Recent developments in superhydrophobic surfaces and their

Transcription

Biofouling, 2006; 22(5): 339 – 360
Recent developments in superhydrophobic surfaces
and their relevance to marine fouling: a review
JAN GENZER & KIRILL EFIMENKO
Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, USA
(Received 12 May 2006; accepted 22 August 2006)
Abstract
In this review, a brief synopsis of superhydrophobicity (i.e. extreme non-wettability) and its implications on marine fouling
are presented. A short overview of wettability and recent experimental developments aimed at fabricating superhydrophobic
surfaces by tailoring their chemical nature and physical appearance (i.e. substratum texture) are reviewed. The formation of
responsive/‘‘smart’’ surfaces, which adjust their physico-chemical properties to variations in some outside physical stimulus,
including light, temperature, electric field, or solvent, is also described. Finally, implications of tailoring the surface
chemistry, texture, and responsiveness of surfaces on the design of effective marine fouling coatings are considered and
discussed.
Keywords: Marine fouling, wettability, superhydrophobic surfaces, responsive/‘‘smart’’ surfaces, amphiphilic surfaces
Introduction
Questions frequently asked are ‘‘why is it so difficult
to design an antifouling (AF) surface’’ and ‘‘why cannot the design of an ‘optimal’ AF surface be based on
what is already know about wettability?’’ This is
because there the various surface-modification methods capable of fabricating both non-stick and very
sticky surfaces are well-known. For instance, it is
known that frying pans have to be coated with Teflon
in order to make them non-stick. Gortex (a specific
version of a Teflon-like material) raincoats protect
the wearer during rainy days. The opposite of wettability, namely very wettable surfaces, is also well
known. For example, before painting a house, a
primer is typically applied, which enables facile
application of the final coating layer. In their quest
to develop effective AF coatings, researchers soon
realised that much more was required than applying
a high quality layer of Teflon. Initial insight into this
complex issue can be seen by surveying the partition
of proteins at surfaces (Norde, 1996; Latour, 2004
and references therein). Being composed of hydrophobic cores and hydrophilic coronas, proteins
typically partition relatively readily on both hydrophilic and hydrophobic surfaces. The quantity of
adsorbed protein is regulated by the conditions of
the surrounding solution; it is highest close to the
protein’s isoelectric point, where charges from
neighboring proteins are effectively eliminated.
Proteins can physisorb on hydrophilic surfaces via
attachment of their coronas to the substratum. When
in contact with hydrophobic materials, proteins can
‘‘open up’’ and place their hydrophobic segments
directly on the surface. The latter phenomenon leads
typically to protein denaturation, i.e. adsorption into
some irreversible conformational state, from which
proteins cannot recover readily. This simple example
illustrates that wettability itself, at least at its very
extremes, cannot aid the design of an efficient
protein-repellent surface. Indeed, it has now been
appreciated that it may not be wettability itself,
but rather the structure of water molecules near
the substratum, which may help in the design of
protein-resistant surfaces. Ethylene glycol-based
surfaces represent examples of such materials
(Mrksich & Whitesides, 1996). They can effectively
bind water molecules and prevent intervening proteins from replacing them, hence making them
protein adsorption-resistant.
When extending this simple example (the emphasis being on ‘‘relatively simple’’ as many outstanding
issues still remain in designing effective proteinresistant surfaces) to more complex cases involving
Correspondence: Jan Genzer, Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA.
E-mail: [email protected]
ISSN 0892-7014 print/ISSN 1029-2454 online Ó 2006 Taylor & Francis
DOI: 10.1080/08927010600980223
340
J. Genzer & K. Efimenko
biomaterial adsorption, the complexity of the problem can be immediately appreciated. One of the
issues is the fact that almost any biomass is made of
hydrophobic, hydrophilic, and charged components.
Moreover, these bio-moieties are adaptable (or
‘‘smart’’); they can adjust their state to the adsorbing
medium very rapidly and efficiently. This, obviously,
makes the task of designing an efficient AF surface
very challenging.
In order to conceive an optimal foul resistant
surface, the driving forces that govern the partition
of biomass on man-made surfaces first have to be
identified and controlled. Wettability is a key parameter that needs to be tailored. As will be discussed
below, wettability is intimately related to both
chemical constitution and the physical topology of
surfaces. There are many examples in nature, some
of which are discussed below, where wettability due
to ‘‘chemistry’’ is fine-tuned by additional topology
effects. Another important issue in creating effective
functional AF surfaces is the ability of surfaces to
change their appearance in response to some external
trigger. The ability of surfaces to respond to variations in outside stimuli will depend crucially on
how fast reconstruction events take place on those
surfaces. Clearly, a very complex set of issues
involving various molecular phenomena, which are
mutually intermingled, are involved (Anderson et al.
2003).
The authors are not proposing to solve the lasting
problem of biofouling in this paper. Instead, the
review will highlight recent experimental developments aimed at understanding wettability of materials. Particular emphasis will be on non-wettable
surfaces as they provide informative insight about
how the structure of the surface influences the partitioning of the liquid phase. The experimental findings
will be put in the context of recent theoretical models.
Some recent case studies will also be reviewed, which
are aimed at designing so-called responsive/‘‘smart’’
surfaces, structures that change their characteristics
as a result of some external stimulus, such as light,
temperature or wettability. Finally, some outstanding
issues relevant to the rational design of an effective
AF surface will be outlined.
Wettability ‘‘101’’
Since many excellent reviews dedicated to this topic
have appeared recently (Feng et al. 2002; Blossey,
2003; Callies & Quéré, 2005; Sun et al. 2005a;
Marmur, 2006a; 2006b; 2006c) only a brief account
of some of the outstanding phenomena in the field
will be discussed. Wettability represents a fundamental property of any material; it reveals information about the chemical structure of the material and
its surface topology. However, as will be discussed
below, decoupling these two effects is not always
straightforward (and in some instances nearly impossible) to do.
More than 200 years ago, the English physician
Thomas Young, identified in a recent biography as
‘‘the last man who knew everything’’ (Robinson,
2006), described the forces acting on a liquid droplet
spreading on a surface (cf. Figure 1a). The so-called
contact angle (y) of the drop is related to the interfacial energies acting between the solid-liquid
(gSL), solid-vapor (gSV) and liquid-vapor (gLV) interfaces via:
cos ðyÞ ¼
gSV gSL
gLV
ð1Þ
The expression given by Equation 1 is a clear
oversimplification of the real situation as it is strictly
valid only for surfaces that are atomically smooth,
chemically homogeneous, and those that do not
change their characteristics due to interactions of the
probing liquid with the substratum, or any other
outside force. Any real surface exhibits two contact
angles, so-called advancing (yADV) and receding
(yREC) contact angle. The difference between them,
referred to commonly as the contact angle hysteresis
(CAH), is a measure of the surface ‘‘non-ideality’’
(Gao & McCarthy, 2006c). As will be discussed
below, the CAH is intimately related to the adhesion
of materials on surfaces. Depending on the value of y,
as measured by water, so-called hydrophilic (y 5 908)
surfaces can be distinguished from hydrophobic
(y 4 908) surfaces. Extremes to those two categories
are superhydrophilic and superhydrophobic surfaces.
