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Hydrophobic materials and coatings: principles of design, properties and applications

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2008 Russ. Chem. Rev. 77 583

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Russian Chemical Reviews 77 (7) 583 ± 600 (2008) # 2008 Russian Academy of Sciences and Turpion Ltd

DOI 10.1070/RC2008v077n07ABEH003775

Hydrophobic materials and coatings: principles of design, properties

and applications
L B Boinovich, A M Emelyanenko

Contents
I. Introduction 583
II. Factors responsible for wetting of material surface 583
III. Highly hydrophobic state of material surface and wetting hysteresis 587
IV. Methods of preparation of textured superhydrophobic surfaces 591
V. Coating of smooth and textured surfaces with hydrophobic agents 594
VI. Ageing and degradation of superhydrophobic coatings 595
VII. Applications of highly hydrophobic and superhydrophobic materials and coatings 597
VIII. Conclusion 599

Abstract. Theoretical grounds of the design of hydrophobic 1208. Among them, a particular place is occupied by super-
materials and coatings and the specific features of the super- hydrophobic materials and coatings characterised by large
hydrophobic state of the surface are discussed. The possibilities contact angles (>1508) and small slope angles of the surfaces
of the formation of various types of ordered textures that and horizon at which water droplets slide from the surface.
ensure high contact angles on the surfaces of hydrophobic Hydrophobic and superhydrophobic materials possess a
materials and prerequisites for thermodynamic stability of the set of unique functional properties, namely, they are water-
heterogeneous wetting regime of such surfaces are analysed. proof, corrosion-resistant and stable against biofouling and
The main groups of methods actively used now to design inorganic and (in some cases) organic pollutants. The fluid
materials and coatings with specified topology and structure flow near hydrophobic surfaces of such materials occurs with
and variable hydrophobicity are presented. The problems of particular ease. Owing to the variety of functional properties,
ageing and degradation of superhydrophobic coatings are the design of hydrophobic materials and coatings has become
discussed. Examples of successful use of superhydrophobic an individual avenue of modern materials science, which has
materials in various fields of industry are given. The bibliog- been rapidly developing in the Russian Federation and abroad.
raphy includes 111 references.
references. This is accompanied by an increase in the number of publica-
tions concerning specific features of wetting of superhydro-
phobic surfaces, description of new methods of preparation of
I. Introduction textured surfaces, design of novel hydrophobic agents, forma-
Traditionally, by hydrophobic materials are meant materials tion of conditions and compositions for the preparation of
and coatings for which the contact angle of water and aqueous surfaces with switchable wetting.
solutions is larger than 908. A feature of such materials is In this review, we will briefly outline the fundamentals of
instabilty of thin wetting water layers on their surfaces. Hydro- the design of hydrophobic and superhydrophobic materials
phobicity is the property that is determined by the properties and coatings, the effect of the chemical structure and specific
and structure of the near-surface layer a few nanometres thick features of topography on the attainable values of the contact
rather than by the characteristics of the bulk material. There- and sliding angles of liquid drops on the surfaces of such
fore, the design of hydrophobic materials and coatings first of materials. We will also dwell on the most interesting methods
all requires analysis of the processes occurring in nanoscale of the design of materials and nanocoatings with specified
systems, which is a typical problem in nanotechnology. topology, structure and variable hydrophobicity. Data on
Practically interesting highly hydrophobic materials are ageing and degradation of superhydrophobic coatings will be
characterised by the water advancing contact angles exceeding briefly analysed in a separate Section. The most successful
fields of application of such materials are also discussed.
L B Boinovich, A M Emelyanenko A N Frumkin Institute of Physical
Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky II. Factors responsible for wetting of material
prosp. 31, 119991 Moscow, Russian Federation. Fax (7-495) 955 44 43, surface
tel. (7-495) 955 46 25, e-mail: boinovich@mail.ru (L B Boinovich),
ame@phyche.ac.ru (A M Emelyanenko) Thomas Young 1 was the first who considered and described
the forces acting on a liquid drop more than two centuries ago.
Received 21 December 2007 He considered an ideal (chemically inert towards the test
Uspekhi Khimii 77 (7) 619 ± 638 (2008); translated by A M Raevskiy liquid), smooth and homogeneous surface (Fig. 1 a,b). It was
shown that the equilibrium macroscopic contact angle y0
584 L B Boinovich, A M Emelyanenko

a b y0 /deg ssv /mN m71

120
1 1 80
slv
2
y0 h 3 y0 100 70
P(h)
ssl ssv 2
80 60

c d 50
60
40
40
y y 30
20
20
Figure 1. A droplet on a substrate.
0 0.2 0.4 0.6 0.8 x
Smooth hydrophobic substrate (a), smooth hydrophilic substrate (b),
homogeneous wetting on a rough substrate (c) and heterogeneous wetting
on a rough substrate (d ). For notations 1 ± 3, see text; y is the effective Figure 2. Water contact angle (1) and the surface energy (2) of a gold
contact angle. substrate covered with a self-assembled monolayer made of
HS(CH2)15COOH and HS(CH2)17CH3 molecules plotted vs. concentra-
tion of HS(CH2)15COOH (x).4
between the meniscus of the bulk liquid and the substrate is
given by
sufficient to make the surface hydrophobic. Below (see Sec-
s ÿ ssl tion V) we will show that not only the chemical composition of
cos y0 ˆ sv , (1)
slv the coating, but also the coating procedure determining the
degree of ordering and orientations of surface molecules
where ssv and ssl are the surface energies at the solid/vapour strongly affect the wettability of materials.
and solid/liquid interfaces, respectively, and slv is the surface Another approach to the description of wetting of smooth
tension of the liquid. Generally, ssv differs from the surface homogeneous surfaces was proposed by Derjaguin and Frum-
energy at the solid/vacuum interface because the solid surface kin. They developed a theory of wetting,6 which makes it
is covered with a thin wetting/adsorption film of the liquid that possible to relate the macroscopic contact angle y0 (see
is in equilibrium with the liquid drop and vapour. Analysis of Fig. 1 b) to the disjoining pressure isotherm P(h), which
Young's relation (1) showed that hydrophobicity can only be characterises the dependence of the forces of the interaction
observed on solid surfaces with low ssv . between phases 1 and 2, which confine the wetting/adsorption
As ssv decreases, the contact angle increases. As an film of liquid 3, on the film thickness h
illustration, Table 1 lists the surface energies and the contact …
?
angles on smooth surfaces of some materials. The following 1 1
cos y0 ˆ 1 ‡ P…he † he ‡ P…h†dh & (2)
should be emphasised: the free energy of a surface is deter- slv slv
he
mined by surface forces that rapidly fade towards the interior
of the bulk phase;6 therefore, hydrophobic properties can be …
?
imparted to the surface by coating a high-energy surface with a 1
&1 ‡ P…h†dh,
nanometre-thick layer of an appropriate material. For slv
he
instance, in Fig. 2 the water contact angles and the surface
energy of a gold substrate covered with a self-assembled where he is the equilibrium thickness of the wetting film at the
monolayer made of HS(CH2)15COOH and HS(CH2)17CH3 disjoining pressure equal to the capillary pressure in the drop.
molecules are plotted vs. concentration of HS(CH2)15COOH. The radii of curvature of the meniscus and drops used for
Coating with a monolayer of HS(CH2)17CH3 molecules is experimental measurements of contact angles usually lie
between 1 and 20 mm, which corresponds to the capillary and
Table 1. Surface energies of solids and water advancing angles on smooth disjoining pressures of at most 1 mPa. In this case, the he value
surfaces of some materials. is almost equal to the film thickness h0 corresponding to the
Material Surface energy Contact Ref. zero-disjoining-pressure point in the isotherm (Fig. 3).
/mJ m72 angle /deg Specific features of the three-phase equilibrium were ana-
lysed in numerous studies (see, e.g., Refs 6 ± 8). It was shown
Poly(methylpropenoxy- that wetting depends on the shape of the disjoining pressure
fluoroalkylsiloxane) isotherms (see Fig. 3), which in turn are determined by the
with the number of nature of surface forces acting in the system under study.9 In
CF2 groups (n) particular, for type 1 isotherms the integral in Eqn (2) is
n=3 14.2 105 2 positive. Such isotherms characterise three-phase systems in
n=5 16.4 102.4 2 which complete wetting with the zero contact angle occurs. In
n=7 12.6 106.7 2 these systems, a liquid droplet placed on a substrate will spread
n=9 12.2 109.3 2 in saturated vapour atmosphere with formation of a macro-
Poly(tetrafluoroethylene) 21 110 2, 3 scopically thick film with the zero contact angle. The 2 and type
Octadecanethiol 20 117 4 3 isotherms correspond to partial wetting.
Perfluoroeicosane 6.7 122 5 A basic difference between systems characterised by type 2
and type 3 isotherms is related to specific features of wetting,
Hydrophobic materials and coatings: principles of design, properties and applications 585

