Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

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ice | science

The terms superhydrophobicity and superhydrophilicity were introduced not very long ago, in 1996 and 2000, respectively.

The former is used to describe exceptionally weak and the latter used to indicate strong interactions of materials and

coatings with bulk water, controlled entirely by surface topography and material chemistry. An explosion of research on

fabrication of superhydrophobic and superhydrophilic surfaces and coatings was noticed almost immediately after the

concepts appeared in the technical literature, with hundreds of reports now published annually. The interest in this new class

of surfaces/coatings is driven by an emerging market for water-repellant, snow- and ice-phobic products and formulations,

water antifogging screens, windows and lenses, antifouling coatings, microfluidic devices, coatings for enhanced boiling

heat transfer, foils for food packaging and many other products. The popularity of this emerging subdiscipline of surface

chemistry can also be attributed to uncomplicated fabrication technologies that can produce superhydrophobic or

superhydrophilic surfaces and coatings, in addition to the simplicity of the testing techniques used, such as contact angle

measurements. In this article, the physics behind superhydrophobic and superhydrophilic effects are reviewed and several

examples of applications of superhydrophobic and superhydrophilic surfaces and coatings are provided.

Notation listfi Fractional area of a component i of the solid surface

fs Fraction of the liquid base in contact with solid surface

r Roughness parameter

rl Roughness ratio of the solid that is in contact with the

liquid

SSLV

Spreading coefficient: mJ/m2

W Wicking parameter: mJ/m2

WS Work of spreading: mJ/m2

Τ Adhesion tension: mJ/m2

γLV

Liquid/vapor interfacial tension: mN/m

γSL

Solid/liquid interfacial tension: mN/m

γSV

Solid/vapor interfacial tension: mN/m

ΔGsl Energy of hydration: mJ/m2

θ Contact angle: degree

θCB Contact angle as per Cassie-Baxter equation: degree

θW Contact angle as per Wenzel equation: degree

θY Ideal (Young’s) contact angle: degree

1. IntroductionAfter more than two centuries of research, capillarity and wetting phenomena still remain popular topics of investigation in many modern surface chemistry laboratories. Curiosity and fascination with rain droplets splashing in a puddle, water droplets decorating flowers, leaves of plants and fruits after rain or watering and colorful soap bubbles shaped at the end of wheat straw have never been stronger and go beyond scientific laboratories. Colorful images of liquid droplets and puddles, illustrating their behavior on a variety of surfaces, thrive in professional journals, popular magazines and Internet. However, terms such as wetting, spreading and contact angle are still puzzling to the layman and beginner researcher. To facilitate

Surface Innovations Volume 2 Issue SI4

Physics and applications of superhydrophobic and superhydrophilic surfaces and coatingsDrelich and Marmur

Pages 211–227 http://dx.doi.org/10.1680/si.13.00017Themed Issue Review PaperReceived 16/07/2013 Accepted 02/10/2013Published online 05/10/2013

ICE Publishing: All rights reserved

Keywords: contact angle/hydrophilic/hydrophobic/ superhydrophilicity/superhydrophobicity/wetting

This article is dedicated to Professor Emil Chibowski on his 70th birthday.*Corresponding author e-mail address: [email protected]

1 Jaroslaw Drelich PhD*Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI, USA

2 Abraham Marmur PhDDepartment of Chemical Engineering, Technion – Israel Institute of Technology, Haifa, Israel

Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

1 2

http://dx.doi.org/10.1680/si.13.00017

mailto:[email protected]


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understanding of these concepts, daily examples of surfaces having controlled or responsive wettability are often much appreciated. Discussing the varying shapes of water droplets on human skin, on a car windshield and/or on the fabric of an umbrella (or raincoat) is always helpful to listeners and readers, and these three examples are briefly reviewed as an introduction to the main topics of this review.

1.1 Example one: introduction to wettability and contact angle

A drop of water (or any beverage) placed on hand does not spread spontaneously but forms a lens, such as in Figure 1(a), with a finite contact angle (roughly defined at this point as the angle that the drop makes with the solid surface at their contact point), most often somewhere between 50 and 80°. Skin contains 70–80% of water (with proteinaceous structures such as collagen),1 and such high contact angle values are rather surprising.2 However, water droplets normally spread to much smaller contact angles when the same experiment is repeated on the same hand immediately after it is washed with soap (Figure 1(b)); washing removes a layer of grease produced by the skin. This simple experiment implies that the state of skin is detrimental to the surface’s strength of interaction with water and demonstrates a close correlation between characteristics of the surface and water spreading.

1.2 Example two: to control liquid behavior by adding artificial coating (surface chemistry)

Glass is considered hydrophilic (‘water loving’) material and water should practically spread spontaneously on its surface.3 However, it is known that rain droplets do not spread to form a water film on the windows and windshields, but spread only partially. The contact angle is typically 10–30° – as a result of surface contamination with dirt and air-borne organics (Figure 2).

