Patterns from Drying Drops Khellil Sefiane † 1 School of Engineering, The University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh, EH9 3JL, United Kingdom. † Email: [email protected]Abstract The objective of this review is to investigate different deposition patterns from dried droplets of a range of fluids: paints, polymers and biological fluids. This includes looking at mechanisms controlling the patterns and how they can be manipulated for use in certain applications such as medical diagnostics and nanotechnology. This review introduces the fundamental properties of droplets during evaporation. These include profile evolution (constant contact angle regime (CCAR) and constant radius regime (CRR)) and the internal flow (Marangoni and Capillary flow (Deegan et al. [22])). The understanding of these processes and the basic physics behind the phenomenon are crucial to the understanding of the factors influencing the deposition patterns. It concludes with the applications that each of these fluids can be used in and how the manipulation of the deposition pattern is useful.
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1.1 History of pure liquid drop evaporation · Web viewfor the early diagnosis of urolithiasis. The mechanisms behind basic drop evaporation, whereby a pure liquid drop evaporates
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Patterns from Drying DropsKhellil Sefiane†
1School of Engineering, The University of Edinburgh,
premature delivery, (g) threatened abortion and (h) hepatitis. The patterns left by samples
from three patients for each condition are shown in figure 9. This figure was modified from
an image in article [32] in which patterns from five patients were shown for each condition.
It can be seen from figure 9 that the patterns left by serum drops with different conditions
vary massively making them easy to differentiate between, however the patterns also differ
slightly from patient to patient with the same condition. This makes it difficult to diagnose
patients with certainty and makes the process very subjective.
Figure 9: Patterns left from dried drops of serum from blood samples of individuals with the
7 listed conditions and one control. Modified from [32]
The investigations of human tear fluid might give indications of ocular disease. In an article
by Filik [37] the importance of the concentrations of different proteins in the tear fluid are
described, they are responsible for keeping the cornea lubricated to allow blinking and
fighting infection etc. A fluctuation in protein levels could give an indication of ocular
disease. This article only describes the use of Raman Spectroscopy, but a similar diagnosis
could be concluded by comparing the patterns from dried drops of tear fluid since the levels
of protein will alter the patterns.
3.3 Nanotechnology
The advent of nanotechnology in the past decade has enticed a large number of people to
research this area. This can be seen by the large increase in the number of publications, there
were 720 papers published in 2010 and 2011 alone [39]. Nanotechnology can utilise the
patterns from drying drops to make new materials, such as gold nanorings or to deposit
materials onto micro-structures, for example gold nanorods spontaneously placed onto single
walled carbon nanotube surface [34]. Patterns formed by drying drops laden either with
nanoparticles or fullerenes have revealed complex formations. The full understanding of the
mechanisms behind these patterns and their exploitation for various technological purposes is
still an open field, Figure 10.
Figure 10 : (a) Patterns from drying Fullerenes, Y.Chen et al., Appl. Phys. Lett. 102, 041911 (2013)]. (b) Patterns formed from Al2O3-H2O nanofluid droplets at various temperature and concentrations [3].
(a) (b)
3. Conclusions
The most common and well understood deposition pattern from a drying drop is the coffee-
ring effect. It is well documented in most papers regarding this topic [1, 20, 21, 25, 29, 40,
41]. Deegan et al. [27] explained that the only conditions required to form the ring are contact
line pinning and evaporation. These are two conditions that are met in most droplets making
the ring such a recurrent phenomenon. As explained previously, this effect is caused by
Capillary flow which sweeps the suspended particles outwards to the edge of the wetted
contact area which can then dry instantaneously to help pin the drop [1]. This is known as
self-pinning where the dried solute particles help to keep the radius of the drop constant.
The suppression or prevention of Capillary flow can be achieved by changing different
factors including; substrate temperature, suspended particle size and solvent type. By
changing the strength of Marangoni and Capillary flow inside the drop the deposition pattern
left can be manipulated. Other types of patterns seen from the drying of drops are; uniform,
central deposits and inner rings. Jung-Hoon Kim et al. [13] concentrated on the manipulation
of the central region of deposits in their experiment and found that a cooler substrate
temperature decreases the Capillary flow and increases the inward Marangoni flow. An
experiment by Yongjoon et al. [1] showed that larger suspended particles do not show the
coffee ring effect as predominantly as smaller particles and tended to be deposited in the
central region.
Biological fluids are incredibly complex, containing numerous different materials that
interact with each other to create very complicated patterns and make them very difficult to
model. Disease and diet can change biological fluid composition which changes the patterns
left from dried drops of infected patients, allowing for its use in diagnosing patients. A good
example of the different types of pattern seen from infected patients is given in figure 9. The
use of patterns from drying drops in diagnostics can start to be more widely used now that a
large database of repeatable patterns has been collected following the increase in research
over the past two decades [3, 26, 36].
The understanding of mechanisms behind patterns left from complex fluids is still lacking.
The experiment carried out by Kajiya et al. [29] can open up avenues to help improve the
existing knowledge of flow regimes inside an evaporating drop through their use of
fluorescent microscopy. It would be nice to see the patterns from drying drops used for the
early diagnosis of patients since it is a cheap and minimally invasive method. The patterns
left from drops of biological fluid of similar condition should be regularly recorded and with
the increase in repeatable patterns it can be hoped that this technique will be used more in the
future and with more confidence.
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