The latter category is particularly interesting as it
characterises surfaces that are nearly completely nonwettable (typically taken as y 4 1508). Depending on
the level of surface roughness two different regimes
can be distinguished. In the so-called Wenzel regime
(Wenzel, 1936; cf. Figure 1b), the liquid wets the
surface, but the measured contact angle (y*) differs
from the ‘‘true’’ contact angle (y):
cos ðy Þ ¼ R cos ðyÞ ¼ R gSV gSL
:
gLV
ð2Þ
In Equation 2, R is the ratio between the actual
surface area of the rough surface and the projected
(apparent) area. A close inspection of the expression
given by Equation 2 reveals that in this wettability
regime, roughness promotes either wettability
(y 5 908) or non-wettability (y 4 908), depending
on the chemical nature of the substratum. When the
surface is made of small protrusions, which cannot
be filled by the liquid and are thus filled with air, the
Superhydrophobicity and marine fouling
341
Figure 1. Liquid droplet spreading on a flat substratum (a) and rough substrata (b) and (c). Depending on the roughness of the substratum,
the droplet is either in the so-called Wenzel regime (b) or the Cassie-Baxter (c) regime.
wettability enters the so-called Cassie-Baxter regime
(Cassie & Baxter, 1944) (cf. Figure 1c):
cos ðy Þ ¼ 1 þ fs ½cos ðyÞ þ 1
gSV gSL
þ1 :
¼ 1 þ fs
gLV
ð3Þ
In Equation 3, fS is the fraction of the surface that
is in contact with the liquid; the remaining fraction
(1 7 fS) is in contact with air. Note that Equation 3
only applies to cases where the liquid touches just the
top of the surface. If partial penetration of the
grooves occurs, a more complex version of Equation 3
is required. Because the pores are filled with air,
which is hydrophobic, the contact angle always
increases, relative to the behavior seen on a flat
substrate having an identical chemical composition.
Hence surface topography may have a very profound
effect on material wettability.
Wettability on physically smooth surfaces
Before discussing the roughness effect on wettability,
it is appropriate to return to flat surfaces and consider how the information about surface wettability
can be used to evaluate surface energies of materials
by using Equation 1. While the surface tension of
the probing liquid (gLV) is known, information about
the interaction energy at the interface between the
substratum and the probing liquid (gSL) is typically
not available. In situations like this, measurements
using several probing liquids are usually performed,
which invoke one of a few approximations, such as
the geometric mean approximation (GMA). For a
two-liquid case (L1 and L2), the corresponding
expressions read:
cos ðyL1 Þ gdL1V þ gpL2V
j
1=2 p
1=2 k
¼ 2 gdL1V gdSV
þ gL1V gpSV
;
ð4Þ
cos ðyL2 Þ gdL2V þ gpL2V
j
1=2 p
1=2 k
¼ 2 gdL2V gdSV
þ gL2V gpSV
:
ð5Þ
In Equations 4 and 5, gd and gp denote the dispersive
and polar components of the surface energy, respectively. Figure 2 demonstrates the applicability
of the GMA approach in determining the surface
energy of a substratum made by co-depositing
carboxy- and methyl-terminated alkanetiols onto a
gold-coated substratum. The composition of the
resulting self-assembled monolayer (SAM) on gold is
adjusted by varying the relative amount of each
component in the deposition solution. By measuring
the advancing contact angles using deionised water
and diiodomethane (cf. Figure 2a) and invoking the
GMA, the surface energy of the SAM as a function of
the composition could be evaluated (cf. Figure 2b).
It has to be noted that the assumptions behind the
GMA are valid primarily for hydrophobic surfaces;
the surface energies corresponding to the SAMs
having a large content of the hydrophilic component
represent only crude approximations. An additional
limitation is that advancing contact angles are used
(instead of ‘‘true’’ contact angles) in evaluating the
surface energy; this is primarily because of lack of
sufficient information. In spite of these limitations,
the GMA provides useful insight into the surface
energetic of materials. For instance, by exploring the
data in the inset to Figure 2b, it can be seen that while
the value of the dispersive component of the surface
energy remains roughly constant, the polar component contribution increases steadily with increasing
the content of the carboxy-terminated alkanethiol in
the SAM. The GMA is not the only equation of state
used to characterise the surface energies of materials.
For instance, Kwok and Neumann derived an expression relating the contact angle of a solid to gSV and gLV
via (Kwok & Neumann, 2000):
1=2
2
g
ebðg1V gSV Þ ;
ð6Þ
cos ðyÞ ¼ 1 þ 2 SV
gLV
where ¼ 0.0001246 m2 mJ71 is a constant that has
been shown to describe the behavior of a large variety
of materials.
Simultaneous co-deposition of two interfacial
modifiers onto substrata, such as those discussed in
the preceding example, typically leads to surfaces
342
Figure 2. (a) Advancing contact angle of water (circles) and
diiodomethane (squares) measured on self-assembled monolayer
(SAM) made of HS(CH2)15COOH (A) and HS(CH2)17CH3
(B) as a function of the concentration of A in A/B solution. (b) The
surface energy of the SAM as a function of the concentration of
A in A/B solution evaluated from the data given in (a) using the
geometric mean approximation. The inset to (b) depicts the
dispersive and polar components of the surface energy as a
function of the concentration of A in A/B solution. The solution
comprised 1 mM of the alkanethiols in tetrahydrofuran.
whose contact angle (ymix) can be described by the
classical Cassie-Baxter equation:
cos ðymix Þ ¼ f1 cos ðy1 Þ þ f2 cos ðy2 Þ;
ð7Þ
where f1 and f2 (¼17f1) are the fractions of the
surface having contact angles y1 and y2, respectively.
The assumption behind Equation 7 is that mixing
two chemically distinct moieties leads to homogeneous surfaces with no in-plane phase separation.
There are instances, however, which require surfaces
with chemically distinct regions (and hence wettabilities) having well-defined spatial dimensions and
distributions present simultaneously on the same
sample. Over the past few years several methodologies have been developed that facilitate the formation
of substrata comprising various wettablity patterns
with lateral dimensions ranging from hundreds of
nanometers to several micrometers. Among the most
widely-practiced techniques are those, which are
based on so-called ‘‘Soft lithography’’ (Xia &
Whitesides, 1998a; 1998b; Xia et al. 1999;
Whitesides & Love, 2001 and references therein).
Applications which utilise such chemically patterned
substrata range from open-air microfluidic channels
(Gau et al. 1999; Gao & McCarthy, 2006b) to mimicking functions of biological species, such as the
Namib Desert Beetle (Zhai et al. 2006 and references
therein). Finally, for some other applications, it is
desirable that wettability changes gradually and
continuously across the substratum. This can be
accomplished by producing surfaces with a positiondependent and gradually varying chemistry (Ruardy
et al. 1997; Genzer 2002; 2005, and references
therein; Genzer et al. 2003). Such wettability gradients have been successfully used to direct motion
of liquid drops (Chaudhury & Whitesides, 1992),
create gradient nanoparticle assemblies (Bhat et al.
2002), and serve as templates for polymerisation
(Wu et al. 2002; Bhat et al. 2006a, and references
therein; 2006b, and references therein).
The previous examples illustrate that surface wettability can be tuned by judiciously choosing the
chemical nature of surfaces and the spatial distribution of various chemistries used. Non-wettable
surfaces will be made typically by close-packing
molecules that possess relatively low surface energies.
The moieties of interest include primarily methylated and fluorinated carbons, whose surface energy
decreases in the following manner: 7CH2 4
CH34
CF2 4CF2H 4
CF3. Hence flat surfaces with the lowest surface energy should be created
by close-packing trifluoromethyl groups. Hare et al.
(1954) predicted that such surfaces should have a
surface energy of 6.0 mJ m72. Nishino and coworkers recently reported on creating surfaces
comprising hexagonally packed 7CF3 groups via
vapor phase epitaxial growth (Nishino et al. 1999).
They reported contact angles of 1198 corresponding
to the surface energy of 6.7 mJ m72. Many other
strategies for creating hydrophobic surfaces containing fluorinated moieties exist and some of these have
been reviewed recently (Nakajima et al. 2001).
The wettability of surfaces depends crucially not
only on the chemical composition of the chemical
modifiers but also on their packing on the surface.
Typical SAMs possess a variety of structural defects;
hence, achieving close-packing over large surface
areas is very difficult. Genzer and Efimenko (2000)
showed that this limitation can be removed by
depositing those modifiers onto flexible substrata
and deforming the substrata mechanically. This can
be achieved by stretching the substratum, depositing
the molecules, for instance, in the form of reactive
organosilanes, and releasing the strain from the
substratum. A schematic illustrating the procedure is
depicted in Figure 3. In their paper, Genzer and
Efimenko (2000) demonstrated that by assembling
fluorinated organosilanes the wettability of the resulting surfaces reached values as high as y 1308 for
water.
