Technological Applications of Superhydrophobic Coatings Needs and Challenges report
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Technological Applications of Superhydrophobic Coatings: Needs and Challenges
Abstract
Superhydrophobic coatings represent an important and interesting field of research. This
report begins with a brief discussion about the causes of superhydrophobicity and characterization
of superhydrophobic surfaces. Some recent synthesis techniques and several applications of these
surfaces and coatings are then described followed by a discussion of the major problems that need
to be solved before these applications become widespread. To implement the results of the
laboratory scale studies on an industrial scale the methods of synthesis have to be modified and
adapted suitably. Although some products like hydrophobic paints, non-stick cookware are being
manufactured and are available commercially, a lot of scope still remains in this field. Further
research is necessary to realize this potential fully.
Introduction
1.1 Background
Superhydrophobicity is a property that governs the extreme water repellency and nonwettability
of a solid surface. On such surfaces water or any other liquid forms nearly spherical
droplets and not continuous films. Superhydrophobic behaviour of many substances has been
known since the 18th century [3]. Examples of such surfaces are found extensively in the natural
world as well. For example, the formation of glistening beads of water on the leaves of many
plants, most notably on the lotus leaves has been a long observed phenomenon. In fact this
property has been given a special name- the Lotus Effect. The ability of some aquatic
creatures like ducks to constantly keep their feathers clean, the survival of some insects like the
Namibian desert beetle in the arid desert regions can all be attributed to the superhydrophobic
characteristics of the surfaces concerned. It is only recently that these naturally occurring
surfaces have been examined in detail ( pioneering work in this field was done by Professor
Wilhelm Barthlott of the University of Bonn , Federal Republic of Germany in 1997) and their
morphological and chemical natures have been analyzed [3]. Earlier the works of Cassie, Baxter
and Wenzel had provided insights into the relation of wettability by liquids and surface
roughness of solids .This has provided us with some insight into the origin of this remarkable
behaviour and the artificial synthesis of such materials. In fact nature itself provides us with
some of the necessary templates that are being used to synthesize these materials [8]. Since the
wettability of solid surfaces affects many industrial processes, hence control over this property
is extremely necessary.
Since the first demonstration of artificial superhydrophobicity by Onda et al. in1996 [2],
researchers across the world have developed many interesting techniques for the creation of
such surfaces. These techniques range from lithography, plasma treatment, chemical vapour
deposition to layer-by-layer techniques and most recently nano-particle-based (usually silica
nano-particles) synthesis routes [2]. Use of fluorinated polymers (though the environmental
impacts of its usage are widely debated), silicones and silanes as materials of low surface energy
is also widespread and most artificially roughened surfaces contain a coating of these substances
to make them superhydrophobic [2],[4].
1
Water repellency, self cleansing and anti-sticking behaviour of these surfaces are very
attractive features that can be exploited in a variety of applications. Self cleaning glass,
superhydrophobic paints and other architectural coatings and textiles are some of the potential
areas of application. Some of these products have already made their way into the commercial
market (Reference: lotusan.de, activglas.com). Intensive research activity is going
on round the world to make self cleaning textiles using less expensive materials (presently
techniques involving silver and titanium dioxide nano-particles have been reported) [6].
Research is also directed at making these surfaces durable, long lasting and mechanically strong.
As these surfaces are water resistant, hence they also resist the growth of microorganisms on
them. Thus their anti-fouling properties are also remarkable. Thus they are being applied as
protective coatings on marine vessels, submarines and oil rigs which are constantly exposed to
harsh saline environment and also get covered by algae and other marine organisms [17]. When
applied as coatings on building material such as marbles and sandstone they can act as
protection from environmental pollution and acid rain [16]. Several reviews of these and other
applications of superhydrophobic coatings are currently available in literature [3].
