubbles
according to many researchers. It is suggested that this is strongly related to hydrogen
bonding at water–gas interface. They reported that the surface of the nanobubble
contains hard hydrogen bonds.
The growing significance of nanotechnology as well as the special properties of
nanobubbles has drawn huge attention in many sectors due to their wide range of
applications, including mine industry, medical applications, food processing and
wastewater treatment.
The effect on biological matter has been studied; water that contains MNBs has been
reported to accelerate the growth of plants. Micro-nanobubble enriched water was used
on soya, oat and wheat plants and their growth rate was examined in comparison with
normal water. All the plants were exposed to the same environmental conditions and
watered with the same volume. These results suggested that NBs in water could
influence its physical properties, which provides an explanation for the effect of NB
promotion on the physiological activity of living organisms. Negatively charged NBs may
influence the bioelectric field of plants, which is strongly related to their elongation
growth. Hyperoxia promotes the growth of plants; air and oxygen-nanobubbles may
affect the growth of life by changing oxygen condition. Furthermore, it is speculated that
larger specific surface area of the microbubbles as well as negative electronic charges
on their surface may promote the growth of plants because microbubbles can attract
positively charged ions that are dissolved in the nutrient solution.
5.2 FURTHER RESEARCH
The purpose of the dissertation was to elucidate the effects of nanobubble suspensions,
produced with nanobubbles generators, and study the nanobubble formation, size
distribution, coalescence, stability and dynamic behavior. Consequently, gain insight
into the properties of nanobubbles. This study discussed the effects of bulk
nanobubbles on the physicochemical properties of water based on research results
from a variety of experiments.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 74 - 2016
While their existence has been confirmed, there are many open questions related to
their formation and dissolution processes along with their structures and properties,
which are difficult to investigate experimentally.
It is considered important to examine nanobubbles under a wide range of pH in order to
gain understanding on their charging mechanism.
Moreover, nanobubbles consist of a condensed gaseous phase with a surface tension
smaller than that of an equivalent system under atmospheric conditions, and contact
angles larger than those in the equivalent nanodroplet case. We anticipate that further
study will provide useful insights into the physics of nanobubbles and will stimulate
further research in the field. For that reasons, surface tension and contact angle
measurements should be conducted.
MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY
Elisavet D. Michailidi - 75 - 2016
6. ABBREVIATIONS – INITIALS
MNB Micro-Nano Bubbles
NB Nano-Bubbles
VP-LP Vapor Phase-Liquid Phase
BP Boiling Point
MP Melting Point
VP Vapor Pressure
DLS Dynamic Light Scattering
SEM Scanning Electron Microscope
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