The aforementioned examples reveal that there is an
upper limit of non-wettability that can be achieved on
flat substrata (y 1308). In order to increase the
substratum non-wettability beyond this limit, a trick
has to be resorted to, based on combining the
‘‘chemical wettability’’ with the effect of the substratum topography, which has a very profound effect on
liquid spreading on such surfaces, as discussed earlier.
The next section will demonstrate that surfaces with
water contact angle 41708 can be generated by
including surface roughness into the picture.
Wettability on physically rough surfaces
Nature offers a diverse wealth of inspiration not only
for artists, but also for scientists and engineers. The
beauty of plants and animals is clearly associated not
only with their visual appearance but also with their
functionality. In many instances the desired functionality is achieved by tailoring the chemistry and
texture on both living and non-living objects. Take,
for instance, the moth’s eye (cf. Figure 4a). Its
unique structure comprising hexagonally organised
microscopic pillars, each 200 nm in height, facilitates very low reflectance for visible light. As a result,
the eye works like a ‘‘black hole’’, i.e. it absorbs
almost all light arriving from nearly any direction. It
may thus be considered as a perfect antireflective
structure. Another example involves one of many
kinds of colourful butterflies (cf. Figure 4b) where
not and all colouring of the wings stems from
pigments. While red and yellow colors typically
result from some color pigments, blue and green
shades originate primarily from light scattered off the
rather complex hierarchically organised scales.
Hence, it is the combination of coloring pigments
and the sizes and spatial arrangement of the scales
(and their ribs), which endow butterflies with their
inherent visual beauty and also hidden functionality.
Reptiles will now be considered briefly. Geckos
can climb steep smooth walls and ceilings very
swiftly. This ability stems from the unique structure
of their feet, which are made of hundreds of
thousands of hairs (‘‘setae’’) decorated with hundreds of submicron-sized pads (‘‘spatelae’’) positioned at each seta tip (cf. Figure 4c). The feet adhere
to the walls via simple van der Waals forces under a
vacuum; the fast locomotion of geckos is facilitated
by the special design of their feet, making them stick
rapidly to a surface and releasing their grasp in a
fraction of a second (Autumn, 2006).
343
In the previous example, a situation was considered where roughness can promote adhesion. Yet,
there are numerous design examples in nature that
facilitate quite the opposite and which are clearly
more relevant to the topic of this review. For instance, the skin of many kinds of fish or reptiles
appears to be clean of any contamination. Many
insects also benefit from the design of their wings and
legs that make them non-wettable. Using this phenomenon it is possible to explain why water striders can
stroll on water (cf. Figure 4d). The unique hierarchical structure of their microsetae along with hydrophobicity (fraction of air ¼ 97%) can support the
weight of the insect; a single leg can support 152
dynes (¼ 1.52 milli-Newtons), which is 15-times the
entire water strider’s body weight.
Lastly, what is perhaps the most well-known
example of water repellency in nature, namely the
leaves of certain plants, will be considered (Neinhuis
& Barthlott, 1997). Plants are quite diverse in their
wettability characteristics. Hence, while floating
leaves are wax-free and wettable, leaves emerging
from the water surface (or growing on land) are
water-repellent. Plants are capable (at least partially)
of regenerating destroyed waxes due, for example, to
rain or mechanical abrasion. But it may take days for
this to happen, so the question arises: Why is water
repellency so important to these plants? One of the
most important reasons for the existence of water
repellent surfaces is that it provides protection against
pathogens supplied by free water, such as bacteria or
fungal spores. Therefore water removal minimises the
chances of infection. In addition, dust particle
removal from leaf surfaces minimises the changes
of, for example, the plant overheating or salt injury.
One of the most studied examples involves the
lotus leaf (Nelumbo nucifeara). Its water repellency
(y 4 1508) stems from a unique surface texture,
which comprises convex microstructures (papillose
epidermal cells) immersed in a sea of a dense layer of
epicuticular waxes. As in other water-repellent
plants, contaminating particles are carried away from
the surface of the lotus leaf by water droplets (cf.
Figure 5). In order for this cleaning procedure to
take place, the adhesion of dirt particulates to the
surface of the leaf has to be smaller than that to the
traversing water droplet. While this phenomenon of
self-cleaning, termed the Lotus effect, has been the
subject of considerable scientific interest over the
past two decades, it represents nothing new to many
Asian cultures, which have been aware of it for
centuries and for this reason considered the lotus leaf
as a symbol of purity.
The few case studies from flora and fauna
discussed above provide examples of the role surface
texture plays in affecting the wettability of materials.
They demonstrate how important roughness is for
344
Figure 3. (a) Schematic illustrating the technological steps leading to the production of mechanically assembled monolayers (MAMs).
A pristine poly(dimethyl siloxane) (PDMS) network is cast into thin ( 0.5 mm) films and subsequently stretched. The stretched
substratum is then exposed to a UV/ozone beam to produce the surface hydrophilic 7OH groups. The chlorosilane molecules are deposited
from the vapour phase on this stretched substratum and form an organised assembled monolayer. Finally, the strain is released from the
modified PDMS substratum, which returns to its original size, causing the grafted molecules to form a densely packed MAM. The lower
panel shows photographs of a water droplet spreading on each of the substrata. (b) Schematic representation of molecular packing in SAMs
(upper) and MAMs. (c) Water droplet spreading on top of PDMS covered with SAMs made of fluorinated chlorosilanes (left) and MAMs
packing the same chemical moieties (right).
Figure 4. (a) SEM image of a moth’s eye (reproduced with permission from Syncroscopy). (b) Butterfly wings are composed of hundreds of
thousands of scales with complex hierarchical structures. The scale details are shown with different levels of magnification (reproduced with
permission from Professor Tina Carvalho). (c) Details of a gecko’s feet (reproduced with permission from Professor Kellar Autumn).
(d) Water strider (Gerris remigis) walking on the surface of a lake (reproduced with permission from Andy Purviance). (e) Close-up of a lotus
leaf (Nelumbo nucifeara), an example of a super-hydrophobic plant. The roughness of the leaf surface results from the coexistence of micronsized bumps and nanoscale hair-like structures (reproduced with permission from Professor Wilhelm Bartholott).
tailoring the wettability/non-wettability of materials
and hence provides inspiration to scientists and
engineers in their quest for designing artificial
non-wettable surfaces. Earlier in the text the two
existing theories of wettability on physically rough
surfaces were discussed. Numerous scientific papers
345
Figure 5. (a) Water droplet rolling on a lotus leaf (reproduced with permission from Zoltan G. Levay). (b) Drop of water rolling off a dirty
tissue with the lotus effect. As the drops fall, the dirt is washed off (reproduced with permission from ITV Denkendorf, Germany). (c) and
(d) Schematic depicting the motion of a liquid droplet on an inclined substratum covered with ‘‘dirt’’. When moving on a flat substratum,
where the adhesion between the ‘‘dirt’’ particles and the substratum is high, the droplet passes through. A different situation occurs on a
substratum that is topographically decorated, where the ‘‘dirt’’ particles have difficulty adhering to it. As the liquid droplet rolls off the
substratum it picks up the ‘‘dirt’’ particles and hence cleans the substratum.
have been dedicated to this interesting and important
topic. About a decade ago, scientists from the Kao
Corporation in Japan demonstrated that complete
control over wettability (from super-wettable to
completely non-wettable) can be achieved by adjusting the fractal dimension of their substrata made of
alkylketene dimers (Onda et al. 1996; Shibuichi et al.