1.2 The causes of superhydrophobicity
A detailed examination of the surface morphology of the lotus leaves using techniques
like Scanning Electron Microscopy (SEM) reveals that the surface is covered by tiny bumps or
stubs which are 5 to 10 µm high and 10 to 15 µm apart. This uneven surface is further coated
with wax crystals (which are substances with low surface energy) with diameters in the
nanometer range. The fine surface structure traps a thin layer of air which reduces the contact
between the water droplet and the solid surfaces. The lotus effect is therefore a physicochemical
property arising out of the combination of surface roughness (on the nanometer and
micrometer scale) and the presence of a coating of low surface energy material. So it is possible
to create an artificial substance that resembles the lotus leaves externally [4]. Since 1990, there
have been continuous efforts directed at creating such surfaces.
1.3 Characterization
Superhydrophobic surfaces can be characterized by the following parameters [3]:
1.3.1 Contact Angle (CA): The most common manifestation of superhydrophobicity is
the formation of nearly spherical liquid drops on it. This is quantified by measuring the contact
2
angle(C A) at the solid-liquid interface. The contact angle is the angle at which the liquid-solid
interface meet and it is defined as the angle made by the tangent to the point of contact of the
solid and the liquid, measured from the liquid side . It is actually determined by the
thermodynamic equilibrium between all the three phases i.e. the gas, solid, liquid phases that
meet along the interfaces. The highest and the lowest C As exhibited by a liquid-solid-gas
combination are respectively known as the advancing and the receding angles and the difference
between the two is called the Contact Angle Hysterisis.
For a surface to be designated as superhydrophobic, it must exhibit C A greater than
150o and the contact angle hysterisis must also be low.
Contact angle measurements are carried out by using an instrument known as the
goniometer.
1.3.2 Angle of Slide: Another feature is that the surfaces must have a low angle of slide
( less than 10o , measured from the horizontal) i.e. the water droplets should roll down the
surface very easily, similar to rolling of rigid spheres.
1.3.3 Self cleaning ability: Self-cleaning behaviour is also an extremely important
behaviour of superhydrophobic coatings or surfaces. On these surfaces the adhesion between the
water droplets and the dust particles is much more than that between the surface and the dust.
Hence most of the dust is picked up by the rolling water drops and no residue is left behind.
Hydrophilic dust particles therefore have a greater chance of removal.
1.3.4 Elastic collision of liquid droplets: Water droplets which fall on a
superhydrophobic surface with some velocity can bounce back, practically without any
deformation after having suffered an almost elastic collision. This is also one reason why these
surfaces remain dry even after coming in contact with some liquid.
Superhydrophobicity of a solid surface is also dependent on the properties if the liquid
concerned, particularly its temperature. The surface tension of a liquid decreases with rise in
temperature. Hence hotter liquids can wet surfaces which exhibit superhydrophobicity towards
the same liquid when it is cooler.
The aim of this seminar and presentation report is to discuss the various ways in which the interesting
properties of superhydrophobic surfaces can be utilized on a large scale. In the report there is a
preliminary discussion of the characterization and preparation of these surfaces which is
followed by a description of some of the interesting applications of these surfaces. Though a
3
large number of researchers are involved in investigating the various properties and applications
of superhydrophobic coatings yet a lot remains to be done as far as large scale manufacture and
utilization of these surfaces are concerned. The present report also looks at these aspects and
suggests some areas on which future research can be directed.
4
Chapter Two
Self-cleaning Glasses
2.1 Introduction
One of the most fascinating practical applications of superhydrophobic materials is in
the field of self-cleaning glasses. Cleaning of window glasses normally requires a lot of labour
and also entails the use of detergents which are harmful to the environment. Self-cleaning
glasses appear to be the answer to these problems. They promise to do away with the need of
regular washing, scrubbing and polishing with chemical agents. It is not that water will no
longer be required for cleaning of these kinds of glass but the amount of water and the effort
needed to accomplish cleaning is certainly reduced.
Normally, most glasses and glass like materials are known to be hydrophobic but these
coatings can impart to them highly water repellent properties. The self cleaning behaviour stems
from the deposition of a superhydrophobic coating on the glass substrate. As discussed
previously, these coatings do not allow water to form continuous films on the glass surface and
the nearly spherical droplets roll off the surface very easily and carry away the dirt particles
with them [4]. Hydrophilic dirt is washed away more effectively than hydrophobic dirt. The
diagram below illustrates this interesting property:
Figure 1: Self cleaning behaviour of superhydrophobic surfaces
(Taken from: Li et.al., Chemical Society Reviews, 2007,36, pp. 1353)
5
The rolling motion of the nearly spherical water droplets on the inclined
superhydrophobic surface causes the dust to be removed easily. The dust also has a higher
affinity for the water droplet than for the surface which has a coating of some extremely low
surface energy material like fluorocarbons or silanes.