h where fi is the proportion of the surface area of the ith type of

smooth areas on the surface of the material under study, which
are characterised by the contact angle yi 0 .
Equation (3) can be used if the size of surface inhomoge-
neities is much smaller than the diameter of the contact area
between the liquid drop and the substrate. From Eqn. (3) it
follows that contact angle on a smooth heterogeneous surface
varies in an interval that is characteristic of each surface type
depending on the ratio of the surface areas of fragments of
3 different chemical composition. If a liquid drop is placed on a
heterogeneous substrate with a smooth surface with deep
1 2 pores, two types of equilibrium are possible. One of them
occurs if pores are not filled with the liquid (pores in which the
h0 liquid contacts the air-vapour phase) while the other occurs
+P 7P when pores are filled with the liquid. Taking into account the
fact that in the former case contact angle on the surface of
Figure 3. Types of disjoining pressure isotherms. pores (liquid/air-vapour phase interface) is 1808, relation (3)
For notations, see text. for the smooth surface with unfilled pores is reduced to
cos y ˆ f1 …cos y0 ‡ 1† ÿ 1 (4)
which characterise the three-phase equilibrium. In systems ( f1 is the proportion of the surface area occupied by the matrix
with type 2 isotherm a `sessile' liquid drop is in equilibrium material), which was first derived by Cassie and Baxter.16
with the `dry' substrate not covered with liquid molecules, In the latter case due to complete wetting (y = 08) of the
whereas systems with type 3 (S-shaped) disjoining pressure surface of pores filled with the liquid, relation (3) takes the
isotherm are characterised by the equilibrium between the drop form 17
and the substrate covered with the wetting/adsorption film of
cos y ˆ f1 …cos y0 ÿ 1† ‡ 1. (5)
finite thickness. In addition, the integral in relation (2) for the
systems with type 2 isotherm is always negative; that is why the The roughness of a wettable surface is the reason for
contact angle differs from zero. The contact angle in the system deviation of the measured contact angle from the contact
depends on the ratio of the area under the disjoining pressure angle on smooth surface. Rough surfaces are characterised by
isotherm to the surface tension of the liquid. In systems with one of the two types of wetting. In homogeneous wetting,18 the
type 3 isotherm, complete wetting is possible if the integral in liquid contacts the whole surface of a solid and completely fills
expression (2) is positive; otherwise, partial wetting occurs. all grooves (see Fig. 1 c). In heterogeneous wetting,16 the air (or
Formally, large water contact angles on hydrophobic and liquid) is trapped into grooves (see Fig. 1 d ). For homogeneous
superhydrophobic surfaces can be attained in systems that are wetting the effective contact angle on a rough substrate is
characterised by both type 2 and type 3 isotherms. Here, the calculated using the Wenzel relation 18
action of rather strong attractive (e.g., structural) forces in the
system is prerequisite. S
cos y ˆ cos y0 ˆ r cos y0 , (6)
Relations (1) and (2) were derived for the equilibrium S0
contact angles, but they can also be used to determine the
quasi-equilibrium receding and advancing angles. Here, one where r = S/S0 is the roughness coefficient equal to the ratio of
should only take into account the thickness difference between the actual surface area S to the apparent surface area S0 . Later
the wetting films in expression (2) [and, hence, ssv in rela- on, Derjaguin 19 reported a thermodynamic substantiation of
tion (1)] that are in quasi-equilibrium with the receding or this relation for a substrate immersed into a liquid. It was
advancing fronts of the liquid. Analysis of numerous exper- shown that relation (6) holds if the height of spikes on the
imental data on the determination of contact angles on smooth surface and the distances between them are small compared to
surfaces showed that the hydrophobic properties of the surface the capillary constant of the wetting liquid
layer can be improved only slightly by varying the chemical  1=2
composition of this layer. Typical values of advancing contact 2slv
aˆ
angles on smooth surfaces of bulk materials lie between 100 Drg
and 1108 (see Refs 3, 10, 11). If a smooth surface is covered with
self-assembling monolayers, the advancing contact angles can (Dr is the density difference between the liquid and vapour and
be as large as 115 ± 1168 for dodecanethiol monolayers (see g is the acceleration of gravity) and to the radius of curvature of
Refs 4 and 12) and 118 ± 1208 for monolayers with terminal the liquid meniscus in the three-phase contact zone.
CF3 groups (see Refs 5 and 13). From relation (6) it follows that in the case of homoge-
However, smooth homogeneous surfaces are rare in prac- neous wetting on a rough surface the contact angle increases
tice; therefore, the applicability of relations (1) and (2) for for hydrophobic surfaces (y0 > 908) and decreases for hydro-
calculations of contact angles on real surfaces is limited. The philic surfaces (y0 < 908) (Fig. 4).
effect of the chemical heterogeneity of the surface on the The second wetting regime occurs on a heterogeneous
measured contact angle can be included based on the relation- surface comprised of fragments with the contact angle y0 and
ship proposed by Cassie 14 (more recently, it was substantiated grooves that are partially or completely filled with air; the
based on statistical physics treatment 15) effective contact angle is given by the Cassie ± Baxter relation 16
X
cos y ˆ fi cos yi 0 , (3) cos y ˆ fr cos y0 ‡ f ÿ 1. (7)
i
586 L B Boinovich, A M Emelyanenko

cos y be a global minimum. Besides, heterogeneous wetting with

partial filling of grooves by the liquid requires meeting an
1.0
additional condition 20
1
d…rf † 1
ˆÿ , (8)
0.5 df cos y0

2 which corresponds to the possibility for the contact angle in the

groove to be determined by the Young equation.
71.0 70.5 0.5 cos y0 Elucidation of a thermodynamically stable wetting regime
in the system under study is simplified if one takes into account
the fact that the smaller of the two possible contact angles
70.5 (corresponding to homogeneous or heterogeneous wetting)
corresponds to the lower Gibbs free energy and, therefore, to
the higher thermodynamic stability.20 ± 23 As an illustration, we
71.0 will consider the plot of the cosine of the effective contact angle
on a substrate textured with rectangular pillars vs. cos y0 (see
Fig. 4). Clearly, condition (8) is not met for this topography at
Figure 4. Effective contact angle cosine on a rough substrate plotted vs.
the Young angle cosine for homogeneous (1 ) and heterogeneous (2) y0 6ˆ 908; therefore, heterogeneous wetting can only occur for
wetting. those drops that do not penetrate into the inter-pillar space.
Calculated for a texture made of square pillars and characterised by a The contact angles were calculated from relation (6) for
roughness coefficient of 1.4 and a wetted surface area proportion of 0.6. homogeneous wetting (solid line) and from expression (5) for
heterogeneous wetting (dashed line). According to the results
of the analysis, 20 ± 22 the intersection point of the solid and
Here f is the proportion of the projection of the wetted surface dashed lines corresponds to such a contact angle on the smooth
area on the substrate surface with inclusion of partial pore surface (the Young angle) where the contact angles character-
filling and r is the roughness coefficient of the wetted surface. istic of the homogeneous and heterogeneous wetting of rough
Clearly, at f ? 1 one deals with transition from heterogeneous surface are equal to each other. In other words, the two wetting
to homogeneous wetting of the surface and relation (7) is regimes are characterised by the same free energy and therefore
reduced to expression (6), whereas at f ? 0 the wettability of each of them can occur equiprobably. Homogeneous wetting is
the surface decreases. It should be noted that relationships (6) thermodynamically more stable for smaller Young angles,
and (7) can be used to analyse the contact angles of finite liquid while heterogeneous wetting is more preferable for larger
drops on substrates only if the characteristic sizes of substrate Young angles.
roughness are much smaller than the diameter of the drop ± If the surface of a system has a complex topography, the
substrate contact area. Gibbs free energy of the system can have several local minima
The possibility for a particular wetting regime to occur in the range of definition of y and f separated by energy
depends on geometric features of the topography of the solid barriers.22, 25 Such systems can exist in metastable states, the
surface, being somewhat different for the surface contacting a transitions to more stable states at low barriers being sponta-
finite drop and the surface immersed into a bulk liquid. Since neous. If the potential barrier is high enough, the transition
the superhydrophobic state with heterogeneous wetting seems from metastable heterogeneous to stable homogeneous wetting
to be quite promising for some practical applications, we will can be induced by vibration or be a result of the pressure of the
briefly outline the conditions for occurrence of both wetting drop against the substrate or due to the kinetic energy of the
regimes. falling drop.23, 26 ± 29
Theoretical analysis of the conditions for occurrence of the It is noteworthy that the angles y for metastable states can
two wetting regimes for a spherical drop on a rough surface differ from the corresponding angles for stable states by tens of
was carried out both ignoring gravity and linear tension { (see degrees; this is apparent attainment of superhydrophobic state.
Refs 20 ± 23) and with partial inclusion of gravity.24 The Moreover, from relation (7) it follows that the existence of
parameters corresponding to stable and metastable states of hydrophobic (oleophobic) metastable states associated with air
the system in question were determined 20, 21 at different values trapping in a rough structure is possible even on hydrophilic
of the parameters y and f by minimising the Gibbs free energy. (oleophilic) textured surfaces,{ which was actually observed in
It was shown that the Wenzel regime of homogeneous wetting the experiments.30 ± 33 However, such a pseudohydrophobicity
corresponds to a boundary minimum of the free energy, is metastable and can hardly be used in practice owing to
whereas the Cassie ± Baxter heterogeneous wetting corre- transition to stable homogeneous wetting regime with the angle
sponds to a local minimum. Heterogeneous wetting on a y < y0 < 908 under the action of external factors. The reverse
hydrophobic surface requires that the local minimum of free transition from metastable homogeneous to stable hetero-
energy (if exists) be lower than the boundary minimum in the geneous wetting was also observed in the systems where the
whole range of definition of f 2 071 and y 2 9071808, i.e., primary droplet was formed by condensation of strongly
supersaturated vapour onto a cooled hydrophobic substrate
textured with pillars.34 ± 36
{ Linear tension characterises the excess energy per unit length of the
three-phase contact line. Rigorous treatment of the equilibrium requires
taking account of the contribution of linear tension to the total free energy
of the system. Analysis and evaluation of this contribution are particularly { One of the referees kindly attracted our attention to the fact that it is the
important when considering equilibria in those systems where the three- apparent hydrophobicity of porous hydrophilic surface that is responsible
phase contact line has a small radius of curvature. for poor wetting of sand and dust with first drops of rain.
Hydrophobic materials and coatings: principles of design, properties and applications 587