1996). Of particular interest is to understand which
regime describes most accurately liquid wetting in
the non-wettability regime. Yoshimitsu et al. (2002)
studied water wettability on surfaces comprising
pillars with different aspect ratios decorated with a
thin layer of a semifluorinated surfactant. By varying
the aspect ratio of the pillars, Yoshimitsu et al. were
able to tailor the surface area of the pillars and hence
the parameter R, defined earlier in Equation 2. They
discovered that at low aspect ratios the wettability
could be described by the Wenzel model; with
increasing R there was a transition from the Wenzel
to Cassie wettabilities. He et al. (2003), Patankar
(2003) and Marmur (2003; 2004) provided independently, theoretical insight into understanding
wettability on such surfaces by assuming an array
of pillars with height h and surface made of squares,
each having a side length a and spaced at a distance
of from one another b. By evaluating the wettability
using the Wenzel and Cassie models, they established the conditions for the existence of the Wenzel
and Cassie regimes in terms of a, b, and h. These
studies revealed that in order to achieve non-wettable
surfaces (Cassie regime), it is necessary to construct
a surface from slender and sparsely spaced pillars
(minimal a/H, maximal b/a).
The wettability in the Wenzel and Cassie regimes
was studied in detail by Bico et al. (1999; 2001),
Quéré et al. (2003), Lafuma and Quéré (2003) and
Callies and Quéré (2005). They showed that there is
a critical value of fS, the fraction of the surface that is
in contact with the liquid, below which the Cassie
regime exists and above which the Wenzel regime is
thermodynamically more stable. The corresponding
transition occurs at a certain critical wetting angle
(yc) defined by:
cos ðyc Þ ¼
fs 1
:
R fs
ð8Þ
Hence at contact angles larger than yc air pockets
should be present beneath the drop, which, in turn,
exists in the Cassie regime. Quéré and coworkers
also discussed the stability/metastability of the two
regimes. They showed that metastable Cassie drops
may form on surfaces, which thermodynamically
prefer the Wenzel regime. The metastability was
demonstrated in several ways. For instance, by
applying small pressure on the metastable Cassie
droplet, the droplet slipped to the stable Wenzel
regime (cf. Figure 6c). Similarly, a Cassie droplet
receded into a Wenzel droplet by allowing a small
amount of the liquid to evaporate (cf. Figure 6d).
Finally, Quéré and coworkers showed that the state
of the droplet depended on the amount of liquid
346
Figure 6. (a) Shapes of 1 mg water droplets spreading on pillar structures with varying roughness factor (R). The corresponding contact
angles (y*) and roughness factors (R) are: (from left to right): 1148/1.0, 1388/1.1, 1558/1.2, 1518/2.0, and 1538/3.1 (reproduced with
permission from Yoshimitsu et al. 2002). (b) A diagram depicting the transition between the Cassie and Wenzel wetting regimes; the
dividing factor between the two regimes is the fraction of the surface that remains in contact with the liquid (fS), which defines the ‘‘critical’’
contact angle, yc. (redrawn from Lafuma & Quéré, 2003). (c) and (e) Water droplet deposited on a surface in Cassie (left drop) and Wenzel
(right drop) regimes. The transition from the Cassie to Wenzel regimes is induced by: (c) applying a small pressure, (d) evaporating some
liquid, and (e) adjusting the volume of the drop (reproduced with permission from Callies & Quéré, 2005).
(cf. Figure 6e) as well as the means of depositing the
liquid on the surface. For instance, when the liquid
was deposited on the surface at once, it formed a
Cassie-like droplet. In contrast, when the liquid was
delivered in the form of a mist, it wetted the surface
instantly as a Wenzel droplet.
A particularly appealing demonstration of generating superhydrophobic surfaces was presented in a
series of papers by Li et al. (2001; 2002) and Feng
et al. (2003) (cf. Figure 7). Surfaces decorated with
vertically standing carbon nanotubes (CNTs) exhibited very high water contact angles. Furthermore,
fluorinating the CNTs led to surfaces having contact
angles 41708. Theses workers also established that
surfaces comprising sparsely spaced polyacrylonitrile needles possessed contact angles 41738. These
studies confirmed the predictions of the wettability
theories in that they demonstrated that nonwettability can be achieved by: (i) decreasing the
area of the substrate that is in contact with the
liquid, and (ii) by hydrophobicizing the substratum
by fluorination.
In the recent literature there are other examples
of creating superhydrophobic surfaces that rely on
manipulating the texture of the solid. These involve
fluorinating plasma-modified polyethylene terephthalate plastic sheets (Teshima et al. 2003), creating
surface clusters via deposition gold on of structured
polyelectrolyte multilayers (Zhang et al. 2004),
fabricating surfaces from plastic micro-sized fibers
(Jiang et al. 2004), solvent casting poly(methyl
methacrylate)/fluorinated polyurethane films (Xie
et al. 2004), or templating engineering plastics, such
as polycarbonate (Guo et al. 2004) or poly(tetrafluoroethylene) (Feng et al. 2004). Non-wettability
can be pushed to its limit as discussed a very recent
report by Gao and McCarthy (2006e), in which an
extreme Cassie regime was documented by reporting
yADV ¼ yREC ¼ 1808. A very appealing method of
creating efficient superhydrophobic surfaces has
been developed by Erbil et al. (2003), who showed
that gel-like porous coatings with water contact
angles as high as 1608 can be fabricated from cheap
polypropylene plastics by tailoring the type of solvent and the deposition temperature, which, in turn,
govern the roughness of the resultant coating.
An issue closely related to the wetting regime
involves adhesion between the liquid and the substratum. While in the Cassie regime, the adhesion is
small and the drop can easily be separated from the
substratum, Wenzel droplets adhere to the substratum more strongly. A first glimpse at this behaviour
can be obtained by exploring the contact angle hysteresis (CAH). CAH is high for the Wenzel regime
and low for the Cassie regime, as demonstrated
experimentally by Bico et al. (1999). Surfaces with
347
Figure 7. (a) Cross-sectional view of aligned carbon nanotubes (CNT) on surfaces. (b) Water droplet on CNT surface. (c) Water droplet on
fluorinated CNT surface (reproduced with permission from Li et al. 2001). (d) Cross-sectional view of aligned polyacrylonitrile (PAN)
nanofibers on surfaces. (e) Water droplet resting on the PAN nanofiber surface (reproduced with permission from Li et al. 2002).
different topographies may have the same area at the
solid/air interface but a different so-called contact
line (cf. Figure 8a and 8b) and hence a different
contribution of the so-called line tension (Buehrle
et al. 2002; de Gennes et al. 2003).
This phenomenon is not captured by either the
Cassie or Wenzel wettability laws. Substrata that
have continuous contact with the liquid may enter
one of many metastable states and hence contribute
to a lower receding contact angle. In contrast,
surfaces made of isolated posts do not provide
continuous contact between the liquid and the substratum; the wettability on such surfaces would
be much closer to the ‘‘true’’ thermodynamic state
characterised by yADV ¼ yREC. One way to examine
the adhesion of the liquid to the substratum is to
evaluate to so-called roll-off (or sliding) angle, a
(Chen et al. 1999):
m g sin ðaÞ
gLV ½cos ðyREC Þ cos ðyADV Þ;
rdrop
ð9Þ
where rdrop is the radius of the drop. The roll-off
angle can be evaluated (very approximately) by
equating the gravitational force acting on a droplet
on an include place to the adhesion, which is
assumed to be roughly proportional to the difference
between cosines of the receding and advancing
contact angle (cf. Equation 9 and Figure 8c). While
recent theoretical study discussed that the use of
advancing and receding contact angles in Equation 9
may not be applicable (Krasovitski & Marmur,
2005), this expression is still used as a good starting
point for discussion. In order to fully appreciate
the effect of the CAH on adhesion, in Figure 8d
estimates are provided of the roll-off angle for a water
drop having a volume of 30 ml as a function of the
cosines of the advancing and receding contact angles.
This result illustrates that increasing the difference
between the advancing and receding contact angle
raises the roll-off angle, which, in turn, increases the
adhesion between the liquid and the substratum.
