1.1 History of pure liquid drop evaporation · Web viewfor the early diagnosis of urolithiasis. The...
Transcript of 1.1 History of pure liquid drop evaporation · Web viewfor the early diagnosis of urolithiasis. The...
Patterns from Drying DropsKhellil Sefiane†
1School 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.
The most commonly seen pattern is the coffee-ring deposit [27] which can be seen frequently
in real life from tea/coffee stains and in water colour painting. This is caused by an outward
flow known as Capillary flow which carries suspended particles out to the edge of the wetted
area. Other patterns that were found were uniform, central deposits and concentric rings
which are caused by inward Marangoni flow. Complex biological fluids displayed an array of
different patterns which can be used to diagnose patients.
Content
1. Introduction............................................................................................................................
1.1 History of pure liquid drop evaporation.....................................................................
1.2 Profile Evolution........................................................................................................
1.3 Flow regimes inside a droplet....................................................................................
1.4 Complex evaporation situations.................................................................................
2. Patterns from Drying Drops.................................................................................................
2.1 Polymers.....................................................................................................................
2.2 Paints..........................................................................................................................
2.3 Biological fluid...........................................................................................................
2.4 Nano-particle suspensions..........................................................................................
3. Applications............................................................................................................................
3.1 Polymers.....................................................................................................................
3.2 Biomedicine...............................................................................................................
3.3 Nanotechnology..........................................................................................................
4. Conclusion...............................................................................................................................
References...................................................................................................................................
1. Introduction
Study of drop evaporation has been growing in number more rapidly since the 1980s. This
increase in activity is due to rising demand for its application in fields such as inkjet printing,
paints, polymers, nanotechnology and medical diagnostic techniques [1]. Two fields of
particular interest recently are nanotechnology, which has appeared in the past decade [2],
and diagnosis methodology, which appeared in the past two decades. Firstly appearing in the
‘Litos Test’ system from Russia [3] for the early diagnosis of urolithiasis. The mechanisms
behind basic drop evaporation, whereby a pure liquid drop evaporates into surrounding still
air, are fairly well understood. Recently improved understanding of the mechanisms behind
more complex drop evaporation has allowed fast improvements in these fields and will give
scope for future study.
1.1 History of pure liquid drop evaporation
The history of the study drop evaporation starts in 1877. When Maxwell published an article
entitled “Theory of the Wet Bulb Thermometer” in which he derived equations for basic drop
evaporation [2, 4-8]. Maxwell believed the drop evaporation process to be diffusion-
controlled [5] due to the difference in vapour concentration between the surface of the drop
and in the surrounding bulk air [9]. Further study in this area has proved his initial
hypothesis, although correct nonetheless incomplete, since it is now accepted that drop
evaporation arises by a combination of heat and mass transfer. Heat is transferred to the drop
from the surroundings by all three modes of heat transfer and mass is transferred from the
drop to the surroundings by convection and diffusion [2]. However, his rudimentary
equations were used as the basis for a lot of the study that followed. Further studies from
Maxwell time have also shown that the evaporation of drops is proportional to other factors
such as: vapour pressure, radius of the drop and surface tension.
Sreznevsky discovered that the evaporation of hemispherical drops from a flat plate was
proportional to the vapour pressure of the evaporating liquid [2]. Following this, Morse added
that evaporation rate was proportional to the radius of a spherical drop [4] and after analysing
his results, Langmuir decided that this was the case based on the relationship between
diffusion and conduction heat transfer [2, 4, 10].
The effect of convection mass transfer, in which, the evaporating vapour from the surface of
the drop is swept away by the surrounding air is known as Stefan flow. It was Fuchs that
introduced this concept into the Maxwell equations [5, 6]. Fuchs was a very influential man
in these early investigations. In 1959, his article entitled “Evaporation and droplet growth in
gaseous media” [11] was translated from Russian and became a crucial resource since it
analysed and criticised all major research work that had been carried out up until then [2].