Now we will consider conditions for transition from The requirements for surface topography to ensure hetero-
heterogeneous to homogeneous wetting for a rough surface geneous wetting of a substrate on deep immersion are specified
immersed into a bulk liquid. Similar problems arise in the by Eqn (9); they are much more rigorous (due to the last term)
design of materials for, e.g., underwater structures, ship hulls, than those specified by inequality (10); however, the effect of
etc. Thermodynamic consideration of the conditions for the term mentioned above becomes negligible for micrometre-
stability of different wetting regimes on underwater surfaces size grooves.
has some specific features. On the one hand, there is no need of
inclusion of the changes in the energy of the free surface of the III. Highly hydrophobic state of material surface
drop due to the change in the shape of the drop upon the
change in the wetting regime. On the other hand, one should
and wetting hysteresis
take into account the change in the potential energy of the In the preceding Section, we noted that by varying the chemical
system as the air bubbles trapped in the grooves on the surface composition of a material or using various hydrophobising
are replaced by the liquid. agents, one cannot achieve the contact angles larger than 1208
Changes in the free energy of a system comprised of a rough on smooth surfaces. To prepare materials with larger contact
substrate immersed into a liquid on going from homogeneous angles, one should use the combined effect of the surface
to heterogeneous wetting in a single groove are given by roughness and chemical structure. It is `tuning' of the surface
texture that allows superhydrophobic states to be attained.
DG ˆ ssv S2 ‡ slv S1 ÿ ssl S2 ‡ DrgHV ˆ (9) Most modern methods make it possible to prepare highly
hydrophobic materials based on disordered surface textures
ˆ …S2 cos y0 ‡ S1 †slv ‡ DrgHV, (see Section IV). In this case, process conditions for surface
coating or treatment are chosen in the course of lengthy and
where S1 and S2 are the areas of the liquid/gas interface and the laborious experiments. At the same time, such methods, as,
unwetted surface of the groove, respectively (Fig. 5), and H e.g., coating from dispersions, photolithographic and template
and V are respectively the depth of immersion and the groove techniques permit preparation of highly ordered and well
volume (it is assumed that the size of the gas bubble is much controllable surface roughness. Here we will analyse the effect
smaller than the depth of immersion). of parameters of ordered textures on the contact angle and
wetting hysteresis in more detail.
The simplest example to be analysed is a texture based on
pillars or holes that obey an ordered pattern on the surface of a
S1
material. Depending on the method of texturing and template
used,13, 23, 25, 26, 34 ± 40 various shapes of cross-sections of these
structures can be obtained. By varying the height, size and
S2 distance between the elements of the texture one can vary the
surface roughness and contact angle over a wide range. As an
example, Table 2 lists the data taken from Ref. 38 in which the
surface was textured with square pillars of size a = 50 mm
Figure 5. An illustration for calculations of changes in the free energy on separated by the distance b = 100 mm. By varying the height c,
going from homogeneous to heterogeneous wetting in a single groove of a one can both vary the contact angle in the Wenzel regime over a
rough substrate on dipping into the liquid. wide range and attain transition to the Cassie ± Baxter wetting
regime for high roughness determined from the relation

Heterogeneous wetting is thermodynamically stable at …a ‡ b†2 ‡ 4ac

rˆ . (11)
negative DG. A positive change in the free energy in the …a ‡ b†2
transition in question corresponds to homogeneous wetting,
i.e., to filling of grooves with the liquid. If the last term in The results of measurements of contact angles and calculations
Eqn (9) is small compared to the first term, one gets a simple based on the Wenzel and Cassie ± Baxter relations are in
but very important expression for the choice of the surface qualitative agreement; quantitative agreement requires the
topography necessary to ensure heterogeneous wetting, inclusion of a rather high roughness of the pillar surface.
namely An important feature of the ordered pillar-like surface
texture is that the advancing angles can be close to 1808.
S2 1 Additional treatment of the surface of pillars with a hydro-
>ÿ . (10)
S1 cos y0 phobising reagent causes significant enhancement of the nano-
structured roughness and makes it possible to reach the
Taking into account the fact that the Young angle is usually receding angles that are also close to 1808 (see Ref. 35).
at most 1208, we obtain that stable heterogeneous wetting can Yet another type of ordered texture, which is widely used
occur only in those systems where the unwetted surface area of for enhancement of roughness, can be obtained by deposition
the groove a priori exceeds the doubled surface area of the of spherical particles on the surface. Here, the r value is
liquid/gas interface. This makes the topography with grooves immediately related to the close-packing coefficient of the
extending towards the bottom more preferable. On the con- particles, the amount of hydrophobising reagent and the
trary, if the height of spikes on the surface is much smaller than presence of fillers in the interparticle space. For instance, if
the spike ± spike separation, homogeneous wetting will occur. the lower hemispheres of particles are immersed into the matrix
However, it should be remembered that here, as in the case of a (binder or hydrophobising reagent) on the surface treated, the
drop on the surface, the position of the liquid/gas interface in topography is formed by hemispheres or truncated spheres. We
the groove should meet the condition (8).37 have derived relations for calculating the key parameters that
characterise wetting of surfaces for different types of close
588 L B Boinovich, A M Emelyanenko

Table 2. Effect of the height of pillars on the wetting regime and contact heterogeneous wetting
angle.
2…1 ‡ cos y0 †
Drop y /deg r c /mm rˆ , (13a)
sin2 y0
p
f ˆ p sin2 y0 ; (13b)
2 3
114 1.0 0
homogeneous wetting ( f = 1)
for hemispheres
p
r ˆ 1 ‡ p , (13c)
2 3
138 1.1 10 for spherical particles
2p
r ˆ 1 ‡ p . (13d)
3

155 1.2 36 Correspondingly, the contact angles on such textured

surfaces for square packing are determined by the following
relationships:
heterogeneous wetting
p 2
cos y ˆ 1 ‡ cos y0 ÿ 1; (14a)
4
151 2.0 148
homogeneous wetting
for hemispheres
 
p
cos y ˆ 1 ‡ cos y0 , (14b)
4
for spherical particles
163 3.1 282 cos y ˆ …1 ‡ p† cos y0 . (14c)
The relations for hexagonal packing are as follows:
heterogeneous wetting
600 mm
p  2
cos y ˆ p 1 ‡ cos y0 ÿ 1; (15a)
2 3
packing of monodisperse particles on the surface. For instance, homogeneous wetting
the roughness and the proportion of the wetted surface area for for hemispheres
square packing of spherical particles and hemispheres can be  
calculated from the following expressions: p
cos y ˆ 1 ‡ p cos y0 , (15b)
heterogeneous wetting 2 3

2…1 ‡ cos y0 † for spherical particles

rˆ , (12a)
sin2 y0  
2p
p cos y ˆ 1 ‡ p cos y0 . (15c)
f ˆ sin2 y0 ; (12b) 3
4
homogeneous wetting ( f = 1) From expressions (14a ± c) and (15a ± c) it follows that the
for hemispheres radius of spherical particles (hemispheres) has no effect on the
p contact angles if the surfaces were textured by deposition of
rˆ1‡ , (12c) monodisperse spherical particles with formation of dense
4
monolayers where adjacent particles contact one another.
for spherical particles This conclusion was substantiated experimentally in a
study 41 of contact angles on the surface made of hexagonally
r ˆ …1 ‡ p†. (12d) packed polystyrene beads. It was found that the effective
contact angle remained unchanged (y = 131  28) as the diam-
The relations for spherical particles were derived assuming eter of particles varied from 270 to 690 nm. It is noteworthy
that at y0 > 908 the meniscus of the liquid for heterogeneous that the y value calculated for heterogeneous wetting using
wetting is above the equator of the particles. relation (15a) and y0 = 1148 is 1328.
The relations for a hexagonal close packing of both hemi- The contact angles calculated from relations (14a ± c) and
spheres and spherical particles have the form: (15a ± c) are plotted in Fig. 6 vs. Young angle. From these
Hydrophobic materials and coatings: principles of design, properties and applications 589