This CAH-induced adhesion is a very commonplace
phenomenon. For instance, during a rainy day some
water droplets remain frequently deposited on the
window. The explanation is that even though the
glass is positioned vertically (i.e. its sliding angle is
908), it is the contact angle hysteresis (yREC yADV),
probably caused by impurities in or on the glass (e.g.
dust particles) or local physical heterogeneities,
which causes small water droplets to adhere effectively to the window surface.
The question may also be asked, how does the
spatial distribution of the texture motif affect the
ability of the drop to roll off the substratum. Again,
nature helps answer this question. For instance, grass
leaves are not very smooth, typically containing lines
of protuberances (veins), which enable the drops of
water to slide down to the ground more easily (cf.
Figure 9a and 9b). Several research groups have
attempted to mimic this by creating surfaces that
possess spatially anisotropic wettability (cf. Figure 9c
and 9d). In particular, Yoshimitsu et al. (2002)
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Figure 8. (a) Schematic representation depicting liquid spreading on two surfaces having the same fraction of the solid phase (honeycomb
and pillar shapes) but very different contact lines. (b) Optical micrograph of a contact line measured on a substratum covered with
honeycomb-like barriers (reproduced with permission from Öner & McCarthy, 2000). (c) Schematic depicting the shape of liquid drops on
hydrophobic (left panel) and hydrophilic (right panel) substrata before (upper panel) and after (bottom panel) tilting. (d) Roll-off angle as a
function of the advancing and receding contact angles (yADV and yREC, respectively) calculated for a droplet having 30 ml volume.
provided evidence that on substrata comprising lines
of pillars, water runs off more easily when moving in
the direction parallel to the pillars, relative to the
orthogonal direction.
Physical roughness effects considered thus far
involve primarily one dominant length scale. Yet,
there are instances, where multiple length scales of
roughness are present and act in concert. Returning
to the example of the lotus leaf, a close examination
of the surface of the plant reveals multiple length
scales of roughness ranging from nano- to micrometers. In a recent publication, Cheng et al. (2006)
addressed the role of the multiple roughness features
on the water repellency of the Lotus leaf and showed
that scales of both lengths are important for the
Lotus leaf effect to be efficient. By baking away the
nanosised hairs on the leaf, Cheng and coworkers
showed that the inherent ability of the Lotus leaf to
self-clean was substantially reduced. The importance
of multiple length scales in the ability to repel water
from lotus leaf-like structures has very recently been
confirmed by Gao and McCarthy (2006a; 2006d).
The effect of multiple roughness length scales on
wettability is not a completely new phenomenon.
Botanists have long been aware of this (Neinhuis &
Barthlott, 1997) and even theoretical models discussing the effect of multiple levels of roughness on
wettability have been presented (Herminghaus,
2000). It still remains to be seen what role the
hairs in certain plants play. Is it just the combination of multiple roughness levels that affects plant
wettabilities, or, as recently proposed (Otten &
Herminghaus, 2004), does the elasticity of the hairs
play also some role in keeping plants clean? The
exact answers to such questions are not yet known.
While some methodologies exist that facilitate the
creation of surfaces with complex multiscale roughness (Efimenko et al. 2005), tailoring the roughness
features independently on all length scales in order to
achieve the best desired effect is still in its infancy.
Hence one possible way to make an efficient superhydrophobic surface is to rely on nature to provide
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Figure 9. (a) Water drops sliding down grass (reproduced with permission from Saskia). SEM image of (b) a rice leaf (Oryza sativa) and (c) a
surface made of aligned carbon nanotubes (reproduced with permission from Feng et al. 2002). (d) SEM micrographs of a one-dimensional
groove structure (left) and pillar-like structure (right) with the same structural dimensions as the one-dimensional groove structure.
Schematic illustration of the measurement direction for the groove structure (below). (e) Dependence of sliding angles on the weights of the
water droplets in parallel direction (&, y ¼ 1358), orthogonal direction (~, y ¼ 1178) on the one-dimensional groove structure, and on the
pillar-covered structure (., y ¼ 1398) (reproduced with permission from Yoshimitsu et al. 2002).
effective templates, which can then be imprinted
using standard molding technologies into almost any
material (Fürstner et al. 2005; Sun et al. 2005b).
Thus far, only surfaces whose wetting properties
are fixed have been considered. In some situations,
a material surface that behaves like a chameleon
may be beneficial, i.e. it adjusts its surface characteristics in response to some external stimulus.
Such surfaces are generally termed responsive (or
‘‘smart’’).
Wettability on responsive surfaces
A responsive/‘‘smart’’ surface changes its physicochemical surface characteristics in response to, for
example, a chemical, electrical or mechanical external stimulus. A detailed discussion of this topic is
outside the scope of this review, so the phenomenon
will be illustrated by a few selected examples of
changing wettability ‘‘on demand’’. A more comprehensive treatment can be found in recent reviews
(Galaev & Mattiasson, 1999; Russell, 2002; Luzinov
et al. 2004) and a monograph dedicated specifically
to this topic (Minko, 2005).
The first case involves wettability changes induced
by light. An example is the well-studied lightinduced cis/trans isomerization of azobenzene. The
ability to change wettability using azobenzene-based
derivatives (specifically, calixresorcinarene modified
with four azobenzene chains) in controlling wettability of materials was used by Ichimura et al.
(2000) in creating real-time wettability spatial patterns on flat surfaces. By irradiating surfaces covered
with self-assembled monolayers of the aforementioned moiety with ultraviolet (UV) light, the
surfaces became wettable. By exposing the surface to
blue light, the azobenzene flipped back to the trans
conformation thus making the substratum less
wettable. By asymmetrically irradiating the substratum with UV and blue light, a wettability gradient
was generated that was capable of moving liquids
along the substratum. However, the rather high
contact angle hysteresis present in the system did not
enable motion of polar liquids; only hydrophobic
liquids, such as olive oil, were transported. Similar
UV-induced wettability changes have been seen in
oxides of certain transition metals, such as TiO2
and ZnO. Wang et al. (1997) reported that when
350
irradiated with UV light, the originally hydrophobic
TiO2 turned hydrophilic. After removing the UV
source, the initial hydrophobicity was recovered.
This work was followed by other studies (Sun et al.
2001; Liu et al. 2004; Wu et al. 2005), which
confirmed the same phenomenon also for ZnO. The
mechanism behind this peculiar behaviour can be
explained in terms of the effect of the UV light on the
sub-surface structure of the oxide. UV illumination
generates electrons, which travel to the surface and
react with adsorbed oxygen molecules. Water molecules then coordinate readily into surface oxygen
vacancies leading to dissociative adsorption of additional water molecules. Sun et al. (2001) reported
that the rate at which the surface renders wettable
increases with increasing UV power (cf. Figure 10).
The process is completely reversible and the material
becomes hydrophobic after removing the UV radiation. This phenomenon of UV-induced variation
in surface structure of oxides has already been
utilised in the fabrication of so-called ‘‘self-cleaning
windows’’, manufactured by Pilkington (‘‘Activ
glass’’) and PPG Industries (‘‘SunClean glass’’). In
the presence of sunlight, nanometer-sized TiO2
particles, incorporated into the glass during the
manufacturing process, act as photocatalysts and
at the same time increase wettability. The photocatalytic process helps to break down organic
material deposited on the surface and the increased
wettability improves removal of the loosened
particulates by water running down readily in
waves.