Sazhin [6, 12], Erbil [2] and Tonini [5] explain the more recent progress made in the
mathematical modelling of drop evaporation, these go into more detail than this review will
cover. Each of these articles gives a thorough history of progresses made in drop evaporation
research starting from Maxwell time. Sazhin concentrates on drops of fuel whereas Tonini
uses general liquid drops and compares new models for unsteady state evaporation with the
more basic Maxwell, Stefan and Fuchs models. Erbil, in his review [2] explains the
evaporation of isolated sessile and spherical drops.
1.2 Profile Evolution
During drop evaporation, two accepted theories for profile evolution are constant radius
regime (also known as constant contact area regime) and constant contact angle regime [13,
14]. In the case for constant radius regime (CRR) the droplet is pinned to the surface and the
height of the drop falls as the fluid evaporates. In constant contact angle regime (CCAR) the
radius decreases but the height remains constant. In reality a combination of both of these
usually occurs since a drop usually follows a CRR until a critical contact angle is reached at
which point the remaining fluid follows a CCAR until the fluid is fully evaporated.
Manipulation of the type of flow inside the droplet can control the type of deposition pattern
left by the drop [13]. Later, this combination of regimes can be used to explain the patterns
left by dried biological fluid drops, for example, a thick outer ring of protein with crystallised
salt in the centre which is seen in experiments carried out by Yakhno et al. [15] and Meloy
Gorr et al. [16].
Guilizzoni et al. [17] investigated the shape of the water droplet before and during
evaporation looking at contact angle, height and contact area as parameters. Their experiment
investigated the effect of surface effusivity, finned surfaces and Weber number on the rate of
evaporation and the droplet shape. The results show that droplets with a higher weber number
(a measure of the relative importance of the fluid's inertia compared to its surface tension)
gave more promising results since they had a larger contact area on finned surfaces which is
as expected. Those with smaller Weber numbers did not have a larger contact area in the
finned surface. The further understanding of profile evolution in this case would have a large
impact in the applications of dropwise cooling since contact area is directly proportional to
heat transfer.
Panwar et al. [18] found that a water sessile drop on a glass or polycarbonate surface
followed the CRR. On the other hand, Xu et al. [19] investigated a complicated system in
which micro-posts were positioned at regular intervals and then coated in the highly
hydrophobic material, Teflon. A water droplet containing varying concentrations of
suspended gold micro-particles was then placed onto the surface and the profile regimes were
recorded. The results showed that the fluid exhibited CCAR for a short while as it passed a
post and CRR between posts until it finally exhibited a fixed mode. This experiment has huge
applications in the deposition of nanoparticles onto surfaces used in areas such as
nanotechnology.
1.3 Flow regimes inside a droplet
A wide area of study is the effects of different flows inside the droplet. Two important flow
regimes are Capillary flow (driven by continuity) and Marangoni flow (driven by surface
tension gradients). The manipulation of these flow regimes can lead to different patterns left
from the evaporation of drying drops [20]. For example, in inkjet printing, the coffee ring
effect caused by Capillary flow provides a sharper image, however in other applications such
as thin film coating the uniform deposition caused by Marangoni flow is more desirable [1].
In Capillary flow it is assumed that evaporation occurs at the base edge of the drop and the
fluid then flows radially outwards to replace the evaporated fluid. In this particular flow
regime, the droplet usually maintains a constant radius and the contact angle/height decrease.
The deposition of particles at the wetted contact line creates the commonly seen ‘coffee ring
effect’ or ring pattern [1, 21, 22].
Marangoni flow can be the opposite of Capillary flow in that it causes the fluid to circulate
inwards. Surface tension decreases as temperature increases. In a drop, the base is cooler than
the apex which creates a temperature gradient and in turn a surface tension gradient. The
stronger surface tension at the base of the droplet pulls the fluid from the apex downwards
which creates circular motions of fluid along the interface. A detailed account of Marangoni
flow will not be discussed in this review, however the reader is referred to some works that
explain this phenomenon in more detailed fashion [1, 23, 24].
The work by Hu et al. [20] described how the manipulation of each type of flow can lead to
different patterns from drying drops. They proposed that the deposition patterns could be
altered by changing the flow inside the droplet by manipulating its temperature profile. They
proposed two methods of controlling the temperature profile; radial heat transfer to its surface
or using resistive micro heaters. Their experimental results proved this theory and show that
the flow can in fact be influenced which can greatly improve applications that require precise
material deposition. An article by Ristenpart et al. [25] also shows that the manipulation of
Marangoni flow inside the droplet can affect the deposition pattern left from drying. Their
experiment proved that Marangoni flow can be influenced by the relative thermal
conductivities of both substrate and liquid and that the direction of flow changed once a
critical contact angle had been reached.