y /deg a y /deg b From relations (16a ± c) it follows that a decrease in the

180 R/D ratio causes an increase in the effective contact angle in the
system for heterogeneous wetting and a decrease in this angle
3 2 3 2 for homogeneous wetting. However, it is the wetting regime
140 1
1
corresponding to the smaller of the two contact angles that will
be thermodynamically more stable (see above). Analysis of
expressions (16a ± c) showed that an increase in the distance
between the centres of spherical particles will cause the effective
100
contact angle corresponding to the equilibrium state to
90 100 110 120 y0 /deg 90 100 110 y0 /deg increase initially (heterogeneous wetting), reach a maximum
value (at equal probability of the homogeneous and heteroge-
neous wetting regimes) and then decrease (homogeneous wet-
Figure 6. Effective contact angles on a textured surface made of mono-
ting). Clearly, it is important to determine the (R/D)t ratio
disperse spheres for square (a) and hexagonal (b) close packing plotted vs.
corresponding to transition from heterogeneous to homoge-
Young angle [calculated using expressions (14a ± c) and (15a ± c)].
Heterogeneous wetting (1) and homogeneous wetting for spherical par- neous wetting at a fixed Young angle. From relations (16a ± c)
ticles (2) and hemispheres (3). The intersection points of lines 1 and 2 (lines for a hexagonal texture made of spherical particles, we get
1 and 3 for hemispheres) correspond to the angles of transition from "p #1=2
homogeneous to heterogeneous wetting.  
R 1 3…1 ‡ cos y0 †
ˆ . (17)
D t 1 ÿ cos y0 2p
data it follows that texturing of weakly hydrophobic surfaces
(y0 < 928 for hexagonal close packing and y0 < 968 for square Substituting the (R/D)t ratio calculated using expression
close packing) causes slight increase in the effective contact (17) into relations (16a ± c), we obtain the angle characterising
angle. In this case, homogeneous wetting appears to be the maximum attainable (by choosing the R/D ratio) hydro-
thermodynamically more stable. An increase in the Young phobicity of the surface with the Young angle y0 (Fig. 7).
angle permits transition to heterogeneous wetting at low According to the calculations, the superhydrophobic state on a
content of binder or hydrophobising reagent, i.e., where the textured surface formed by monodisperse particles can be
topography is formed by spherical particles. In this case, the reached using two scaling factors associated with the particle
square close packing ensures larger effective contact angles. At size and interparticle distance. However, the requirements
the same time, if coating of a substrate was accompanied by imposed in this case on the Young angle (y0 > 113 8C) and
embedding of the particles into the polymer matrix (binder on the inter-particle distance (R/D < 0.3) are quite rigorous
and/or hydrophobising reagent) and the topography is deter- and therefore the desired surface textures are prepared using
mined by truncated spheres, the wetting will be governed by the special technological procedures.
Wenzel law as in the case of small Young angles.
Thus, comparison of the data in Figs 6 a and b shows that
yt /deg (R/D)t
the highest hydrophobicity on dense texturing using mono-
180 0.5
disperse particles is attained for a looser square packing and
shallow immersion of particles into the polymeric matrix.
However, one should keep in mind that such a texture is less 0.4
stable to mechanical action. Finally, an important result is the
fact that a superhydrophobic surface cannot be prepared at 140
close packings of monodisperse particles irrespective of par- 0.3
ticle size.
In loosely packed systems, a new degree of freedom 0.2
appears. For instance, the contact angles on the surface for 100
loose hexagonal packing are given by: 0.1
heterogeneous wetting 90 100 110 120 y0 /deg
 
2p R 2  2
Figure 7. R/Dt ratio calculated using Eqn (17) and effective contact
cos y ˆ p 1 ‡ cos y0 ÿ 1; (16a)
3 D angle yt corresponding to wetting transition plotted vs. Young contact
angle.
homogeneous wetting
for hemispheres
   Another situation occurs in the case of particle aggregation
2p R 2 on the surface with formation of multimodal rough-
cos y ˆ 1 ‡ p cos y0 , (16b)
3 D ness.12, 42 ± 45 In the simplest case this is a bimodal texture
prepared using two types of particles of strongly different size,
for spherical particles namely, the surface is coated with large particles covered with a
    monolayer of small particles. As the roughness coefficient
8p R 2 significantly increases for close packings, in this texture, on
cos y ˆ 1 ‡ p cos y0 , (16c)
3 D the one hand, homogeneous wetting becomes thermodynami-
cally unfavourable even on weakly hydrophobic particles and,
where R is the radius of the spherical particle and D is the on the other hand, the proportion of the wetted surface area
distance between the centres of adjacent spheres in the lattice. decreases, which in turn leads to an increase in the contact
angle for heterogeneous wetting. It should be emphasised that
590 L B Boinovich, A M Emelyanenko

difference between the advancing angle (yadv ) and the receding

a
angle (yrec )

Dy ˆ yadv ÿ yrec ,

therefore we will dwell on the effect of surface texture on Dy.

In their fundamental study, Dettre and Johnson 47 system-
atically investigated the effect of roughness on the receding and
advancing angles (Fig. 9). The experimentally observed hyste-
resis of the contact angle for the surfaces coated with fluoro-
carbon and paraffin waxes was 158 and 88, respectively.
Initially, as the roughness of both surfaces increased, the
advancing angle increased and the receding angle decreased.
As a result, the hysteresis increased to 1008 on fluorocarbon
10 mm waxes and to 858 on paraffin waxes. In the second step, upon
attainment of a certain critical roughness, the receding angle
behaves in different manner, namely, an increase in roughness
b was accompanied by fast increase in the receding angle and
simultaneous much slower increase in the advancing angle.
Finally, the third step was characterised by almost constant
value of the advancing angle and a slight increase in the
receding angle. As a result, the contact angle hysteresis is
much smaller than for smooth surfaces and corresponds to
sliding angles of a few degrees.

yrec , yadv /deg a

170

150
3 mm

130
c
110

1
90 2
20 nm

70

50

yrec , yadv /deg b

20 mm 160

Figure 8. Surface textures: the leaf of lotus Nelumbo nucifeara at differ- 130
ent magnification (a, b) 46 and the legs of water strider Gerris hemigis (c).31

100
almost all superhydrophobic surfaces based on both artificial
disordered textures and textured natural materials (Fig. 8) are
characterised by multimodal roughness. 70
Up to this point we have discussed the effect of specific
features of textures on the contact angle. However, the most 1
attractive for practical use are those superhydrophobic materi- 40 2
als that are characterised by not only large advancing contact
angles, but also small sliding angles, a, of liquid drops (see
Introduction). It is the small value of the angle a that 10
unambiguously points to predominance of heterogeneous r (rel.u.)
wetting where the surface area of the substrate ± water contact Figure 9. Effect of roughness on the receding angle (1 ) and advancing
is minimal. The angle a is first of all determined by the mass of angle (2) on the surface covered with fluorocarbon (a) and paraffin (b)
the drop and by the contact angle hysteresis defined as the waxes.
Hydrophobic materials and coatings: principles of design, properties and applications 591

This pattern of changes in the contact angle hysteresis with

a b
an increase in roughness was observed in numerous more
recent studies. It is associated with manifestation of homoge-
neous wetting in the region where the receding angle decreases
and heterogeneous wetting in the region where yrec rapidly
increases. Interestingly, to a first glance the initial decrease in
the receding angle from values greater than 908 to those lesser
than 908 contradicts the Wenzel relation (6). However, this 300 mm 300 mm
apparent contradiction can be explained with ease from the
standpoint of the Derjaguin ± Frumkin theory. c
As mentioned above, relations (1) and (2) were derived to
describe the equilibrium angles, they can correctly be used for
calculations of quasi-equilibrium receding and advancing
angles taking into account the actual thicknesses of wetting
2
films that are in equilibrium with the receding and advancing
fronts of the liquid. In the case of homogeneous wetting, there
is a rough structure with grooves partially filled with the liquid
1
behind the receding front; this leads to an increase in the
effective thickness of the wetting film that coexists with the a /deg d
receding drop. Therefore, calculations of the effective receding
angle from relation (6) require the use of the receding angle for 70
the drop that is in quasi-equilibrium with the thicker film 1
rather than the receding angle y0 rec on the smooth surface 2
determined for the drop that coexists with the film. According 3
50
to relation (2), this in turn leads to a decrease in y0 rec for the
three-phase systems described by type 2 and type 3 isotherms
(see Fig. 3). However, on going to heterogeneous wetting at
30
larger roughness due to the absence of the liquid in grooves, the
effective thickness of the liquid film after the receding front
rapidly decreases; this causes the angle to increase. Finally, in
10
the case of the limiting attainable receding angles correspond- 10 20 30 m /mg
ing to the absence of liquid after the receding front the differ-
ence between the advancing and receding angles is only
Figure 10. SEM images of the groove structure (a) and pillar-like struc-
determined by the capillary pressure difference between the
ture (b); schematic illustration of the sliding angle measurement direction
receding and advancing fronts of the drop. The residual parallel (1) and orthogonal (2) to grooves (c) and the dependences of the
hysteresis depends on the shape of the disjoining pressure sliding angles on the droplet mass m (d ).
isotherm in the region of small negative P values and is usually (d ): parallel direction (1), orthogonal direction (2), and pillar-like struc-
very small. ture (3).38
The effect of the mass of the drop (m) on the sliding angle
can be estimated as follows. As the tilt angle of the substrate
becomes equal to the sliding angle (here the drop is still in IV. Methods of preparation of textured
equilibrium!), the sum of the projections of the forces acting on
the drop along the sliding plane on the inclined plane should be
superhydrophobic surfaces
equal to zero. Therefore, the projection of the gravity force, Systematic research on the design of specific textures suitable
which depends on the tilt angle of the substrate, will be for imparting superhydrophobic properties to surfaces of
compensated by the sum of wetting tension along the drop ± materials began in the 1950s.47, 49 In the study by Dettre and
substrate contact line. To a first approximation, in this case one Johnson 47 (see above), the surface was textured by spraying
has 3, 48 the substrate with either dispersed paraffin (or fluorocarbon)
waxes or heated dispersion of micrometre-size glass beads in a
mg sin a ˆ pRslv …cos yrec ÿ cos yadv †, (18) wax solution in hexane. Different roughness was imparted by
subsequent fusion of the surface. The effect of surface rough-
(R is the radius of the drop ± substrate contact), which shows ness on the contact angles and on wetting hysteresis was
that the sliding angle decreases as the mass of the drop studied systematically.47
increases and wetting hysteresis decreases. More recently, a large number of approaches to prepara-
Thus, it is the small wetting hysteresis associated with tion of textured surfaces necessary to attain the superhydro-
wetting transition to heterogeneous wetting, which is respon- phobic state of surfaces were proposed. The techniques most
sible for instability of drops on oblique hydrophobic surfaces. widely used at present are as follows:
Experimental studies of the effect of the mass of the drop on the Ð coating with thick layers of alkylketene dimers by
sliding angle revealed 38 the sliding angle anisotropy on aniso- removal of substrate from melt followed by crystallisation of
tropic textures. The light sliding direction coincided with the the coating and formation of a fractal structure;50, 51
direction characterised by continuous drop ± substrate contact Ð solution polymerisation of coatings with the formation
line (Fig. 10). of a porous phase on various surfaces;52, 53
Ð chemical vapour deposition of ordered structures fol-
lowed by treatment with hydrophobic materials;3, 54 ± 58
Ð plasma etching of polymer surfaces;3, 10, 11, 59 ± 61
592 L B Boinovich, A M Emelyanenko