Temperature can also be used to regulate the
wettability of materials. For example, when oil is
poured into a frying pan and heated to a higher
temperature, the oil forms less wetting droplets. This
is relevant to a certain class of polymeric systems that
exhibit a so-called lower critical solution temperature
(LCST) behavior in aqueous solutions. At low
temperatures the polymers are hydrophilic and
dissolve readily in water. Upon raising the temperature, the polymer solubility decreases and finally at a
certain temperature, called the critical temperature
(TC), the polymers collapse and turn more hydrophobic. The temperature, at which this so-called
coil-to-globule transition occurs, varies from polymer to polymer. Probably the most current studied
system that exhibits this behavior is poly(N-isopropyl
acylamide) (PNIPAAm) (Schild, 1992). The
temperature-induced wettability variation has been
confirmed by several groups on systems involving
surface-grafted PNIPAAm. In a recent paper from
the authors’ laboratory, some studies are summarised that demonstrate the coil-to-globule transition in surface-grafted PNIPAAm is affected by the
presence of salt (Jhon et al. 2006). The latter effect
may be particularly important when considering
application of PNIPAAm-based smart coatings in
salty waters (e.g. ocean waters). Several groups have
combined the surface-responsiveness of PNIPAAm
Figure 10. (Top panel) Water droplet spreading on top of ZnO surface before (left) and after (right) illumination with ultraviolet (UV) light.
Bottom panel) Water contact angle on ZnO (left) and TiO2 (middle) surfaces as a function of the illumination time having different
intensities (.: 0.1, : 2.0, and 4: 50 mW cm72) and the recovery to hydrophobicity for both surface as a function of time in the dark (right)
(reproduced with permission from Sun et al. 2001).
with the ability of physically rough surfaces to
magnify the wetting effects (cf. the Wenzel regime
discussed earlier) in creating coatings capable of
switching reversibly from superhydrophilic to superhydrophobic states (Fu et al. 2004; Sun et al. 2004a).
As demonstrated by the data in Figure 11, the resultant surface can change wettability reversibly over
extremely large range with almost no hysteresis (Sun
et al. 2004a).
An electric field represents another external force
that can be used to alter the performance of surfaces.
351
Lahann et al. (2003) used an electric field for dynamical control of conformation of alkane molecules
attached to solid supports. By applying a positive (or
negative) potential on the substratum Lahann et al.
(2003) were able to attract (or repel) the carboxylated terminus of the alkane molecules towards (or
away from) the substratum, which, in turn, altered
the alkane conformation from gauche to trans states
casing variations in wettability. Based on the seminal
work of Lippmann (1875) and Vallet et al. (1999)
it is known that wettability of materials can be
Figure 11. Thermally responsive wettability for a flat PNIPAAm-modified surface. (a) Change of water drop profile of PNIPAAm polymer
brushes on flat substratum upon elevating the temperature from 258C (left, y ¼ 63.58) to 408C (right, y ¼ 93.28). (b) Diagram of reversible
formation of intermolecular hydrogen bonding between PNIPAAm chains and water molecules (left) and intramolecular hydrogen bonding
between C¼O and N
H groups in PNIPAAm chains (right) below and above the LCST. (c) Wettability of PNIPAAm on rough surfaces
having groove spacing D low temperature (~, 258C) and at high temperature (&, 408C). The groove spacing of ? represents flat substrata.
(d) Water drop profile for thermally responsive switching between superhydrophilicity and superhydrophobicity of a PNIPAAm-modified
rough surface with groove spacing of about 6 mm, at 258C and 408C (reproduced with permission from Sun et al. 2004a).
352
enhanced by applying an electric field between the
liquid and the substratum. For example, Krupenkin
et al. (2004) used electrowetting on rough surfaces to
generate responsive surfaces capable of changing the
wettability in real time from completely non-wettable
to completely wettable using relatively small voltages.
This work illustrates how by judiciously combining
the interplay between the chemical wettability and
the effect of substratum geometry the wettability of
materials can be varied repetitively.
Finally, a brief consideration of wettability-induced
non-wetting. While in a typical case the presence of
polar liquids, such as water, promotes the segregation
of hydrophilic groups to material surfaces, as demonstrated by a series of papers (see e.g. Carey &
Ferguson, 1996; Koberstein, 2004; Crowe & Genzer,
2005). Makal and Wynne (2005) designed a surface
that becomes more hydrophobic when exposed to
water. Their ‘‘contraphilic’’ material comprises a
polyurethane block decorated with semifluorinated
and 5,5-dimethyhydantoin groups. In the dry state,
the inter- and intra-molecular hydrogen bond interactions dominate. However, exposing the material to
water induces the amide-water hydrogen bonding,
which, in turn, releases the semifluorinated groups
that travel to the surface and render it hydrophobic.
So, the presence of water can induce non-wettability;
the surface is ‘‘smart’’ and ‘‘schizophrenic’’ at the
same time. While it still remains to be seen how useful
this type of coating is for the design of efficient AF
surfaces, there are clearly many other applications in
the area for example of microfluidic devices and
switches.
Implications to biofouling
After considering a variety of effects influencing
wettability of materials, it is appropriate to return to
the original question: Can superhydrophobic surfaces reduce bioadhesion? The answer is not clear.
Baier (e.g. 1970; 1972) published a series of papers,
in which he indicated that the amount of bioadhesion
does not correlate well with the surface energy of the
substratum. He reported that there was a window
in surface energies (20 – 30 mJ m72), within which
adhesion was minimal. Substrata having surface
energies below 20 and above 30 mJ m72 exhibited
appreciable amounts of adsorbed biomass. Baier’s
findings have been confirmed by other groups (see
e.g. Dexter et al. 1975; Dexter, 1979; Schrader,
1982). There are some indications, however, that
point to direct correlations between the surface
chemistry and adhesivity of biomolecules.
Absolom et al. (1983) conducted a series of experiments aimed at uncovering the thermodynamic
driving forces governing the adsorption of bacteria
on surfaces. They reported that adhesion of bacteria
on solid substrata was dictated by the interplay
between the surface energies of the three phases
involved, namely, the surface energy of the bacteria,
the surface energy of the substratum, and the surface
tension of the suspending liquid. Absolom et al.
(1983) stated that bacteria adsorbed readily to
hydrophilic substrata when the bacterial surface
energy was larger than that of the suspending liquid.
In contrast, increasing the surface energy of the surrounding liquid beyond that of the bacteria caused
the bacteria to adsorb preferentially onto hydrophobic surfaces. However, as pointed out by Fletcher
and Pringle (1985), the situation is more complex. In
particular, these authors stressed the importance of
the liquid medium on attachment of bacteria to
surfaces. Specifically, they argued that the actual
interfacial energy contribution due to the liquid
medium is greatly affected not only by the interaction
of the liquid with the surface but also by the presence
of any dissolved macromolecules which adsorb on
surfaces, and surface active agents that themselves
influence the surface tension of the liquid and hence
the thermodynamics of adhesion. Whilst in general,
adhesion of bacteria involves, at least in part,
hydrophobic interactions (a water exclusion mechanism), additional interactions, such as van der Waals
forces, hydrogen binding and electrostatic interactions may also mediate partitioning of bacteria
on surfaces (Sorongon et al. 1991). Wiencek and
Fletcher (1995) carried out a systematic study
utilizing SAMs with a variable composition of
hydroxy- and methyl-terminated alkanethiols on the
adhesion of estuarine bacteria. They reported that
the number of attached cells and the slowest cell
detachment increased steadily with increasing substratum hydrophobicity (cf. Figure 12a). They also
attempted to provide molecular insight into the
relative effects of adsorption and desorption on net
bacterial cell adhesion and concluded that desorption, rather than adsorption, determined the net
number of attached cells at any given time. The
study of Wiencek and Fletcher (1995) provided
evidence that substratum wettability affects the
number of cells that attached reversibly of irreversibly but it had no effect on the residence time
required for development of irreversible adhesion.
The observed decrease in cell detachment on more
hydrophobic surfaces was attributed to the increased
degree of irreversible adhesion on the SAM surfaces.
Wiencek and Fletcher (1995) also speculated that
water molecules preferentially attached to surfaces
with a higher degree of hydrophilicity, which in turn
acted as a barrier for irreversible adhesion of cells.