Self-cleaning glasses are already available in European markets and the manufacturers
include Saint Gobain and Pilkington Activ„¢.
2.2 Methods of preparation
There are generally two routes to creating self-cleaning glasses “ one of these involves
utilizing the lotus effect i.e. the ability of the leaves of the lotus plant to keep itself clean ,
especially after rainfall due to the presence of micro and nano-level surface roughness and a
water-resistant wax-like coating [10]. The rough glass surface is created by a variety of
techniques like plasma etching, chemical vapor deposition etc. The other technique is based on
the photocatalysis of organic impurities deposited on glass surface, by a catalyst layer, usually
TiO2. The catalyst must be activated by ultra-violet light which causes the electrons to be
promoted to the conduction bands and hence oxidize the impurities accumulated on the surface.
Another technique reported by Bravo et. al. utilizes a layer-by-layer technique to deposit
silica nano-particles of differing sizes on a glass substrate to obtain the necessary degree of
roughness that makes the surface superhydrophobic. Since silica is a natural constituent of most
glasses hence this technique appears to be promising and very compatible with most glass
substrates. A brief description of the method is given below [7] :
Reagents used : Poly(sodium-4 styrene sulfonate) (SPS), Poly- (allylamine hydrochloride)
(PAH), colloidal silica (7 and 20 nanometer particle size)
Method: Glass slides or silicon wafers were first dipped in PAH solution, rinsed with deionized
water and then dipped in SPS solution for creating an adhesion layer. For the body
layers the silica colloidal dispersion (particle size 50 nm) was used. A top layer having silica
particles of 20 nm was created, and finally the specimens were calcined at 550oC for 4
hours.
Measurements: The properties that were measured include Film thickness, Refractive
index, Root mean square roughness, Contact angle, Optical transmittance.
Results and conclusion:
6
Figure 2: AFM image of the silica nano-particle coated glass substrate
(Taken from: J Bravo, L Zhai, Z Wu, R.E. Cohen and M.F. Rubner, Langmuir, 2007, 23,pp
7297)
Figure 2 shows the surface roughness generated on the glass slide after the deposition of
silica nano-particles. The structure is reminiscent of the lotus leaf structure and hence explains
the superhydrophobic behaviour of the glass.
Figure 3: Contact angle measurement versus surface roughness
(Taken from: J Bravo et. al., Langmuir, 2007, 23,pp 7296)
Figure 3 gives the quantitative measures of the contact angles and the contact angle
hysteresis observed on the glass substrate due to the deposition of layers of silica nano-particles
of differing thicknesses. It is observed that contact angle rises with increasing roughness of the
surface but then progressively settles down to a value of about 160o .
7
It is not enough to have superhydrophobic glass alone, it is also important to maintain
transparent nature of the glass. Hence measurement of the optical transmittance of the glass
sample with the multiple layers of silica deposited on it was also carried out.
Figure 4: Optical properties of the films of silica nano-particles on glass substrate
(Taken from: J Bravo, L Zhai, Z Wu, R.E. Cohen and M.F. Rubner, Langmuir, 2007,
23,pp 7295)
It was observed that the maximum transmittance was about 95% for light of wavelength
584 nm. The transmittance decreased with increasing thickness of the silica layers. The
transmittance has also been compared with that of a plane glass surface and it has been found
that at 584 nm the transmittance of a plane glass surface is actually less than that of the coated
substrate.
An important conclusion that can be drawn from this study is that for high levels of
transparency and sufficiently high superhydrophobic coatings there can be no more than 20-25
bilayers of silica nano-particles. If higher levels of superhydrophic behaviour are desired then it
can be achieved at the cost of reduced optical transmittance. Thus such glasses could be suitable
for the outer facades of office buildings but its use is slightly limited as far as optical
instruments, especially those used for underwater photography and other similar purposes are
concerned.