1.4 Complex evaporation situations
In more complex evaporation situations, there are numerous different factors that must be
considered which make it much more complicated to model. Modelling more complex
situations requires a connection between several mechanisms that act within the drop and
since the modelling of pure liquid systems is still being explored, the modelling of complex
systems is very difficult. It appears that many current models ignore fundamental
mechanisms inside the droplet to simplify the situation allowing it to be modelled. For
example, lubrication theory neglects the vapour dynamics and models evaporation purely on
the evaporation of a liquid film [3]. Some experiments use simplified fluids to model more
complex ones, as seen in the experiments carried out by Meloy Gorr et al. [16] who use a
solution of lysozyme and NaCl to represent a biological fluid.
Works by Sazhin [6, 12] explain very clearly the problems faced in advanced modelling of
droplet heating and evaporation very clearly. Sazhin et al. [6, 12] review current models of
droplet evaporation and then identify unsolved questions. The review identifies further areas
of study that will be vital for further improvements in the modelling of droplet evaporation. It
explains how the use of computational fluid dynamics (CFD) is difficult because some
models may be too simple and others too complex. Models based on the complete Navier-
Stokes equations are the most complicated and are rarely used since the methods required to
solve them are too cumbersome.
In many cases, the fluid can be a suspension. This creates problems since factors such as
particle size, electrostatic interaction between different components in the fluid on the
substrate and each other have an effect on the patterns left from evaporation. A distinction
must be made between a suspension and a phase change system. In the evaporation of a
suspension such as coffee, the liquid evaporates leaving the solid deposition, the pattern of
which can be altered by changing the composition of the suspension [3]. In phase change
systems, such as salt or protein solutions, the residue changes phase to a gel or a solid as the
fluid evaporates. This is an important step in the biomedical applications since blood follows
this regime [15, 26]. Varying the concentration of different components within the fluid can
have a large effect on the behaviour of the drop.
Another area of investigation is the effect of the hydrophobicity of the substrate which has
been reported in numerous studies, including the one by Xu et al. [19]. As too is the effect of
contact line pinning and its dynamics. An everyday example of contact line pinning is when
small raindrops on a window occasionally stick to the surface which appears to contradict
gravity [27]. This situation occurs due to roughness on the glass surface which the water can
grip to [28]. As explained previously, the effects of Marangoni and Capillary flow also have
to be included in the modelling attempt.
2. Patterns from Drying Drops
There are a number of different patterns that can be left from the drying of a fluid drop. The
pattern depends on a number of factors, some of which have been highlighted previously.
Other factors that need to be considered are the effect of atmospheric temperature, substrate
temperature and surface roughness and pattern e.g. Xu et al. [19]. This section will describe
deposition patterns left from similar types of fluid such as polymers, paints, suspensions and
biological fluids and discuss the factors that affect the pattern left.
2.1 Polymers
Polymers exist in colloid suspensions. A lot of previous study has only involved Newtonian
suspensions, however, a polymer suspension is a Non-Newtonian fluid and the mechanisms
involved in its drying are poorly understood. This is an important area to gain understanding
in since polymers are regularly added to fluids used in applications such as inkjet printing to
manipulate the patterns left by the dried drops [1]. Further applications are explained later on.
It is accepted that the pattern left by a dried drop of colloid suspension is most commonly the
prominent coffee-ring effect [29]. Deegan [27] explains 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 a very common 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.
In an experiment carried out by Kajiya et al. [29], the change in polymer concentration inside
a drop of solution was recorded using fluorescent microscopy. This allowed the internal
transport processes to be seen clearly and is crucial to improving the understanding of fluid
dynamics inside the drop. In this experiment they used a solution of fluorescent polystyrene
and anisole. The results from the experiment showed that early in the drying process the
concentration of polystyrene increased at the contact line. After this the concentration in the
central region remained fairly constant throughout drying, until later stages. Capillary flow
was responsible for this early movement towards the outer edge and explains why the dried
central region can be very thin. The experiment also proved that the rate of Capillary flow is
proportional to evaporation rate. Therefore, the thickness of the coffee ring can be controlled
by changing the surrounding or substrate temperature. The ability to record the movement of
particles in the fluid opens up avenues to improve the understanding of the mechanisms
taking part in droplet evaporation.