Ð coating with films of sublimation materials;62, 63 ces CF2 radicals, which undergo polymerisation and form a
Ð electrodeposition and electrochemical deposition of thin poly(tetrafluoroethylene) layer on the nanotube surface.
nanoparticles and films followed by treatment with hydro- Once these procedures completed, the advancing and receding
phobic materials;64, 65 contact angles on the surfaces coated with the `forest' of
Ð use of organic and inorganic fillers with multimodal hydrophobised nanotubes can be as large as 170 and 1608,
particle size distribution for particles in the matrix of the respectively.
hydrophobic material;42 ± 44, 66 ± 71 An interesting example of attainment of the superhydro-
Ð template methods for preparation of rough surfaces phobic state by substrates of different nature using plasma
with subsequent removal of templates and treatment with etching was reported by Woodward et al.10 A silicon (or
hydrophobic materials;72 ± 76 potassium bromide) substrate was coated with a polybutadiene
Ð controllable aggregation of nanoparticles on the sur- film in toluene solution and annealed to remove traces of the
face, which leads to multimodal roughness, followed by treat- solvent. Then the sample was placed in a vacuum chamber
ment with hydrophobic materials;12 (261074 atm) filled with carbon tetrafluoride and an electric
Ð photolithographic techniques followed by treatment discharge was switched on. This treatment gave a rough
with hydrophobic materials;13, 23, 25, 26, 39, 40 and surface of the polybutadiene film, and fluorination of the
Ð etching of the surface of materials followed by treatment surface layer occurred simultaneously. The degree of rough-
with hydrophobic materials.77, 78 ness depended on the discharge power (Fig. 12) and on the time
Now we will consider specific features of different techni- of exposure to the discharge. The superhydrophobic state
ques in more detail taking some studies cited above as required that the root-mean-square roughness be at least
examples. 46.5 nm. The maximum water contact angle on this surface
Plasma-assisted chemical deposition of carbon nanotubes was 1758 and wetting hysteresis was at most 18.
was used for texturing the surface of silicon covered with oxide
film.54 The two main steps of the process include coating of a a b
substrate with a catalyst (in the form of nickel islets) by melting
a thin nickel film and growth of carbon nanotubes on the nickel
islets in d.c. plasma discharge (acetylene/ammonia gas mix-
ture) at a pressure of 4 mm Hg. Plasma-assisted chemical
depositon provides the desired orientation of nanotubes (nor-
mal to the substrate). The diameter and number of nanotubes
per unit surface area are specified by the size and distribution of
the catalyst islets, while the height is proportional to the
5 mm 5 mm
deposition time. The texture thus prepared exhibits reasonable
homogeneity with respect to the distribution of nanotubes over
the substrate surface; the shorter the nanotubes the better the c d
homogeneity with respect to height (Fig. 11). However, the
contact angles of water droplets on the nanotube textured
surfaces lie between 84 and 868. In particular, it is the fact that
the contact angles are smaller than 908 that is responsible for
the possibility of water condensation to occur in the space
between nanotubes on thermal equilibrium. Here, the capillary
effects cause single nanotubes to form bundles,54 which has a
negative effect on the operating properties of nanotubes. 5 mm 5 mm
Subsequent treatment of the surface of carbon nanotubes also
makes use of chemical vapour deposition, namely, thermally
activated decomposition of hexafluoropropylene oxide produ- e f

a b

50 nm
5 mm 5 mm

Figure 12. AFM height images of the surface of a polybutadiene film after
plasma etching (CF4 atmosphere, duration = 5 min) at plasma discharge
1 mm 1 mm power of 0 (a), 10 (b), 20 (c), 30 (d ), 40 (e), 80 W ( f ).10

Figure 11. SEM images of a surface covered with carbon nanotubes.54 Coating of the surface of glass with films of sublimation
Non-hydrophobised surface (a) and the hydrophobised surface coated materials containing titania inclusions 62, 63 allowed three prac-
with poly(tetrafluoroethylene) obtained by chemical vapour deposition tically important problems to be solved at once, namely, to
during thermal decomposition of hexafluoropropylene oxide (b). attain the superhydrophobic state, to retain transparency of
the coated glass and to perform photocatalytic cleaning of the
Hydrophobic materials and coatings: principles of design, properties and applications 593

surface when operating in air. Figure 13 presents a flow grooves on the final texture. The texture is made hydrophobic
diagram of the process 62, 63 of preparation of similar hydro- by treatment with hydrophobic materials with low surface
phobic films based on aluminium hydroxide or silica. energy. In particular, the surface of a textured silicon wafer was
coated (by adsorption from vapour phase) with methylated or
fluorinated organosilane layers.13 The advancing contact
Al(C5H7O2)3 TiO(C5H7O2)2 AlO2H C2H5OH angles on such surfaces were as large as 1768; however, a large
hysteresis was observed, which strongly depended on the shape
of the cross-section of pillars (Table 3). A drawback of photo-
Sonication of a suspension lithographic techniques is that this is applicable only to small
surface areas. In addition, textures with pillars are mechan-
ically unstable, which also reduces the field of application.
Spin-coating on a substrate at 1500 rpm Structures with grooves are mechanically more stable but
inappropriate for attainment of the superhydrophobic state.
Five cycles
Calcination at 500 8C for 20 s
Table 3. Contact angles on surfaces textured using variously shaped pillars
of height 40 mm and hydrophobised with dichlorodimethylsilane.13
Coating with a hydrophobising reagent
Pillar shape and Texture Contact angle /deg
arrangement parameters
Drying at 140 8C for 1 h advancing receding

x
Transparent superhydrophobic coating x/2 staggered 176 156
rhombus
Figure 13. Flow diagram of the processing of titania-containing trans- 4x pillars,
parent superhydrophobic coating.62 x = 8 mm

2x
Titanium acetylacetonate, which undergoes thermal
decomposition with the formation of titania, is mixed with x
aluminium hydroxide or silica powder, aluminium acetylacet- x
onate Al(C5H7O2)3 and ethanol to produce a suspension. 2x star-like 175 149
Sonication of the suspension leads to homogeneous distribu- pillars,
tion of components and dissolution of Al(C5H7O2)3 . The x = 8 mm
suspension is coated on a glass substrate and dried at room
temperature to afford a matt coating. Short-term heat treat-
ment of the coated substrate at 500 8C causes decomposition of 2x
titanium acetylacetonate, sublimation of Al(C5H7O2)3 and x
enhancement of film transparency. The transparency of this
system in the visible spectral region strongly depends on both x
the surface roughness due to sublimation and the content of 2x indented 175 143
titania. The coating, drying and calcination of suspension are square pillars,
repeated several times until uniform coating of the substrate is x = 8 mm
achieved. Hydrophobic properties are imparted to the coating
by storing samples in a solution of heptadecafluorodecyltri- 2x
methoxysilane in methanol at room temperature for 1 h and
subsequent drying at 140 8C also for 1 h. Studies of the x
coatings thus prepared revealed a small loss of hydrophobicity x square pillars,
after long-term operation of the films containing TiO2 (2 mass 2x x = 16 mm 173 134
%); these films also exhibited photocatalytical self-cleaning of x = 23 mm 175 146
the surface and high transparency (*90%) throughout the x = 32 mm 173 154
visible spectral region. x = 56 mm 121 67
Recently, template methods of surface texturing have
2x
become popular.13, 23, 25, 26, 39, 40 This makes it possible to
prepare both surfaces covered with pillars and porous sub-
strates with a regular system of grooves. The advantage of
these methods is the possibility of controllable variation of not Template methods are widely used in nanotechnologies,
only the density, but also the size of grooves and pillars. but template-based design of superhydrophobic surfaces of
Usually, the preparation of textured surfaces by these techni- materials is in the early stage of its development. For instance,
ques involves a number of steps.13 First, the substrate surface is bimodal roughness on glass substrates was formed using the
covered with a photoresist layer. Then, a template is made on template technique.72 A monolayer of spherical polystyrene
the photoresist surface using a contact lithographic mask/ latex beads 5 mm in diameter used as the template was
pattern prepared with a high-resolution printer or using an suspension coated on the substrate by spin coating. The
electron beam. The template is transferred to the substrate monolayer was impregnated with a silver acetate (AgOAc)
using chemical etching; the duration and regime of this solution by capillary forces and a thin AgOAc layer was
procedure determine the height of pillars or the depth of formed after subsequent drying of the lower surface of the
594 L B Boinovich, A M Emelyanenko