8
The level of aggregation of the silica nano-particles was also found to have an effect on
the properties of the film. The nature of surface roughness is controlled by this factor. If a nanoporous
film is formed due to certain types of aggregation then superhydrophobicity is enhanced.
2.3 Application of self cleaning glasses
Self-cleaning glasses have several potential applications.
¢ These glasses when used as window-glass will stay cleaner for much longer (as
organic dirt can be broken down by them by photo-catalysis).When these are
required to be cleaned they require much less labour, water and surfactants
(which are also harmful to the environment). In fact they can be cleaned just
after a moderately heavy spell of rain. The roofs of conservatories and glass
houses which are difficult to clean can be made out of these glasses [21].
¢ These glasses can also be used in automobile and aircraft windshields. As they
are highly water repelling, hence they prevent the accumulation of ice and frost
on the surface, so they are especially beneficial in cold climates. They lead to
better visibility in inclement weather and can significantly reduce the probability
of road- accidents that might occur due to poor visibility.
¢ Optical instruments like lenses of cameras for underwater photography which
are exposed to saline water conditions for long durations can benefit from this
kind of glass immensely. Staining of the glass due to the effects of the salts can
be reduced to a great extent by these glasses.
¢ Appliances such as touch screens are becoming increasingly common at many
public places. They often get contaminated by the dirt and grime from the userâ„¢s
fingers. The substitution of the normal glass or polymer screens by self-cleaning
glass can ensure that they retain their cleanliness for much longer durations than
usual.
¢ Anti-reflection coatings for solar panels should be preferably superhydrophobic
in order for them to withstand the effects of atmospheric moisture [14].
9
2.4 Some technological challenges and future developments
The field of self cleaning glasses is a very interesting and exciting one but there are
several problems that are yet to be overcome before these glasses find extensive usage in
different fields. Some of the major challenges facing researchers today are the following:
The major problem that needs to be solved is to find ways of applying
superhydrophobic coatings on the glass surface easily and without defects. Most
of the methods involve very specialized and delicate techniques that are suitable
for laboratory scale studies. On a large scale these methods are not very feasible.
Spraying techniques, sol-gel techniques and in some cases plasma treatment have
been found to be most effective in coating large areas of glass surfaces quickly.
So these techniques need to be developed in such ways that they can be utilized
on an industrial scale.
The coating must also adhere to the glass surface properly. Hence it is necessary
to choose the material combination properly so that they form a continuous and
compatible interface. The development of Janus or two-sided films needs to be
studied further [18].
Superhydrophobicity and transparency of a surface are two conflicting
requirements. This is because a rough surface microstructure is a necessary
condition for superhydrophobic surfaces but this also leads to more scattering of
light, thereby reducing the amount of light transmitted and hence affecting its
transparency. Thus it is necessary to ensure that the presence of
superhydrophobic coatings on glass surfaces do not significantly disturb its light
transmitting capacity. Usually an optimum level of roughness has to be ensured
to obtain both the properties.
Glasses which depend on the photocatalysis action to be self cleaning usually
need to be activated by the ultraviolet rays of sunlight before they can achieve it.
Thus they are suitable for outdoor applications such as glass houses, office
buildings etc. Efforts are being made to develop glasses in which the selfcleaning
behaviour can be developed even indoors i.e. the range of wavelengths
of light that can be successfully used to bring about the photo-destruction of
bacteria and dirt must be extended. Materials which absorb light in the visible
ranges are required to be developed [10].
10
Most of the self-cleaning glasses that are currently available have the ability to
destroy organic dirt and grime but they are not really effective towards the
inorganic impurities. Hence this is also an aspect that needs further investigation.
Efforts are also being made to impart bactericidal properties to the glasses so that
they can be used in hospitals and health care facilities to destroy harmful
pathogens.
Though there are some European glazing companies which have started the
commercial production of self-cleaning glasses, these materials are still at least
20% more expensive than regular glasses (Reference: various Internet sites). So
cheaper materials and methods of applying these coatings without affecting how
the glasses appear externally need to be developed.