Controlling the central region contained within the outer ring is more complicated. Research
by Jung-Hoon Kim et al. [13] looked at the deposit patterns left by evaporating drops of
water and polymer solutions. The polymer used was Poly(3,4-ethylenedioxythiophene)-
poly(styrenesulfonate) known as PEDOT-PSS. The deposition patterns were affected by
changing the substrate temperature which controlled the direction of internal flow during the
later stages of evaporation. They found that on a cooled substrate (temperature lower than the
droplet temperature), the Marangoni flow inwards was stronger than the outward Capillary
flow. This led to a larger concentration of deposits in the central region. On the room
temperature or heated surface, the Capillary flow was stronger creating the prominent coffee
ring pattern. This coffee ring pattern is concurrent with Kajiya et al. [29], who also saw this
pattern at room temperature. Using the conclusion from Kajiya and co-workers experiment,
Capillary flow is proportional to evaporation rate which is faster at higher substrate
temperatures, explaining the stronger capillary forces at higher temperatures. The ability to
control the deposition pattern has large applications in ink-jet printing, later explained. Figure
1 shows a summary of the cross sectional deposition patterns left by a dried drop at varying
substrate temperatures. The simple diagram below clearly shows how the central region has a
thicker deposits layer at lower temperatures and that the coffee ring effect appears at higher
temperatures.
Figure 1: Cross sectional interpretations of deposits left from dried drops of a solution of
water and PEDOT-PSS at different substrate temperatures
An experiment carried out by Yongjoon et al. [1] investigated the effect of adding different
sized particles into polymer solutions. They added particles of Polystyrene (1 μm and 6 μm)
and hollow glass beads (9-13 μm) separately into three different solutions: pure water,
polyethylene oxide (PEO) and xantham gum (XG). PEO is a flexible polymer and XG is semi
flexible. The results of this experiment highlight the effect of: Newtonian versus Non-
Newtonian fluids, polymer flexibility, polymer elasticity and suspended particle diameter on
the patterns left from the dried drops. Their results showed that suspensions containing
smaller particles, i.e. the 1 μm Polystyrene particles, produced the most noticeable coffee
stain pattern upon drying. Larger particles suspended in the water and in the PEO solution
were not deposited on the edge and formed a more uniform pattern in the middle. Larger
particles suspended in the XG solution did form the coffee stain pattern upon drying.
Therefore, changing the polymer solution can have a large effect on the pattern and so can the
size of particle suspended within it. Figure 2 shows a table of photos of the final pattern from
the fully dried drop of each solution containing each size of particle. This figure has been
created from the figures contained within [1] to give a summary of the different patterns. The
original article shows a timeline of pictures for the evaporation of each drop.
Figure 2: Summary of final patterns left from drops of three solvents (water, PEO and XG)
with three different sized suspended particles as labelled on the diagram. Modified from [1]
A possible explanation for the difference in patterns seen between the two polymer solutions,
PEO and XG, is the difference in their viscosity. XG is a shear thinning fluid, so during low
shear drying, the viscosity remains high since it has a very high zero shear viscosity. The zero
shear viscosity is the viscosity exerted by a fluid while it is stationary. Capillary flow pushes
large particles towards the rim at the beginning of the drying process that are then trapped.
Therefore larger particles in higher viscosity fluids exhibit the coffee ring effect after drying.
This is backed up by the fact that solutions with lower concentration of XG do not exhibit
such dominant coffee stain outer ring patterns [1]. The collection of particles within the ring
can be explained by the attraction of the positive glass substrate and the negative suspended
particles.