latex particles. Subsequent slow heating at 360 8C for 3 h led to presence of dichlorodimethylsilane on the surface of a glass
thermal decomposition of AgOAc and formation of silver substrate was followed by their aggregation. The shape of
nanoparticles and their sintering, the template being removed aggregates strongly depended on the structure of the substrate.
simultaneously. The structure thus fabricated (Fig. 14) has a For instance, an islet-shoped film made of aggregated particles
bimodal roughness with the characteristic size of silver nano- was formed on a smooth surface, while the deposition after
particles of 150 nm (mean size) and 5 mm. The water contact preliminary polishing of the surface gave a uniform distribu-
angle on this textured surface reached a value of 1698 after tion of the aggregated particles across the substrate surface
coating with a hexadecanethiol monolayer. with two characteristic roughness intervals, 0.2 ± 1 mm and
40 ± 80 nm. To impart hydrophobic properties to the texture
thus prepared, its surface was coated with a polymeric film of a
a commercial hydrophobising reagent FC735 (blend of a fluo-
roacrylate polymer and fluoroalkyl ethers). It should be noted
that the superhydrophobic state of the substrate was attained
only in the case of uniform distribution of the aggregated
particles across the substrate surface; the advancing contact
angle was 1708 and hysteresis was at most 28.
The promise of the methods of preparation of superhydro-
phobic coatings based on the use of organic and inorganic
fillers with multimodal particle size distribution is first of all
due to the relative technological simplicity of coating of large
5 mm
surface areas. In addition, one can use composites; this makes
it possible not only to impart hydrophobic properties to the
b surface of the texture, but also to chemically bind it to the
substrate surface, which makes the hydrophobic coating more
stable under the operating conditions.
Summing up, emphasise again that the key requirements
for textures used to attain the superhydrophobic state of
surfaces of materials include the maintenance of multimodal
roughness of the surface and mechanical strength of the
texture.

2 mm V. Coating of smooth and textured surfaces

with hydrophobic agents
Figure 14. SEM images of a film made of silver nanoparticles with Numerous experiments showed that the surface energy and
bimodal roughness at different magnification.72 wettability depend not only on the chemical structure and
texture of the surface. The contact angle also strongly depends
on the orientation of molecules on the surface and on the
Now we will consider in more detail a widely used method degree of their ordering. The hydrophobising monolayers are
of preparation of superhydrophobic coatings that involves the most often coated using dip coating, spin coating, adsorption
use of organic and inorganic fillers with multimodal particle from solutions or from the vapour phase and the evaporating
size distributions,42 ± 44, 66 ± 71 such as silica,44, 67 glass beads,47 drop techniques.
fluoride latexes,66, 69 polystyrene latexes and carbon nano- The dip coating (or dipping) technique involves immersion
tubes.68 On the surface of substrates, these particles either of a substrate into, and subsequent removal from, a solution
self-assemble to form a monolayer or aggregate with the containing a hydrophobising reagent or a dispersion of hydro-
formation of bimodal or multimodal roughness after evapo- phobic particles. In the case of solution coating, the possibility
ration of the dispersion medium. In particular, silica particles of preparing coating with uniform thickness is governed by the
with sizes of 700 and 70 nm were used.67 Large particles were possibility of immersing and removing the substrate at a
coated with a film containing epoxy groups, while the smaller constant velocity, whereas the thickness of the coating depends
particles, with amino groups. The reaction between epoxy and on the velocity of the motion of the receding (advancing)
amino groups led to covalent bonding of the small particles to meniscus of the liquid, the concentration of hydrophobising
the surface of the large ones. The composite particles thus reagents and the viscosity of the solution.79 ± 81 In addition, the
prepared were deposited from dispersion in ethanol on an diameter of particles is important when using dispersion coat-
aluminium substrate coated with a film containing epoxy ing.82 Finally, the quality and structure of the coating are to a
groups; this led to formation of bimodal roughness. The great extent determined by the character of the interaction of
reaction between non-consumed amino groups on the surface the hydrophobising components (hydrophobised particles of
of the small particles with the composite particles led to the dispersion) with the substrate, the solvent and with one
covalent binding of the latter to the surface of the epoxy another.
coating of the substrate. Subsequent curing of the epoxy film In the case of spin coating or centrifugation of a drop of a
resulted in embedding ot the particles into the epoxy matrix; solution (or dispersion) on a rotating substrate, the possibility
binding to the surface of the monoepoxy-functionalised poly- of prepareing coating with uniform thickness strongly depends
dimethylsiloxane ensured the hydrophobic properties of the on the rotation frequency and viscosity of the solution. In
texture prepared. addition, an important role is played by the volatility of the
Silica particles of primary diameter 16 nm were also used.44 solvent (dispersion medium), which determines both the equi-
Deposition of these particles from dispersion in hexane in the
Hydrophobic materials and coatings: principles of design, properties and applications 595

librium character of the coating structure and its homogeneity value of Young contact angle. For instance, for a gold
with respect to thickness and the chemical composition.80, 81 substrate coated with alkanethiols by adsorption from solu-
Adsorption from solutions or vapour phase is mainly used tions, the contact angle on the surface of the coating can be as
for preparation of monolayered hydrophobic coatings based large as 115 ± 1168, being only 100 ± 1108 when using the dip
on, e.g., self-assembled monolayers. Specific features and coating technique. The contact angles on fluorinated chlorosi-
problems that arise when using this technique have been lane monolayers coated using different techniques also vary
discussed in literature;83 ± 89 therefore, we will not dwell on from 100 to 1148. Here, the problem of choice of the method for
them. We only point that the quality of the coating, its spatial applying hydrophobic coatings to attain the superhydrophobic
homogeneity and roughness of the coated substrate depend to state arises naturally. Analysis of experimental data on the
a great extent on the solvent used and the presence of water in contact angles on hydrophobised textured substrates showed
the solvent, on the surface and in the surrounding atmosphere, that small variations of the Young angle do not preclude the
as well as the temperature and pretreatment of the substrate obtaining of the effective contact angles larger than 1508 on
surface. The contact angle attainable on a particular surface textures with multimodal roughness. However, obtaining
upon adsorption of a hydrophobising monolayer strongly small sliding angles for drops on such hydrophobic surfaces
depends on the density and regularity of the packing of the in each particular case requires thorough choice of the coating
monolayer. Usually, self-assembled monolayers have many technique that provides the chemical and structural homoge-
structural defects that deteriorate the hydrophobic properties. neity of the coating. Yet another prerequisite for hydrophobic
A method of preparation of mechanically assembled mono- coatings is their chemical stability and wear resistance, because
layers on relatively smooth surfaces, for which the contact it is these properties that are responsible for long operating life
angle reached a value of 1308, was proposed.87 Figure 15 of the coatings.
schematically shows the necessary process steps. A polydime-
thylsiloxane (PDMS) substrate (1 ) is pre-stretched (2) and VI. Ageing and degradation of superhydrophobic
exposed to ozone and UV irradiation to produce surface
hydroxyl groups (3). In the next step (4) the surface is coated
coatings
with a self-assembling chlorosilane monolayer by adsorption The ability of hydrophobic and superhydrophobic materials to
from the vapour phase. After stress relief, a closely packed retain hydrophobic properties under operating conditions is of
monolayer of grafted molecules is formed on the substrate (5); paramount importance when assessing potential fields of their
the number of structural defects in the monolayer is much application. Clearly, the ageing and degradation (loss of
smaller than in the self-assembled monolayer on the non- hydrophobic properties) are governed by both the properties
pretreated substrate. The coating thus prepared is character- of the coating and specific features of the operating conditions
ised by the largest contact contact angle for smooth surfaces. and the character of its interaction with the environment.
The evaporating droplet technique is the fastest and Unfortunately, the problem of durability of hydrophobic
simplest. It can be used for rough assessment of the properties materials has not been cosidered in detail in the literature. In
of coatings. A drop of a solution (dispersion) containing a this Section we will briefly outline a number of studies on the
hydrophobising reagent is placed on the surface of a substrate subject.
where it spontaneously spills over. Films thus prepared usually When materials are used in air, the loss of hydrophobic
have different thickness at different points and inhomogeneous properties is due to atmospheric pollution. Usually, surfaces
chemical composition even if the substrate is completely wetted become significantly more hydrophilic upon deposition of dust
by the solution or dispersion. Similarly to the methods men- and organic chemicals. To reduce this effect, it was proposed 62
tioned above, here the properties of the coating depend on the to add titania to the hydrophobising coatings for glass ele-
state of the surface, the solvent used, the temperature and ments of buildings. Titania possesses photocatalytic activity
humidity. Nevertheless, in some cases simplicity of the techni- and organic residues exposed to UV radiation in the presence
que plays the determining role, because it allows the chemical of TiO2 are oxidised to CO2 . Outdoor exposure tests of
composition, solvent, temperature, etc. to be chosen with ease coatings with different content of titania were carried out in
in the design of novel coatings.45 Tokyo (Japan) in winter at a height of 20 m from the ground
Coatings prepared by the methods considered above usu- level. The tests showed that degradation of the coatings caused
ally differ not only in thickness, but also in the degree of by stains occurs much more slowly in the presence of small
ordering and orientation of molecules. This in turn affects the amounts of TiO2 (*2 mass %) and faster in the presence of