As discussed later by Ista et al. (1996), the situation
is quite complex and they reported on the adsorption of Cobetia marina (syn. Deleya marina) and
Staphylococcus epidermis on SAMs made of
353
Figure 12. (a) Number of cells attached to surfaces comprising mixed SAMs made of CH3 and OH terminated thiols. The number of
cells attached after 2 h increases with increasing hydrophobicity of the substratum (reproduced with permission from Wiencek & Fletcher,
1995). Attachment of (b) Staphylococcus epidermis and (c) Deleya marina to SAMs formed from thiol-based SAMs with different terminal
groups including CF3, CH3, (CH2 CH2 O)6, and COOH. While S. epidermis adsorbs primarily on the carboxy-terminated SAMs,
D. marina attaches preferentially to the hydrophobic (trifluoromethyl- and methyl-terminated) SAMs (reproduced with permission from Ista
et al. 1996). Attachment of (d) Cobetia marina and (e) Ulva linza zoospores to mixed SAMs made by co-depositing either CH3/
OH or
CH3/
COOH terminated thiols. In both cases, the number of attached cells increases with increasing the substratum hydrophobicity.
C. marina shows similar behavior on both types of surfaces, however, U. linza zoospores appear to respond differently to OH vs COOH
termini present in the SAM (reproduced with permission from Ista et al. 2004).
methyl-, carboxy-, and PEG-terminated moieties
(cf. Figure 12b and 12c).
Ista et al. (1996) observed that while the quantity of
C. marina adsorbed was highest on the methylterminated SAMs, S. epidermis attached preferentially
to carboxy-terminated SAMs. In both cases, no
attachment was observed on PEG-terminated SAM
surfaces. Substrata comprising mixed SAMs made of
co-depositing methyl- and hydroxyl-terminated thiols
have been tested against zoospores of the green
macrofouling alga Ulva (syn. Enteromorpha) (Callow
et al. 2000). Positive correlation between the number
of spores that attached to the SAM surface and the
substratum hydrophobicity was reported. Ista et al.
(2004) also reported on an experiments aiming at
systematically exploring the attachment of C. marina
and Ulva linza to SAMs made by mixing alkanethiols
with hydrophobic and different hydrophilic (hydroxylvs carboxyl-) tail groups. Whilst C. marina attached
with an increased abundance to SAMs having a
higher content of the hydrophobic (methyl-terminated) alkanethiol (cf. Figure 12d), attachment of
Ulva spores was found to be more complex (cf. Figure
12e). Specifically, these researchers reported that
while Ulva spores also attached in higher numbers on
hydrophobic surfaces, the density of adhered zoospores on hydroxylvs. carboxyl- terminated surfaces
was different, suggesting a direct effect between the
physico-chemical properties of the surface and the
glycoprotein adhesive secreted by the spores on
settlement that facilitates adhesion to the surface
(see Callow & Callow, 2006).
Some work has also been done on exploring
marine fouling on surfaces decorated with responsive
materials. In particular, Ista and López (1998) and
Ista et al. (1999) published a series of papers reporting on the utilisation of PNIPAAm in preventing
biofouling. Ista et al. (1999) described experiments
354
aimed at studying the adsorption of C. marina (syn.
Halomonas marina) on PNIPAAm surfaces. They
observed that while the organism attached when the
PNIPAAm-covered substrata were in their hydrophobic state (above its TC) it desorbed when
PNIPAAm transitioned to a more hydrated state.
Concurrently, C. marina was also seen to attach in
great quantities to hydrophilic PNIPAAm (below its
TC) but was removed when the opposite transition
occurred. These observations imply that when
C. marina attaches to a hydrophilic surface it does
so in a different manner than it attaches to a more
hydrophobic material. This means that different
adhesion mechanisms are used under different environmental conditions and that a single organism may
posses a variety of different adhesins.
In addition to the correlations between substratum
wettability and bioadhesion, there are indications that
contact angle hysteresis (CAH) has some effect on
biofouling. Working with perfluoroalkyl-containing
acrylate coatings, Schmidt et al. (2004) confirmed
that while the amount of biofouling did not correlate
with the surface energies of the coatings (tuned by
varying the composition of the coating), in agreement
with the earlier observation of Baier they reported that
the adhesive properties of the surface correlated with
water CAH; coatings with the best release properties
exhibited the lowest CAH.
Does this result mean that rather than trying to
make superhydrophobic surface efforts should be
concentrated on designing strategies that would minimise the CAH? This question cannot be answered
unambiguously at this time. Clearly, there are many
issues that need to be resolved. Some interesting and
inspiring ideas have been presented in recent works
by Marmur (2006a; 2006c). The first strategy for
solving the biofouling problem is based on utilising
superhydrophobic surfaces for decreasing the contact
area between the solid surface and water, thus
minimising the chances of biofoulers reaching the
surface. The second approach benefits from designing surfaces that prefer contact with water rather than
biomass. Such superhydrophilic surfaces would then
act as an analog to an ethylene glycol (EG)-based
protein-resistant surface. While this latter strategy
seems feasible, the actual sample design has not yet
been worked out. Even the EG-based surface may
not represent the most generic type of material
capable of preventing protein adsorption. Indeed,
recent work has revealed that EG may not be
performing that well in all instances. Some proteins,
such as lysozyme, are capable of displacing water
molecules from the vicinity of EG coatings and
adsorb rather readily (Lord et al. 2006a; 2006b).
In spite of these shortcomings, correlations between foul-release properties of surfaces and their
surface energy still exist. Brady (2001) and Brady
and Singer (2000) found out that the relative bioadhesion of pseudobarnacles (a proxy for barnacles)
on various polymeric surfaces is related to (gCE)1/2,
where gC and E are the critical surface energy and the
elastic modulus, respectively (cf. Figure 13). These
researchers also pointed out the effect of coating
thickness on adhesion. Adhesion appears to decrease
with increasing coating thickness; coatings whose
thickness was 4 100 mm did not exhibit any
marked improvement in decreased bioadhesion.
These findings thus suggest that the mechanical
properties of materials may also affect the extent of
bioadhesion, which is supported by recent work
employing live barnacles (Sun et al. 2004a; Wendt
et al. 2006) and ‘soft’ algal fouling (Chaudhury et al.
2005). The role of stiffness on bioadhesion is not
completely new and has been known to cell biologists
for some time (see e.g. Discher et al. 2005 and
references therein).
Experiments revealed that when cultured on solid
substrata (e.g. tissue-culture polystyrene dishes),
cells proliferate readily relative to situations involving
more compliant materials. Whilst understanding
of this behaviour is still far from complete, recent
observations indicate that at least part of the reason
for such behavior may be associated with the fact
that cells may alter their biological functions (e.g.
signaling pathways) based on the substratum stiffness
(Kong et al. 2005). Hence surface chemistry and the
mechanics of materials act in concert in regulating
biological functions. While these findings are very
exciting and important for developing novel genedelivery strategies, they also reinforce the aforementioned notion that partitioning of biological species at
man-made surfaces very likely depends on much
more than merely the surface energetic of materials.
In addition to various chemical approaches, surface topography has also been shown to play a role
in mechanical defense against macrofouling on a
larger scale, which may be hindered by certain surface structures, e.g. spicules (Wahl, 1989). Callow
et al. (2002), Hoipkemeier-Wilson et al. (2004), and
Carman et al. (2006) demonstrated that engineered
topographically corrugated surfaces are capable of
reducing biofouling. The degree to which biofouling
was reduced was found to depend on the dimensions
of the geometrical protrusions as well as the chemistry of the surfaces. Detailed studies pertaining to
examining the effect of surface topography on
biofouling has also been reported by other groups
(Bers & Wahl, 2004; Scardino & deNys, 2004;
Scardino et al. 2006). While these studies demonstrated the antifouling potential of microtopographical surfaces, the underlying mechanism responsible
for reduced fouling (i.e. settlement and attachment)
still remains unclear. One conclusive implication of
these studies is the fact that adhesion strength is
355
Figure 13. (a) Schematic of the Baier curve indicating the surface energies of typical organic layers and polymers: semifluorinated SAM (SFSAM), poly(tetrafluoro ethylene) (PTFE), poly(dimethyl siloxane) (PDMS), polystyrene (PS), poly(ethylene terephthalate) (PET), and
polycarbonate (PC). The yellow area denotes the approximate region of minimal bioadhesion. (b) Relative adhesion plotted as a function of
(gCE)1/2, where gC and E are the critical surface energy and the elastic modulus, respectively, for poly(hexafluoropropylene) (PHFP), PTFE,
PDMS, poly(vinylidene fluoride) (PVDF), polyethylene (PE), PS, poly(methyl methacrylate) (PMMA), and Nylon-66 (adapted from Brady
& Singer, 2000).
related to the number of attachment points of the
marine organism on the surface (Scardino et al.