Work by Uno et al. [30] investigated the effect of hydrophobic and hydrophilic substrates in
the pattern left by drying drops of polymer latex solution. As expected, on the hydrophilic
surface, the drop followed CRR while drying and produced a circular coffee-ring stain upon
drying. Because the particles are ‘attracted’ to a hydrophilic surface, they adsorbed to it
which creates the ring pattern. On the hydrophobic surface the drop maintained a more
spherical shape which followed CCAR during evaporation. The particles did not adsorb onto
the surface initially, however as the drop continued to evaporate the concentration of particles
increased which caused the particles to clump together and form aggregates. These
aggregates then adsorbed onto the surface to leave a random pattern of ‘spots’ upon drying as
the aggregates were deposited. Figure 3 is a clear illustration depicting the situation explained
here.
Figure 3: Explanation of pattern formation of latex polymer solution of hydrophilic and
hydrophobic substrates
2.2 Paints
Paint is used in small scale applications, such as artwork, and in larger scale applications such
as decorating or protecting metals against corrosion such as in bridges. Artists experiment
with the different patterns left from the drying of paint drops in their work. This is most
commonly used in watercolour paintings where the coffee ring pattern can be seen clearly.
The addition of solvents and other ingredients to paint can create a uniform drying pattern
which is preferential to form an even coating. In these cases, the addition of the ingredients
lessens or prevents the coffee ring effect that is usually seen in suspensions.
The different ingredients each have their own influence on the finish. For example, some are
made from a mixture of water and a thicker solvent. Upon drying, the water evaporates
quickly leaving the pigment carrying particles in the solvent which is highly viscous stopping
the particles from moving outwards. This creates a uniform drying pattern [21].
In an experiment by Abbasian et al. [31], the Marangoni flow during the evaporation of three
solvents that are commonly added to paints was investigated. These three solvents were
xylene, MEK and MiBK. It was found that the patterns left by the pure solvents and solutions
of different solvent ratios had different levels of unevenness. This experiment investigates
film drying, rather than droplets and this tends to be the case in most of the articles on the
patterns left by drying paint.
With new advances in the manipulation of capillary and Marongoni flow during drop
evaporation [20, 23], there could be new avenues of research opening up that investigate the
effect of different solvents on the patterns left from paint droplets. This is an important
application in spray painting.
2.3 Biological fluid
Biological fluids are very complicated since they contain a variety of different proteins and
electrolytes which all interact with each other and affect the mechanisms inside a droplet as it
dries. This makes the understanding of pattern formation very difficult, however, research has
been more concentrated on this area for the past two decades and repetitions of different
patterns are now being found [3, 16].
Yakhno et al. [15, 26, 32] have carried out a lot of investigative work into the patterns left
from biological fluids. In one article, they discussed the results from patterns left from dried
drops of four different fluids: pure water, 0.9 wt% NaCl solution, bovine serum albumin
(BSA) solution and a solution composing of 7 wt% BSA and 0.9 wt% NaCl. Although BSA
is a serum albumin derived from cows, it has a lot of applications in biomedicine including
Enzyme-linked immunosorbent assay (ELISA). This test uses antibodies and colour change
to detect a substance. The patterns left from the dried drop of each sample are shown in figure
4 below.
Figure 4: Patterns left from dried drops of; (a) water, (b) 0.9 wt% NaCl solution, (c) BSA
solution and (d) 7 wt% BSA and 0.9 wt% NaCl solution. Modified from [15]
It is clear from figure 4 that the NaCl solution (b) dries to leave a distinct outer ring of
crystals with some clusters randomly forming inside the ring. The BSA solution (c) dries to
leave a smooth, thick outer ring. It can be assumed that these patterns are formed by the NaCl
or BSA since there is no pattern left from the drop of pure water (a). When NaCl and BSA
are both in solution with water (d), the pattern left by the NaCl is very different since it
crystallises uniformly in the centre of the thick outer ring of deposited BSA, the pattern of
which remains unchanged from the pattern formed by the BSA solution.
The crystallisation of salts within a protein outer ring is further documented by Meloy Gorr et
al. [16] who investigated the effect of lysozyme and NaCl concentrations on the patterns left
from the dried drops of the solutions. Lysozyme is a protein commonly found in tear drops
and saliva of humans. The combination of lysozyme and NaCl offers a very simplified
version of a biological fluid. In pure lysozyme solutions, a thick smooth ring of deposits was
left at the outside of the wetted area as described in the two previous experiments discussed.