UV irradiation
and ozone
Dx treatment

PDMS

1 2 3 4 5

y = 508 y = 1108 y = 1308

Figure 15. Process steps leading to production of mechanically assembled monolayer.87 For notations (1 ± 5), see text.
596 L B Boinovich, A M Emelyanenko

larger content of titania (Fig. 16). At the optimal concentra- Ageing of superhydrophobic anticorrosion coatings with
tion of TiO2 , the water contact angle decreased from 150 to different compositions on the carbon steel surface was
1408 after 75 days (cf. the range from 1488 to 1008 for the studied.66 The surface of a steel sample was coated with either
hydrophobic coating with no TiO2 additives). a layer of Ni7P composite or a phosphate film with complex
chemical composition to provide corrosion resistance and
specify the topography. Then the dip coating technique was
y /deg
used to coat the corrosion resistant layer with a solution of a
160 hydrophobising reagent that contained a fluoride latex and the
system was heated at 150 ± 200 8C for 1 ± 2 h. The contact
angles on the freshly prepared coatings based on the phosphate
120
film were in the interval 155 ± 1688 and changed only slightly on
storage under laboratory conditions. For instance, the contact
80 angle on these films decreased by *38 after 45 days, while the
1 sliding angle considerably increased from 2 ± 3 to 15 ± 208.
2 Storage of the coatings in water led to a decrease in the contact
3
40 angle by 48 after 24 h and by 68 after 48 h. The sliding angle
increased by 28 after 48 h. The results of water erosion test and
the character of the interaction of the samples with 0.5% NaCl
0 solution showed that the coatings with superhydrophobic
500 1000 1500
surface layer exhibited much better corrosion resistance com-
Time /h pared to the coatings having no hydrophobic layer.
Superhydrophobic surfaces with multimodal roughness
Figure 16. Changes in contact angle on the surface coated with a compo- (Fig. 17) can be prepared by coating various materials with a
site alumina/titania hydrophobising film plotted vs. film exposure time textured system based on silica nanoparticles treated with
under atmospheric conditions.62
TiO2 content /mass %: 0 (1), 2 (2) and 20 (3).
a
961.71 nm

The problem of ageing of hydrophobic coatings was also

considered in another study.89 The surface of a sample was
hydrophobised by keeping of the sample in carbon tetrafluor-
ide plasma. Two types of samples were studied, (i) PDMS
containing no fillers (they were removed by centrifugation) and
(ii) PDMS filled with large amount of quartz. Keeping in
plasma led to hydrophobisation of the surface due to fluorina-
tion and to simultaneous enhancement of roughness. At
residence times of no longer than 15 min, a layer containing
CmFn (m > 1) or CFxH37x (0 < x < 3) groups was formed on
the PDMS surface and the surface energy considerably
decreased. Ageing of this surface in air was studied by 1 mm
elemental analysis (Table 4). In particular, it was found that
on long-term treatment of samples the content of carbon and
oxygen increased, that of fluorine decreased, whereas the 0 nm
b
amount of silicon remained virtually unchanged. After keeping
in air for 5 weeks, the atomic concentration of fluorine was
93% with respect to the initial concentration in the samples
with the filler and 89% in the samples with no filler. The
smaller loss of hydrophobicity of the Teflon-like surface with
quartz filler was explained by rigid binding of fluorosilane
groups to the filler surface.

Table 4. Elemental composition (in %) of the surface of etched poly-

dimethylsiloxane sample (CF4 plasma, etching duration 15 min) cured in
air.89

Curing PDMS with quartz filler PDMS with no filler

duration
C Si O F C Si O F
400 nm
Freshly 18.7 5.8 10.8 64.7 18.9 4.8 18.8 57.5
prepared
sample Figure 17. AFM images of the a surface of superhydrophobic coating
5 days 19.7 5.9 11.2 63.2 21.2 5.2 19.5 54.2 prepared by deposition of dispersed aerosil on a silicon latex substrate
15 days 19.3 6.5 12.0 62.2 22.4 5.3 19.6 52.7 followed by hydrophobisation with fluoroxyaminosilanes obtained at
35 days 20.9 6.4 12.4 60.3 23.7 5.3 20.1 50.9 different magnification.45
Hydrophobic materials and coatings: principles of design, properties and applications 597

fluoroxyaminosilanes. Tests for retention of superhydropho- larger than 1568 and the sliding angle of water is *48. At the
bic properties of these coatings revealed a decrease in the same time the surface remained oleophilic. Water ± oil emul-
contact angle from 158 to 1568 after storage under atmospheric sion in this sieve was rapidly separated, because oil droplets
conditions for 60 days, while the sliding angle of the drop passed through the openings in the sieve to the lower tank.
remained unchanged (*58). The results of investigations of the Textile fibres with nanostructured surface are suitable for
dynamics of hydrophobicity loss on long-term contact with production of hydrophobic cloth based on cotton fibres 91 and
water suggest that the coating prepared provides contact manufacturing of self-cleaning ties and shirts (no wash is
angles that are larger than 1508 and completely protects the required).92
surface from water for 10 ± 15 h. However, the hydrophobic The use of superhydrophobic coatings in electrical engi-
properties of the surface are little changed even on longer time neering appeared to be highly efficient. For instance, coating
of contact with water. For instance, the contact angle on the conductors of power lines with superhydrophobic films con-
coating decreased by 188 over a period of 48 h. Tests of the siderably reduced the noise due to corona discharges produced
same coatings in salt, weakly acidic and weakly alkaline by water drops on the surface of conductors.93
solutions revealed slight changes in the degree of hydropho- We have developed hydrophobic coatings with multimodal
bicity over a period of 2 ± 4 h of continuous contact with the roughness for the surface of silicon insulators for high-voltage
solution (Fig. 18). power lines. This makes it possible to significantly reduce the
intensity of the interaction of the material of the insulator with
atmospheric precipitates and, therefore, to reduce the leakage
y /deg
currents flowing across the surface of the insulators.45
165 Now we will dwell on the use of textured hydrophobic
materials as adaptive materials that can change their properties
under the action of external factors. The chemical properties
160 and wetting of such materials can vary upon irradiation,94 ± 96
1
2 in an electric field,97 ± 99 on heat treatment,100 treatment with
3 solutions 61, 101 and as a result of change in the pH value.64
155
Reversible changes in the wetting properties of surfaces
from superhydrophobicity to superhydrophilicity (Fig. 19) on
exposure to UV radiation were observed for zinc,55, 103 ± 105
150
titanium,106 ± 108 tin 109 and tungsten 110 oxides. They were used
by researchers from PPG Industries (USA) and Pilkington
0 2000 4000 t /s
(UK) for fabrication of self-cleaning glasses. There are two
mechanisms of self-cleaning. In the daytime, exposure to UV
Figure 18. Dynamics of changes in the contact angle of aqueous solution
droplets on the surface covered with a textured superhydrophobic coat-
ing.45 y /deg a
Alkaline solution with pH 7.45 (1), 0.5 M solution NaCl (2) acid solution
with pH 6 (3). 120

1
It should be noted that on prolonged contact the hydro- 80 2
phobic coating reacts with water the degree of the reaction 3
being dependent on the chemical structure of the hydrophobis-
ing reagent. Therefore, a prerequisite for extension of the
40
durability of a hydrophobic coating is minimisation of the
surface area of the contact with the liquid. This can be attained
in the case of heterogeneous wetting.
0
20 40 60 80 t /min
VII. Applications of highly hydrophobic
and superhydrophobic materials and coatings y /deg b
120
Hydrophobic materials are widely used at present. Now we will
give a few examples of the use of materials the hydrophobic
and superhydrophobic properties of which are attained by
choosing the chemical composition of the surface and specific 80
texturing.
Treatment of the surface of satellite antennas with super-
hydrophobic coatings developed by researchers at NTT 40
(Japan) led to significant reduction of the adhesion of snow
to metallic antenna surfaces 89 and, as a consequence, the
number of satellite communications' breakdown events
0
decreased. 2 4 6 8 10 t /days
An efficient device for separation of aqueous and oil phases
was made using tailored textured coatings on microsieves 90 Figure 19. Time dependences of contact angle of ZnO surface exposed to
The operation of such sieves is based on the use of strongly UV radiation with a light intensity of 0.1 (1), 2.0 (2) and 50 mW cm72 (3)
different contact angles of water and oil on the surface of the (a) and on storage of the irradiated samples in the dark (b).102
sieve. For instance, the advancing contact angle of water is
598 L B Boinovich, A M Emelyanenko

a b

O O
H H H H
O O O O H N H N
N H N H
H H
N N O O
heating
cooling O O
H H H H
O O O O H N H N
N H N H
H H
N N O O