Self-cleaning glasses therefore represent an interesting field of research and it still has a
lot of potential waiting to be realized.
11
Chapter Three
Self-cleaning Textiles
3.1 Introduction
The property of superhydrophobicity of certain surfaces has led them to be
extended to the domain of textiles. A lot of research has been directed towards the
creation of textiles which are water repellent and stain resistant [9]. The main application
of these textiles is towards waterproofing. But now water and dirt resistant apparel to be
used in a wide variety of situations are being considered. Typically this involves the
treatment of the target fabric with some kind of superhydrophobic coating which ideally
does not affect the other properties of the fibres.
Self-cleaning textiles are actually much better repellers of dirt than conventional
fabrics and the can also degrade several kinds of stains based on some catalytic
mechanisms.
The application of repellency techniques for the creation of self-cleaning textiles
had been proposed way back in the 1940s [5]. Most of the current techniques in use
depend on the coating of the textile with low surface energy materials like fluorinated
polymers. The surface roughness of the fibres is their own natural characteristic and so
are their diameters. For the synthetic fibres, this property can be controlled by the
spinning method or by controlling the diameter of the fibre. The creation of
superhydrophobicity on natural fibres like cotton (which is mainly composed of
cellulose and has very good water absorption capacity due to the presence of water
absorbing hydroxyl groups on the surface) is different from the creation of
superhydrophobicity on artificial textiles [5].
3.2 Methods of production
Presently the synthesis of self-cleaning textiles is achieved using the assembly of
certain nano-particles (like those of TiO2 in the anatase phase or carbon nano-tubes) on
to the textile fibres. But a much more feasible method appears to be the plasma treatment
of the fibres in order to change their surface morphology and make it sufficiently rough
12
to be superhydrophobic. Some of the recent techniques for the creation of
superhydrophobic textiles are described below:
o Liu et al. have reported a technique of depositing multi-walled carbon nano-tubes
on the surface of cotton fibres by a simple ultrasound irradiation process using an
aqueous dispersion of the nano-tubes in water[5]. The nano structures achieved
by this process have been reported to resemble the lotus leaf morphology very
closely. But the superhydrophobic behaviour of these fibres was found to be
exhibited for a short duration as the water contact angle progressively decreased
with the passage of time. In order to increase the affinity of the nano-tubes for
the cotton substrate, the tubes were modified with poly-butyl acrylate coatings.
This step imparted long standing water repellency to the fibres.
o Silica and silica based nano-particles have also been extensively used for the
purpose of making textiles hydrophobic. Bi et. al. have very recently reported the
synthesis of hydrophobic textiles by the creation of silica and zinc oxide nanorod
arrays on cotton fabrics by a typical dip-pad-dry technique in an alcoholic
medium [12]. This coating was subsequently modified by a coating of
n-dodecyltrimethoxysilane. Water contact angles of upto159o have been reported.
o A simple one-step technique has been reported by Zimmermann et. al which
involves depositing a layer of polymethylsilsesquioxane (PMSQ) on a variety of
substrates , both natural and artificial in a gas or solvent phase coating set-up [9].
This method, which is generally known as silicone nano-filament coating has
been proven to give excellent superhydrophobic properties to the fibres and it
also has the benefit of being a simple technique. This filament coating is not
sensitive to heat up to a temperature of 200o C but it shows mechanical wear
when cleaned in a washing machine or even under lower mechanical stresses.
Especially good results were obtained with the polyethylene terephthalate (PET)
fibres.
13
Figure 5: Morphology of textile fibres, before and after plasma treatment
(Taken from: J Zimmermann et. al., Advanced Functional Materials, 2008, 18, pp 3663)
o The non-thermal plasma treatment of textile is a very good method of imparting a
rough surface morphology and hence superhydrophobicity to it. This is
particularly suitable for textiles which get damaged very easily by heat. This is
usually followed by the application of a coating of a low surface energy material.
This is also a method of generating multi-functional textile fibres e.g. fibres with
both fire retardant and water resistant properties [13].