As NaCl concentration increased, rough crystalline structures appeared in the centre of the
outer protein ring in dendritic patterns. However, in this experiment, unlike the one carried
out by Yakhno et al., at higher salt concentrations a clear ring formed inside the outer
lysozyme ring with the crystallised salt forming within that. This second ring contained larger
groups of lysozyme molecules.
Although this experiment is carried out using a very simplified version of a biological fluid,
the patterns produced are very similar to those formed by more complex fluids. Therefore,
investigating these simpler fluids might help understand the mechanisms behind the patterns
formed in more complex fluids and further enhance their uses in biomedical applications later
discussed.
Meloy Gorr et al. suggest that the general pattern of salts crystallising inside a protein ring is
due to the separation of the two fluids early on in the evaporation process [16]. They also
noted that initially the drop was pinned and followed a constant radius regime during drying.
Once the outer ring had formed, the drop then changer regime to a constant contact angle
regime until the drop had fully evaporated [16]. This phenomenon was only seen in solutions
with NaCl concentrations of lower than 0.5 wt%. The pattern seen in both of these
experiments can therefore be explained by coupling the suggestion of an early fluid split with
the presence of the two different regimes. If the protein separates from the fluid early, it
would experience Capillary flow, seen in constant radius regime, and would be deposited on
the edge of the initial contact area creating the thick outer ring. As evaporation continues, the
salt that is still in solution begins to crystallise but by this point the drop now experiences
constant contact angle regime so a uniform layer of crystals would be deposited inside the
outer protein ring.
Investigative work has been carried out on women who have just given birth, comparing
those who experienced normal, premature and premature to the point of threatened abortion
childbirth. Yakhno et al. [26, 32] have carried out some of this research. The patterns left
from drops of plasma of women who have just undergone childbirth are similar however a
difference was seen in the thickness of and size of crystals present in the ‘transition zone’ of
the dried drop. Those who experienced premature and extremely premature childbirth (b) had
a wider circle containing larger crystals compared to those who experienced normal
childbirth (a), the circle of crystals being talked about here are indicated by the black arrows
on the photos in figure 5 below.
Figure 5: Patterns left from drying drops of plasma from post childbirth women who
experienced normal childbirth (a) and premature child birth (b). Modified from [26]
Although it is not a biological fluid, a very similar phenomenon where the salt crystallises
inside a ring of potato starch was found by Choudhury et al. [33]. In this situation, the salt
crystallised into dendritic fractal formations with a thick outer ring of pure starch surrounding
it (the ring of starch is denoted by arrow A, near the bottom edge of figure 6).
Figure 6: Picture of a dried drop of starch and NaCl gel showing enlarged photos of certain
areas of the deposited pattern. Taken from [33]
2.4 Nano-particle suspensions
Many experiments have been carried out using gold particle suspensions [34, 35]. One
experiment by Budhadipta et al. [34] found a method for dispersing gold nanorods onto a
single walled carbon nanotube (CNT) macrostructure. Their method used the drying of drops
of nanorod suspensions on a single walled carbon nanotube substrate in ambient conditions.
They found that the factor controlling the pattern of GNRs on the substrate was the
anisotropic interaction between the nanorod and the single walled carbon nanotube substrate
which means that gold nanorods aligned with the direction of the single walled carbon
nanotube microfibers. The experiment was also carried out using triangle and polygonal
shaped gold particles and the same result was found. The spontaneous alignment with the
single walled carbon nanotube substrate is due to a combination of forces acting on the
nanoparticles. As the drop continues to evaporate, the volume of liquid present in the drop
falls until the nanoparticles are sitting in a very thin film of fluid. The capillary force then
promotes the particles to lie horizontally. Van der Waals forces promote the particles to
attach to the substrate with maximum contact area, in the case of nanorods this is horizontally
in the micro channels on the single walled carbon nanotube. Figure 7 shows the pattern of
gold nanorods on the single walled carbon nanotube substrate and clearly shows the
alignment in the micro channels. To achieve higher concentrations of gold nanorods on the
surface, the drop evaporation process was repeated on the same area.