Figure 20. Reversible conformational transition with formation of stretched (a) and helical (b) conformations of poly(N-isopropylacetamide) chains on
heating and cooling.

radiation causes (i) photocatalytic decomposition of organic molecules grafted to the surface (Fig. 20). It was shown 100
and (ii) hydrophilisation of the surface due to additional that this conformational transition on heating causes contact
adsorption of water molecules initiated by radiation. Stains angle on the PNIPAAm-coated surface to change from 63 to
are washed out from such surfaces either by rain or by artificial 938. By texturing the surface with micrometre-size grooves, it is
irrigation. Storage of the surface in the dark for some time possible to induce the transition from superhydrophobicity to
recovers the superhydrophobic state and the second mecha- superhydrophilicity with very small wetting hysteresis. The
nism operates; it is associated with absorption of stains having system is characterised by multiple cycling of the wetting
weak adhesion to glass by water drops sliding across the regimes without degradation of coatings (Fig. 21).
superhydrophobic surface. Here, the contact angle decreased
on exposure to UV radiation from 109 to 108 on smooth ZnO
a
surface and from 163 to 08 on the textured surface. y /deg
Photoswitchable wetting has found an interesting applica- 160
tion in microfluidics and in organic synthesis. Namely, a
macroscopic motion of small liquid droplets across a photo-
120
sensitive surface used for the delivery of components to the
reaction zone can be driven by irradiation.94 A monolayer
based on azobenzene derivatives made the surface photo- 80
sensitive. UV irradiation of the surface at l = 360 nm caused 1
photoisomerisation with an increase in the proportion of cis- 40 2
isomers, which impart the hydrophilic properties to the sur-
face. Irradiation with blue light at l = 436 nm caused cis ± 0
trans-isomerisation and the surface returned to the hydro- 20 30 40 T / 8C
phobic state. If a droplet a few millimetres in size is placed on b
such a surface and asymmetrical (with respect to the wave- y /deg
length) illumination is created, the surface energy difference 160
50 8C
between different points along the perimeter is sufficient for the
droplet to move. This motion can be controlled with high
120
accuracy.
Yet another application of photoswitchable wetting was
proposed.111 The surface of nanoporous aluminium mem- 80
branes was modified with a mixture of spiropyran and hydro-
phobic molecules. A non-illuminated membrane contained the 40
non-polar form of spiropyran, which ensured nonwetting of
the membrane by aqueous solutions. On UV irradiation,
0
spiropyran undergoes transition to the polar merocyanine 20 8C
form and the water molecules and dissolved ions can pass
0 4 8 12 16 20
across the membrane. Thus, the membrane operates as a
photosensitive valve, which switches the water and ion trans- Number of cycles
port and changes the ionic conductivity.
Figure 21. Temperature dependence of the water contact angle on
Temperature-switchable wetting observed in a number of
PNIPAAm film (a) and cyclic switching between wetting regimes on a
systems characterised by the lower critical solution temper- textured surface on heating and cooling (b).100
ature can be employed to preclude biofouling of surfaces. The Smooth surface (1 ) and textured surface with micrometre-size grooves (2).
best studied polymer possessing this property is poly(N-iso- Arrows denote the directions of changes in the angle y due to surface
propylacetamide) (PNIPAAm). The wetting transition is asso- texturing.
ciated with conformational changes in the PNIPAAm
Hydrophobic materials and coatings: principles of design, properties and applications 599

10. I Woodward, W C E Schofield, V Roucoules, J P S Badyal

VIII. Conclusion Langmuir 19 3432 (2003)
In this review, we considered the key principles of the design of 11. A Tserepi, E Gogolides, K Tsougeni, V Constantoudis,
E Valamontes J. Appl. Phys. 98 113502 (2005)
highly hydrophobic and superhydrophobic materials and coat-
12. Y Zhao, Q Lu, D Chen, Y Wei J. Mater. Chem. 16 4504 (2006)
ings. The design of such materials implies the use of low-energy 13. D Oner, T J McCarthy Langmuir 16 7777 (2000)
coatings along with surface texturing characterised by multi- 14. A B D Cassie Discuss. Faraday Soc. 3 11 (1948)
modal roughness. A prerequisite that ensures the superhydro- 15. P S Swain, R Lipowsky Langmuir 14 6772 (1998)
phobic properties of a material is the surface texturing in such a 16. A B D Cassie, S Baxter Trans. Faraday Soc. 40 546 (1944)
manner that heterogeneous wetting of the surface be thermo- 17. A M Emelyanenko, L B Boinovich, in The 13th International
dynamically stable. Meeting this condition allows a number of Conference on Surface Forces (Book of Abstracts), Moscow, 2006
technological problems to be solved at once. First of all, this p. 43
18. R Wenzel Ind. Eng. Chem. 28 988 (1936)
makes it possible to reduce wetting hysteresis and, therefore, to
19. B V Derjaguin Dokl. Akad. Nauk SSSR 51 357 (1946) c
ensure self-cleaning properties of the hydrophobic surface
20. A Marmur Langmuir 19 8343 (2003)
because water drops sliding from the surface carry away 21. A Marmur Langmuir 20 3517 (2004)
waste characterised by low adhesion towards the material. In 22. N A Patankar Langmuir 19 1249 (2003)
addition, in this regime the surface area of the contact between 23. B He, N A Patankar, J Lee Langmuir 19 4999 (2003)
the coating material and the liquid is minimal and therefore 24. O Werner, L Wagberg, T Lindstrom Langmuir 21 12235 (2005)
degradation of the hydrophobic state as a result of the reaction 25. L Barbieri, E Wagner, P Hoffman Langmuir 23 1723 (2007)
with water occurs more slowly. 26. J Bico, C Marzolin, D Quere Europhys. Lett. 47 220 (1999)
We believe that even the short list of studies on methods of 27. E Bormashenko, R Pogreb, G Whyman, M Erlich Langmuir 23
6501 (2007)
preparation of textured hydrophobic surfaces of different
28. S Daniel, S Sircar, J Gliem, M K Chaudhury Langmuir 20 4085
nature cited in this review and a few examples of application (2004)
of highly hydrophobic and superhydrophobic materials pro- 29. D Quere, A Lafuma, J Bico Nanotechnology 14 1109 (2003)
vide a clear idea of a great potential of this field of materials 30. L Feng, Y Song, J Zhai, B Liu, J Xu, L Jiang, D Zhu Angew.
science. Chem., Int. Ed. 42 800 (2003)
However, a number of problems to be solved should also be 31. T Sun, L Feng, X Gao, L Jiang Acc. Chem. Res. 38 644 (2005)
mentioned. The key factors that limit the use of hydrophobic 32. M E Abdelsalam, P N Bartlett, T Kelf, J Baumberg Langmuir
materials are insufficiently high wear resistance and chemical 21 1753 (2005)
33. A Tuteja, W Choi, M Ma, J M Mabry, S A Mazzella,
instability of the hydrophobic layers. In some cases, the surface
G C Rutledge, G H McKinley, R E Cohen Science 318 1618
texture and coatings that ensure optimum hydrophobic prop-
(2007)
erties of the surface are characterised by insufficiently high 34. A Lafuma, D Quere Nat. Mater. 2 457 (2003)
mechanical strength. Of particular importance for long-term 35. K Wier, T J McCarthy Langmuir 22 2433 (2006)
durability of hydrophobic materials is the development of 36. C Dorrer, J Ruhe Langmuir 23 3820 (2007)
methods for surface cleaning from organic waste that pollute 37. A Marmur Langmuir 22 1400 (2006)
the surface of materials under atmospheric conditions. Slowing 38. Z Yoshimitsu, A Nakajima, T Watanabe, K Hashimoto
down degradation of hydrophobic state due to the reaction Langmuir 18 5818 (2002)
with water in the case of underwater and underground 39. E Martines, K Seunarine, H Morgan, N Gadegaard,
C D W Wilkinson, M Riehle Nano Lett. 5 2097 (2005)
operation of hydrophobic materials is still topical. It seems
40. E Martines, K Seunarine, H Morgan, N Gadegaard,
likely that major efforts of researchers and materials scientists C D W Wilkinson, M Riehle Langmuir 22 11230 (2006)
who work in the field of design of hydrophobic and super- 41. J Y Shiu, C W Kuo, P Chen, C Y Mou Chem. Mater. 16 561
hydrophobic materials and coatings will be focused on solving (2004)
these problems in the nearest future. 42. M Hikita, K Tanaka, T Nakamura, T Kajiyama, A Takahara
The review is based on the results of studies carried out by Langmuir 21 7299 (2005)
the authors with the financial support from the Russian 43. G Zhang, D Wang, Z-Z Gu, H Mohwald Langmuir 21 9143
Foundation for Basic Research (Project No. 08-08-12130) (2005)
44. M Ferrari, F Ravera, L Liggieri Appl. Phys. Lett. 88 203125
and the State Programme for Support of Leading Scientific
(2006)
Schools (under Contract No. 02.516.11.6081).
45. L B Boinovich, A M Emelyanenko, A M Muzafarov,
A M Myshkovskii, A S Pashinin, A Yu Tsivadze, D I Yarova
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