3.3 Some applications
The self-cleaning textiles have been a topic of research for several decades round the
world. While hydrophobic textiles have been in use for several years as in raincoats etc. these
are not endowed with self cleaning ability. Several applications of these textiles have been
proposed:
¢ As in the case of self-cleaning glass, these textiles have the potential to save
water and energy needed in cleaning and the need to use chemical based
cleaning agents can also be reduced significantly. With reduced washing
requirements, these textiles are expected to be more durable and longer lasting.
¢ They can be applied to create apparel for use by hospital workers because the
catalytic mechanisms behind their self cleaning behaviour may also lead to the
14
destruction of pathogens, odours etc. Water repellency also reduces the survival
of micro-organisms on such surfaces. Thus they can limit the chances of
infection and the spread of contagious diseases.
¢ The defense sector also stands to benefit from these textiles. Defense personnel
working in difficult and inhospitable areas often do not have the luxury of being
able to wash clothes often and wear clean garments. Self-cleaning garments will
certainly be able to provide cleaner and more hygienic conditions for them.
¢ Textiles used as upholstery are difficult to clean frequently and take a lot of
time and effort. Self-cleaning textiles may therefore find application in this field
as well.
3.4 Technological challenges
Self-cleaning textiles hold several promises but it could take some time before garments
made of these are commercially produced. According to some estimates it could be well over
five years before self cleaning garments actually appear in the markets. Some of the difficulties
that remain to be addressed are as follows:
o The first and probably the most important point to be taken care of is that selfcleaning
textiles take a long time to clean themselves. It has been reported by Qi
et. al. that for titania (TiO2) coated white cotton fabric to get rid of coffee stains
under UV illumination, approximately 20 hours were required [6]. More active
catalysts are therefore needed to speed up the cleaning process for these textiles
to be feasible for the manufacture of self cleaning garments.
o Titania (TiO2 ) based textiles will also have to deal with the additional problem
that the high oxidation power of the catalyst will not only degrade the stains but
will also adversely affect the fibres themselves. The mechanical strength (i.e.
tearing strength) and the durability get reduced considerably. The catalyst is also
a skin irritant. Hence research is required in order to develop safer catalysts as
well as newer techniques of binding the existing catalysts to the surfaces of the
fibres in ways that do not cause any harm to them.
15
o Carbon nano-tubes based self cleaning coatings have limited applicability as they
turn dark in colour after the coating.
o Studies are required to monitor the effects of fluorinated polymer-based
superhydrophobic coatings
o There is a requirement of making the superhydrophobic coatings on the textiles
more long lasting and mechanically stable i.e. more resistant to abrasion and
wear. Presently many of the coatings are damaged during machine washing.
While these textiles might require less frequent washing, it is not that they will
never require to be washed, and under these conditions it is important to ensure
that they retain their properties [9].
o These textiles are good at repelling atmospheric dusts but they are often not very
effective towards oils. So surfaces which can repel both dirt and oil-based
impurities are required.
o Plasma based techniques seem promising as far as large scale manufacture is
concerned and their use in creating superhydrophobicity and self cleaning
behaviour in textiles is extremely essential as they are much more environment
friendly [13].
o These textiles should be created in such a way that the texture and feel of the
garments are not altered significantly i.e. the garments should remain
comfortable to wear even after the attachment of the nano-particles to it.
The transfer of self-cleaning behaviour to the textile fibres leads to many attractive
possibilities for the future. Presently these textiles are being tested at the laboratory scale
and industrial scale studies are also to be carried out shortly. Refinements in the
technology are still required before self cleaning clothes make their way into the retail
market.
16
Chapter Four
Some Special Applications of Superhydrophobicity
4.1 Introduction
The special surface chemistry and the unique micro- and nano-level surface roughness
that is responsible for the superhydrophobicity of certain substances are the bases behind several
interesting applications. Apart from the self cleaning glasses and textiles that have been
previously discussed in this report, there are many other areas on which researchers across the
world have focused their attention. Some of these are briefly discussed in the following sections.