Figure 7: SEM image of: (a) single walled carbon nanotube fibre [scale bar: 40 μm] and
(b,c) spontaneous gold nanorods alignment on single walled carbon nanotube fibre [scale
bar: 0.2 μm]. Gold nanorods deposited from dried drops of dilute gold nanorods solution.
From [34]
Darwich et al. [35] used the natural evaporation of nanodroplets to produce nanoring patterns
of gold particles. It can be seen that the gold particles form a ring spontaneously upon drying
or they can be introduced into a system which allows the diameter and placement of the
particles to be controlled. The ring of gold particles that forms around the edge of the wetted
area is the same as the coffee-ring effect that is seen in many other fluids explained
previously in section 2. The nanodroplets form quickly and reliably on the hydrophobic
substrate. The advantage of this technique over others which use nanosized moulds and
templates is that the rings can be made much quicker since the nanodroplet formation is fast.
A disadvantage seems to be that it is slightly more difficult to control the size of the rings
which using a mould ensures. Figure 8 shows the nanoring deposition pattern of gold
particles.
Figure 8: AFM images of gold nanoring structures formed by the drying of nanodroplets of
gold suspension (a) Frame size 1 μm and (b) 0.3 μm
3. Applications
3.1 Polymers
The work explained previously, by Uno et al. [30], started due to the demand of materials to
coat building exteriors in, that would exhibit ‘self-cleaning’ properties. These materials
would therefore keep maintenance costs to a minimum since stains would be more easily
removed. The experiment carried out in their work aimed to give a better understanding of
why the striped stain patterns that form on building exteriors appear more commonly on
hydrophobic surfaces rather than hydrophilic. To do this they created a polymer latex solution
from styrene and p-styrenesulfonate to signify the rain drop since they believed the pattern
formation to be caused by the different pollutants present in the rain drop. The results from
this experiment proved that the aggregates adhered more firmly to a hydrophilic surface than
the hydrophobic and means that study into a suitable material should begin with hydrophobic
substances.
A second application for polymer solutions is in ink-jet printing. The manipulation of the
pattern of solute left on the substrate is important in this application [29]. The commonly seen
coffee ring pattern affects the thickness of the deposited polymer film created during ink jet
printing and also enhances the sharpness of the image.
3.2 Biomedicine
The composition of biological fluids are altered by disease and diet [3], and as earlier stated,
the patterns left from drying drops are altered by the composition of the fluid drop. This
means that the patterns left from the drying of biological fluids could be used as a cost
effective and fast means of diagnosis of certain diseases. Different biological fluids have been
investigated such as blood, serum (blood plasma with the blood clotting components
removed) and tear drops in various studies [15, 16, 26, 32, 36, 37].
The development of this diagnosis technique began with the ‘Litos’ test system which was
established in Russia. The use of this system arose after it was realised that urine salts of
patients suffering from urolithiasis, or more commonly known as kidney stones, would
crystallise in the biological fluid and therefore leave a distinct pattern [3, 38]. Since the
discovery of this phenomenon, it has attracted others to study the possibility of this diagnosis
technique. However, the technique relies on comparing a drop pattern from an infected
patient to that of a healthy patient (control). This requires the collection of numerous samples
for each disease/condition before a good comparison can be made. This explains why it has
taken a couple of decades for big improvements in diagnostics, since repeatable and distinct
patterns are now being seen in the residue left by drops of similar biological fluid.
The problem with using the deposition patterns as a means of diagnosis is the subjective
nature of the work since there are no exact patterns to look for, it is very much a judgment
made by eye. An advantage of this technique is that certain diseases, such as cancer and
kidney disease, that are difficult to diagnose early might be detected sooner since they can
cause high levels of certain proteins in the blood [3]. This change in protein production and
concentration can be spotted early using drop patterns since it will have an effect on the
pattern left from drying.
Work by Yakhno et al. [32] has proven that this attractive idea of diagnosis from deposition
patterns of blood drops is realistic. It is not only cheap and fast, but it is not invasive for the
patient and the procedure can be used by less qualified personnel. The experiment carried out
by Yakhno et al. looked at blood samples from different patients with one of eight conditions:
(a) control, (b) breast cancer, (c) lung cancer, (d) paraproteinemia, (e) in-time delivery, (f)
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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