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Recent developments in superhydrophobic surfacesand 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)

AbstractIn this review, a brief synopsis of superhydrophobicity (i.e. extreme non-wettability) and its implications on marine foulingare presented. A short overview of wettability and recent experimental developments aimed at fabricating superhydrophobicsurfaces by tailoring their chemical nature and physical appearance (i.e. substratum texture) are reviewed. The formation ofresponsive/‘‘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 surfacechemistry, texture, and responsiveness of surfaces on the design of effective marine fouling coatings are considered anddiscussed.

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 can-

not the design of an ‘optimal’ AF surface be based on

what is already know about wettability?’’ This is

because there the various surface-modification meth-

ods 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 wett-

ability, 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 hydro-

phobic cores and hydrophilic coronas, proteins

typically partition relatively readily on both hydro-

philic 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 pro-

teins from replacing them, hence making them

protein adsorption-resistant.

When extending this simple example (the empha-

sis being on ‘‘relatively simple’’ as many outstanding

issues still remain in designing effective protein-

resistant 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]

Biofouling, 2006; 22(5): 339 – 360

ISSN 0892-7014 print/ISSN 1029-2454 online � 2006 Taylor & Francis

DOI: 10.1080/08927010600980223

biomaterial adsorption, the complexity of the pro-

blem 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 para-

meter 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 varia-

tions 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 develop-

ments aimed at understanding wettability of materi-

als. Particular emphasis will be on non-wettable

surfaces as they provide informative insight about

how the structure of the surface influences the parti-

tioning 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 & Quere, 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 funda-

mental property of any material; it reveals informa-

tion 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 im-

possible) 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 inter-

facial energies acting between the solid-liquid

(gSL), solid-vapor (gSV) and liquid-vapor (gLV) inter-

faces 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 (y5 908)surfaces can be distinguished from hydrophobic

(y4 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 non-

wettable (typically taken as y4 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

(y5 908) or non-wettability (y4 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

340 J. Genzer & K. Efimenko

wettability enters the so-called Cassie-Baxter regime

(Cassie & Baxter, 1944) (cf. Figure 1c):

cos ðy�Þ ¼ �1þ fs½cos ðyÞ þ 1�

¼ �1þ fs

gSV � gSL

gLV

þ 1

� �: ð3Þ

In Equation 3, fS is the fraction of the surface that

is in contact with the liquid; the remaining fraction

(17fS) 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 con-

sider 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 ðyL1Þ gdL1V þ gp

L2V

� �¼ 2 gd

L1V gdSV

� �1=2þ gpL1V gp

SV

� �1=2j k

; ð4Þ

cos ðyL2Þ gdL2V þ gp

L2V

� �¼ 2 gd

L2V gdSV

� �1=2þ gpL2V gp

SV

� �1=2j k

: ð5Þ

In Equations 4 and 5, gd and gp denote the dispersive

and polar components of the surface energy, res-

pectively. 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 compo-

nent 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 expres-

sion relating the contact angle of a solid to gSV and gLV

via (Kwok & Neumann, 2000):

cos ðyÞ ¼ �1þ 2gSV

gLV

� �1=2

e�bðg1V�gSVÞ2

; ð6Þ

where¼ 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

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.

Superhydrophobicity and marine fouling 341

whose contact angle (ymix) can be described by the

classical Cassie-Baxter equation:

cos ðymixÞ ¼ f1cos ðy1Þ þ f2cos ðy2Þ; 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 homoge-

neous surfaces with no in-plane phase separation.

There are instances, however, which require surfaces

with chemically distinct regions (and hence wett-

abilities) having well-defined spatial dimensions and

distributions present simultaneously on the same

sample. Over the past few years several methodolo-

gies 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 mimic-

king 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 position-

dependent and gradually varying chemistry (Ruardy

et al. 1997; Genzer 2002; 2005, and references

therein; Genzer et al. 2003). Such wettability gra-

dients 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 wet-

tability can be tuned by judiciously choosing the

chemical nature of surfaces and the spatial distri-

bution 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 methy-

lated and fluorinated carbons, whose surface energy

decreases in the following manner: 7CH2��4��CH34��CF2��4CF2H4��CF3. Hence flat sur-

faces 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 co-

workers 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 contain-

ing 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

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.


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 result-

ing 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 substra-

tum 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 func-

tionality 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, facil-

itates 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 hun-

dreds of submicron-sized pads (‘‘spatelae’’) posi-

tioned 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).

In the previous example, a situation was consid-

ered 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 ins-

tance, 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 pheno-

menon it is possible to explain why water striders can

stroll on water (cf. Figure 4d). The unique hierarch-

ical structure of their microsetae along with hydro-

phobicity (fraction of air¼ 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

(y4 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


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

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 micron-

sized bumps and nanoscale hair-like structures (reproduced with permission from Professor Wilhelm Bartholott).

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).


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 adjust-

ing 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 inde-

pendently, 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 estab-

lished 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),

Quere et al. (2003), Lafuma and Quere (2003) and

Callies and Quere (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 ðycÞ ¼fs � 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. Quere 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, Quere and coworkers showed that the state

of the droplet depended on the amount of liquid

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.