4.2 Architectural materials
Architectural coatings like paints are used not only to improve the appearance of the exterior
walls of the buildings but also to protect them from the effects of the weather. In damp and cold
areas the outer walls of buildings often suffer from corrosion, growth of mould and mildew and
water seepage. In general the exterior of the buildings are exposed to the maximum amounts of
dust, acid rain, snow etc. Superhydrophobic coatings if combined with the paints applied on
these surfaces can effectively safeguard them from these problems. Superhydrophobic paints are
already available commercially in several European countries. They are manufactured using
certain patented techniques but at the heart of it is the lotus effect [19]. These paints are claimed
to be highly water and dirt repellent, UV stable and contain natural biocides and may be used
not only on walls, but also on signboards, automobiles and aircrafts.
Apart from paints, hydrophobic cements are also being developed. These basically consist of
some hydrophobic substance mixed with the other constituents of the cement. Normally the
water repellent additives are mixed with the clinker and are ground together during the
production of cement (Reference: several Internet sites) . They are useful in resisting the effects
of water seepage in damp areas.
The effect of environmental pollution, especially the damage brought about by acid rain is
most often seen on the walls of buildings. This is particularly true for buildings which have used
stones such as marble and sandstone as construction materials. Water which seeps into the
cracks in the exteriors of the buildings also affects the structure during alternate cycles of
freezing and melting, and widens or deepens the existing cracks. Studies carried out by several
17
researchers like Manoudis et.al have shown by experiments carried out on various grades of
marbles, that the modifications carried out in the commercially available siloxanes protective
agents for surfaces by incorporation of silica nano-particles is a good method of preventing the
degradation of these stones [16].
Thus superhydrophobic coatings also have an important role to play in the field of
building construction.
4.3 Anti-fouling coatings for marine vessels
Marine vessels and structures like oil rigs and jetties are exposed to the saline water of
the sea for prolonged durations and therefore they are subject to serious corrosion problems. In
addition to this they are also affected by several kinds of microorganisms that hasten the
degradation, especially of wooden structures that are exposed to water. Therefore a protective
coating that repels water and prevents microbial attack is a necessity for these structures [17].
Previously toxic chemicals were used to prevent the formation of films of bacteria on these
surfaces but now due to strict regulations these substances are being replaced by hydrophobic
paints and coatings. As they are water repellent they automatically resist the growth of bacteria
as well. The use of superhydrophobic behaviour in the development of anti-fouling coatings is
an area of intense research all over the world today.
4.4 Oil-water separation
The water repellency of these coatings can be effectively utilized to separate oil and
water phases. Sun et al. have reported the fabrication of a Teflon„¢ coated mesh on which water
exhibits high contact angles of about 157o but oil spreads very quickly forming a continuous
film and then penetrates through the mesh very easily, thereby getting separated [19]. This
special form of wettability can be successfully applied in the effluent treatment plants of
petroleum refineries where the first treatment step involves the phase separation of oil and water
in order to recover the oil.
18
Chapter Five
Summary and Conclusions
This report presents a brief discussion about superhydrophobic coatings, their methods
of preparation and their various applications. Some of the challenges yet to be overcome before
these applications become widespread have also been discussed. A lot of areas still remain
unexplored in this particular field. There is therefore plenty of scope for research on
superhydrophobic materials.
Most of the current techniques applied for the synthesis of these coatings for various
applications as diverse as self cleaning glasses, textiles to anti-fouling coatings are only suitable
for laboratory operations. The methods are very delicate and their success often depends on the
careful control of many parameters. The results have mostly been obtained under test conditions
indoors but most of the applications areas are outdoors. Moreover the basic raw materials used
are often very expensive e.g. researchers at the Clemson University, have developed
superhydrophobic textiles using silver nano particles. Quite naturally such apparel will be
several times more expensive than conventional ones. But such apparel will also have a niche
market where such specialized clothing is required (e.g. medical purposes) and hence the higher
prices might not have that big an impact on their use. Therefore there is a need to obtain cheaper
but equally effective raw materials before more extensive use of these coatings is possible.