2006). Notwithstanding any influences of surface
topography in terms of disrupting the searching
behaviour of motile spores or larvae, it would be
expected that fouling organisms that are larger than
the primary length scale of the surface texture would
exhibit reduced adhesion strength as there would be
fewer attachment points. Conversely, when settling
on surfaces with topographic features of larger
length dimensions, i.e. larger than the cell or organism, more attachment points are presented, thereby
facilitating stronger adhesion.
Because biofouling encompasses a very diverse
range of marine organisms with settling stages (cells,
spores, larvae) that span several orders of magnitude,
a topographical pattern having a single length scale
will not likely perform as a generic AF surface.
Rather, surface corrugations having multiple length
scales acting in concert should be utilised in the
design of an effective AF surface. The hierarchically
wrinkled topographies developed by Efimenko et al.
(2005) may represent a convenient platform for
designing such surfaces. Preliminary experiments,
currently in progress (Efimenko et al. personal communication), indicate that coatings based on such
topographies may indeed be efficient in preventing
biofouling of some marine species, such as barnacles
(so-called ‘‘hard fouling’’). While more work still
needs to be done to fully understand this phenomenon, initial observations suggest that coatings
comprising roughness features on multiple length
scales may represent a new and promising platform
for fabricating efficient foul-release marine coatings.
What still needs to be established is the ‘‘appropriate’’ level of roughness needed to minimise
bioadhesion of various marine organisms. Referring
again to the earlier discussion, it might be expected
that the roughness should be smaller than the size of
the settling cell/organism.
Various marine organisms may also settle differently on various kinds of surfaces. This presents a
confusing and complex situation when considering
what should ‘‘the ideal’’ AF surface be made of
chemically. Perhaps the notion of the responsive/
‘‘smart’’ surface can provide what is required, but
regardless of how ‘‘smart’’ the surface is, it will not
likely ‘‘outsmart’’ all marine organisms. In common
with other living matter, marine organisms are quite
adaptable. However, some of the design strategies
coming from various research groups working on
responsive materials should be helpful here.
One possible scenario was recently explored by
Gudipati et al. (2005) who synthesised a series
of polymeric networks comprising hyperbranched
fluoropolymers (HPFP) and poly(ethylene glycol)
(PEG) chains and studied their resistance against a
variety of proteins, including bovine serum albumin,
lectin, and lipopolysaccharides (cf. Figure 14). They
also explored the performance of the HPFP-PEG
coatings against settlement and release of zoospores
and young plants of the green fouling alga
Ulva. Earlier work by Gan et al. (2003) and Gudipati
et al. (2004) established that the surfaces of the
HPFP-PEG networks comprised both compositional
and topographical structures, which stemmed from
356
Figure 14. (Left) A Schematic illustrating the structure of polymer networks comprising hyperbranched fluoropolymers (HPFP) and
poly(ethylene glycol) chains (PEG). (Right) HPFP-PEG network that contain 45% of PEG exhibit the highest resistance towards protein
adsorption (reproduced with permission from Gudipati et al. 2005).
phase separation between the HPFP and PEG
components. The HPFP-PEG network surfaces
exhibited surface reconstruction when placed in
polar media, such as water, which resulted in surface
segregation of the more hydrophilic PEG component
to the coating/liquid interface. Their recent work
revealed that the best coatings capable of minimising
protein adsorption and also facilitating a high degree
of removal by hydrodynamic forces of Ulva zoospores and young plants were those that comprised
45 weight % of the PEG component. These
researchers attributed this observed behavior to the
compositional and topographical heterogeneity of
their coatings. They also hypothesised that while
‘‘hard fouling’’ (e.g. adhesion of barnacles) may
indeed be correlated with the surface energy and
bulk properties, e.g. elastic modulus of the coating,
‘‘soft fouling’’, involving the adhesion of for instance,
Ulva zoospores, does not exhibit such strong
correlations.
Another strategy was recently suggested by
Krishnan et al. (2006) who designed amphiphilic
block copolymers (ABCP) based on polystyrene (PS)
and polyacrylate (PA) blocks and used the latter
block as an anchor for an amphiphilic chain comprising a hydrophilic polyethylene glycol part and a
hydrophobic tetrafluoethylene part (cf. Figure 15).
When assembled on substrata, the PS block acted as
a substratum binder while the PA block exposed the
amphiphilic side chains to the exterior. The materials
were tested against Ulva and cells of a diatom,
Navicula, two organisms with very different settlement and adhesion characteristics; Ulva sporelings
(young plants) weakly adhere to hydrophobic fouling
release whilst Navicula is known to adhere strongly
to hydrophobic surfaces (Holland et al. 2004;
Chaudhury et al. 2005). While both organisms were
found to adhere to the ABCP surfaces, they were
both easily removed by applying a simple water jet
cleaning. This behaviour was explained by considering the amphiphilic nature of the surfaces. Hence,
the presence of both blocks clearly made an impact
on the rather unusual long term resistance of these
materials to both organisms, with such diverse attachment mechanisms and bioadhesives (see Callow &
Callow, 2006; Chiovetti et al. 2006). This work and
the experiments of Gudipati and coworkers referred
to earlier suggest that in order to design effective
marine coatings capable of resisting multiple marine
species, surfaces that are amphiphilic in nature may
need to be designed.
Before concluding, it is appropriate to elaborate on
one further point. The limited number of examples
discussed in this review indicates clearly that designing surfaces that resist marine fouling is a very
complex task. They reveal that controlling surface
chemistry, topology, and perhaps also surface dynamics is important in the design of efficient marine
coatings. Considering the broad range of marine
organisms, their adaptable nature and the range of
bioadhesives they employ (see Smith & Callow,
2006), it may be impossible to come up with an
environmentally benign coating design that is completely and universally non-fouling. However, some
progress in protecting ships in an environmentally
benign way has been made through the use of the
silicone fouling release coatings. One possible
avenue in this endeavour would involve designing
357
Figure 15. (a) Comblike block copolymer with amphiphilic side chains. (b) Proposed mechanism for surface reconstruction of the
ethoxylated fluoroalkyl side chains upon immersion of the surface in water. The picture on the left indicates the orientation of side chains in
air whereas that on the right shows the effect of water immersion. (c) Settlement of Navicula on glass, PDMS, and amphiphilic polymer
surfaces. (d) Percentage removal of Navicula from each substratum. A60 denotes the amphiphilic polymer surfaces annealed at 608C.
(e) Settlement of Ulva spores on glass, PDMS, and amphiphilic surfaces. (f) Percentage removal of Ulva sporelings from each substratum
after exposure to a shear stress of 53 Pa in a water channel. A60 and A120 are the amphiphilic surfaces annealed at 60 and 1208C,
respectively (reproduced with permission from Krishnan et al. 2006).
more robust and efficient foul-release coatings by
incorporating some of the strategies discussed in this
review.
Acknowledgements
The authors gratefully acknowledge the financial
support from the Office of Naval Research. The
authors are grateful to Professor Avi Marmur
(Technion – Israel Institute of Technology) for
reading the manuscript and providing invaluable
insight and comments. The authors also want to
express their gratitude to Dr Maureen Callow
(University of Birmingham) for her support, helpful
comments, and invaluable assistance in finalising the
text.
358
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Recent developments in superhydrophobic surfaces and their

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