(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 generat-

ing 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) exhib-

ited very high water contact angles. Furthermore,

fluorinating the CNTs led to surfaces having contact

angles41708. Theses workers also established that

surfaces comprising sparsely spaced polyacryloni-

trile needles possessed contact angles 41738. These

studies confirmed the predictions of the wettability

theories in that they demonstrated that non-

wettability 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 tereph-

thalate 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(tetra-

fluoroethylene) (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

yADV¼ yREC¼ 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 sol-

vent 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 sub-

stratum. While in the Cassie regime, the adhesion is

small and the drop can easily be separated from the

substratum, Wenzel droplets adhere to the substra-

tum more strongly. A first glimpse at this behaviour

can be obtained by exploring the contact angle hys-

teresis (CAH). CAH is high for the Wenzel regime

and low for the Cassie regime, as demonstrated

experimentally by Bico et al. (1999). Surfaces with

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 & Quere, 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 & Quere, 2005).


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 sub-

stratum; the wettability on such surfaces would

be much closer to the ‘‘true’’ thermodynamic state

characterised by yADV¼ 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 ðaÞrdrop

� gLV½cos ðyRECÞ � cos ðyADVÞ�; 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 effec-

tively 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)

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).


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 micro-

meters. 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 dis-

cussing 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 combina-

tion 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 rough-

ness (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 super-

hydrophobic surface is to rely on nature to provide

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 Oner & 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.


effective templates, which can then be imprinted

using standard molding technologies into almost any

material (Furstner 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 charac-

teristics in response to some external stimulus.

Such surfaces are generally termed responsive (or

‘‘smart’’).

Wettability on responsive surfaces

A responsive/‘‘smart’’ surface changes its physico-

chemical surface characteristics in response to, for

example, a chemical, electrical or mechanical ex-

ternal 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 com-

prehensive 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 light-

induced cis/trans isomerization of azobenzene. The

ability to change wettability using azobenzene-based

derivatives (specifically, calixresorcinarene modified

with four azobenzene chains) in controlling wett-

ability of materials was used by Ichimura et al.

(2000) in creating real-time wettability spatial pat-

terns on flat surfaces. By irradiating surfaces covered

with self-assembled monolayers of the afore-

mentioned 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 substra-

tum 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

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 (&, y¼1358), orthogonal direction (~, y¼1178) on the one-dimensional groove structure, and on the

pillar-covered structure (., y¼1398) (reproduced with permission from Yoshimitsu et al. 2002).


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 mole-

cules then coordinate readily into surface oxygen

vacancies leading to dissociative adsorption of addi-

tional 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 radia-

tion. 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 photo-

catalytic 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 tempera-

ture, the polymer solubility decreases and finally at a

certain temperature, called the critical temperature

(TC), the polymers collapse and turn more hydro-

phobic. The temperature, at which this so-called

coil-to-globule transition occurs, varies from poly-

mer 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 sum-

marised that demonstrate the coil-to-globule transi-

tion 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, and4: 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 super-

hydrophobic states (Fu et al. 2004; Sun et al. 2004a).

As demonstrated by the data in Figure 11, the resul-

tant 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.

Lahann et al. (2003) used an electric field for dyna-

mical 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 carboxy-

lated 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, y¼63.58) to 408C (right, y¼93.28). (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).


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 demon-

strated 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 inter-

actions 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 sur-

faces 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 experi-

ments 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 sur-

rounding liquid beyond that of the bacteria caused

the bacteria to adsorb preferentially onto hydropho-

bic 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 mechan-

ism), additional interactions, such as van der Waals

forces, hydrogen binding and electrostatic interac-

tions 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 sub-

stratum 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 deso-

rption, 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 irrever-

sibly 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 adsorp-

tion of Cobetia marina (syn. Deleya marina) and

Staphylococcus epidermis on SAMs made of


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 methyl-

terminated 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 (hydroxyl-

vs carboxyl-) tail groups. Whilst C. marina attached

with an increased abundance to SAMs having a

higher content of the hydrophobic (methyl-termi-

nated) 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 zoos-

pores on hydroxyl- vs. 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 Lopez (1998) and

Ista et al. (1999) published a series of papers report-

ing on the utilisation of PNIPAAm in preventing

biofouling. Ista et al. (1999) described experiments

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).


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 hydro-

phobic 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 envi-

ronmental 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 mini-

mise 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 design-

ing 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 be-

tween foul-release properties of surfaces and their

surface energy still exist. Brady (2001) and Brady

and Singer (2000) found out that the relative bio-

adhesion 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 gene-

delivery strategies, they also reinforce the aforemen-

tioned 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, sur-

face topography has also been shown to play a role

in mechanical defense against macrofouling on a

larger scale, which may be hindered by certain sur-

face 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 chemi-

stry 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 demon-

strated the antifouling potential of microtopographi-

cal 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


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 orga-

nism, 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 com-

munication), 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 phenom-

enon, 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 ‘‘appro-

priate’’ 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 differ-

ently 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

Figure 13. (a) Schematic of the Baier curve indicating the surface energies of typical organic layers and polymers: semifluorinated SAM (SF-

SAM), 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).


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 zoo-

spores 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 com-

prising 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 settle-

ment 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 consider-

ing 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 attach-

ment 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 design-

ing surfaces that resist marine fouling is a very

complex task. They reveal that controlling surface

chemistry, topology, and perhaps also surface dy-

namics 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 com-

pletely 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

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).


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.

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).


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