There are a few major issues to consider whenever we are concerned about the
technological applications of superhydrophobic coatings. These are as follows:
Large scale manufacture of these coatings on the specific substrates. Most of the
methods developed are very specific to the application and the kind of substrate that is
being used. Manufacture of these films on an industrial scale, without any defects like
cracks and if possible, the simultaneous application of these onto the required surfaces
are two areas to which attention must be paid. The adherence of the film to the
respective substrate is also necessary and this aspect is crucial to the success of these
coatings. The techniques which seem to be most suitable for the application of these
coatings to large areas at a time are the plasma treatment techniques, sol-gel methods
and some spraying techniques. The method for the application of Teflon„¢ coatings on
19
non-stick cookware is a well developed technique for a specific type of substrate. More
such techniques need to be generated.
The second point pertains to the long term stability of these coatings. These coatings
function on the basis of micro and nano-level surface roughness which is easily damaged
by mechanical stresses. Therefore mechanical stability needs to be ensured. Also the
thermal stability of these coatings must be ensured. In most of the applications these
coatings are not exposed to very high temperatures. But the surface tension of liquids
generally decreases with temperature which means that their wetting capacity increases
with increasing temperature. Hence surfaces which are water repellent towards cold
water will show an increased affinity for hot water [1]. So one should be aware of thus
fact when designing a particular coating.
Superhydrophobicity appears to hold the solution to several problems that are commonly
encountered by us as far as the problems of corrosion, bio-fouling and dirt accumulation are
concerned. Thereby this field needs to be investigated further and new, innovative
applications must be designed before its full potential can be realized.
20
References
1.Y Liu, X Chen and J.H. Xin , Journal of Materials Chemistry, 2009,19,5602-5611
2. M Ma, R.M. Hill, Current Opinion in Colloid and Interface Science, 2006, 11,193-202
3. X Zhang, F Shi, J Niu, Y Jiang, and Z Wang, Current Opinion in Colloid and Interface
Science, 2008, 18,621-633
4. X Li, D Reinhoudt, and M Crega-Calama, Chem. Soc. Rev., 2007, 36, 1350-1368
5. Y Liu, J Tang, R Wang, H Lu, L Li,Y Kong, K Qi and J.H. Xin, Journal of Materials
Chemistry,2007, 17, 1071-1078
6. K Qi, W Daoud, J.H. Xin, C.L. Mak, W Tang and W.P. Cheung, Journal of Materials
Chemistry, 2006, 16, 4567-4574
7. J Bravo, L Zhai, Z Wu, R.E. Cohen and M.F. Rubner, Langmuir, 2007, 23, 7293-7298
8. R Fürstner, W Barthlott, C Neinhuis and P Walzel, Langmuir, 2005, 21, 956-961
9. J Zimmermann, F.A. Reifler, G Fortunato, L.C. Gerhardt and S Seeger, Advanced
Functional Materials, 2008, 18, 3662-3669
10. I.P. Parkin and R.G. Palgrave, Journal of Materials Chemistry, 2005, 15, 1689-1695
11. A Nakajima, K Hashimoto, T Watanabe, K Takai, G Yamauchi and A Fujishima,
Langmuir, 2000, 16, 7044-7047
12. Bi Xu, Z Cai, W Wang and F Ge, Surface and Coatings Technology, 2009, doi:
10.1016/j.surfcoat.2009.09.086 (accepted manuscript)
13. R Morent, N. De Geyter, J Verschuren, K. De Clerck, P Kiekens and C. Leys, Surface
and Coatings Technology, 2008, 202, 3427-3449
14. W.L. Min, B Jiang and Peng Jiang, Advanced Materials, 2008, 20, 3914-3918
15. G.R.J. Artus, S Jung, J Zimmermann, H.P. Gautschi, K. Marquardt and S Seeger, Adv.
Mater.,2006, 18, 2758-2762
16. P.N. Manoudis, A Tsakalof, I Karapanagiotis and C Panayiotou, Surface and Coatings
Technology, 2009, 203, 1322-1328
17. L,D. Chambers, K.R. Stokes, F.C. Walsh and R.J.K. Wood, Surface and Coatings
Technology, 2006, 201, 3642-3652
21
18. M.M. Kulkarni, R Bandyopadhyaya and A Sharma, Journal of Materials Chemistry,
2008, 18, 1021-1028
19. T Sun, L Feng, X Gao and L Jiang, Accounts of Chemical Research,2005, 38, 644-652
20. lotusan.de
21. activglas.com
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