Everyone who owns a car takes it to a carwash for periodic cleaning and waxing. Waxing of the car windshield is important as it provides better visibility in the rain. Because of curvature effects, light bends on rain droplets causing reduction in visibility for the driver and passengers. During a heavy rain, wavy streams of water are formed on the windshield of a moving car through which visibility is often even worse (waving is accelerated by the speed of moving car). A waxed windshield forces the rain droplets to remain spherical in shape (large contact angle, ~90° and larger), significantly reducing the adhesion to the glass. Consequently, droplets easily drift to the edges of the windshield under the pressure of air produced by the moving car. These droplets are also more easily removed by the windshield wipers. Waxing is nothing but coating of the glass with a hydrophobic (‘water fearing’) material (i.e., hydrocarbons, fluorinated hydrocarbons or silicone polymer/oil). Coating a surface with hydrophobic films to eliminate or reduce water pick up by hydrophilic materials is not a new idea. In fact, the concept dates back to early seafarer’s painting of their wooden boats with tar for waterproofing and resistance to rotting.

1.3 Example three: to control liquid spreading by manipulating surface texture

Modern rainwear and umbrellas use a polyester textile for a few reasons. It is an inexpensive fabric with sufficient mechanical properties that can be colored and possesses hydrophobic properties. The water contact angle reported for a smooth surface of monolithic polyester is about 80°,4 which makes water drops stick relatively well to the surface. However, on the umbrella fabric, most of the droplets roll off easily during rain. A closer look at the droplet shape suggests a contact angle that appears to be >120°, much larger than on a smooth surface of polyester (Figure 3). This is simply the result of fabric texture. Fibers (threads) of the polymer are knitted together to form a 2D network with openings in between the individual strands. The shape of a water drop residing on top of such a textile is affected by the thread network density and the size of individual fibers. More openings in the network and fibers of smaller diameter force the rain drops

Figure 1. Water drop placed on hand (a) before and (b) after soap

washing. Inserts at the bottoms present water droplets zoomed from

major photographs and additional droplets colored with red dye

captured on separate photographs

(a)

(b)


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to assume a shape close to a perfect sphere when on the surface of the umbrella. This is an excellent example of the manipulation of material geometry in the early stages of design and development of products with enhanced water-resistance. Cassie and Baxter5 are considered the founders of this approach and the authors will return to their work in the further part of this article.

The three examples briefly discussed above provide a general introduction to wetting phenomena and surfaces with controlled wettability. These days, however, the scientific community and practitioners of wettability, focus their debate on the science and applications of superhydrophobicity and superhydrophilicity. The roots of the former term date back to 1996, when Onda et al.6,7 published two articles on wettability of fractal (rough) surfaces in which the terms of superhydrophobic and superwetting surfaces were proposed. The term superhydrophilicity appeared for the first time in the technical literature in 2000, in four articles published by three different research groups from Japan.8–11 In 1997, Fujishima et al.12 demonstrated the superhydrophilic effect on a glass slide

coated with a thin titanium dioxide polycrystalline film. The spreading of water was the result of both the hydrophilic properties of anatase exposed to UV radiation and submicroscopic roughness of the coating, although the effect of water spreading was entirely attributed to photoinduced self-cleaning capability of titanium dioxide at that time, and the term superhydrophilicity was not used. Why were these new terms introduced and why have they attracted so much attention in recent years? These questions will be answered later. Before that, the major terminology and physics behind wetting phenomena will first be summarized.

2. What is the contact angle and what does it represent?

Although the ancient civilizations well explored the benefits of waterproofing coatings and lubricants, the formal scholastic studies of wetting and capillary phenomena (wetting and capillary phenomena directly relate to each other in all three-phase systems) began in the eighteenth century, with the first reports appearing in

Figure 2. (a) Rain droplets on a window. (b) Photo of the outdoor

chair on a porch through window covered by the rain droplets. (c)

Photo of the same outdoor chair through clean window

(a) (b) (c)

Figure 3. Water droplets on the fabric of umbrella


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the early nineteenth century. It was Laplace13 and Poisson14 who established the fundamental background for capillarity. Young15 described the relationship between contact angle and surface energies for a liquid and a solid surface based on mechanical considerations. This concept was thermodynamically proven on a more rigorous basis by Gibbs.16 From that time it has been known that the behavior of a liquid on a solid surface is controlled by the competition between the liquid–vapor (γ

LV), solid–liquid (γ

SL) and

solid–vapor (γSV

) interfacial tensions. A spontaneous and complete spreading of liquid occurs over the solid surface when the spreading coefficient (S

SLV) is greater than or equal to zero:

1.

During partial spreading (SSLV

< 0), the liquid forms a lens with a finite contact angle (θ) at the solid surface. The contact angle is defined as the angle between the solid surface plane and the tangent to the liquid surface plotted at the point of contact between three phases (Figure 4). The contact angle defined by Young’s Equation 2 is referred to as the ideal contact angle (θY where the Y refers to Young’s contact angle) and is valid only for solids whose surfaces are homogeneous, isotropic, smooth, rigid and surrounding fluids are inert to such solid (no chemical reaction or specific adsorption or dissolution or swelling or rearrangement of phases, molecules and functional groups):

2.

3.

In the case of microscopic and submicroscopic droplets, a tension component associated with the three-phase contact line needs to be added to Young’s equation.17,18

Solids examined in research laboratories that can be described as ideal solid surfaces, such as required by the Young equation, are very rare. They are often either rough, at the micro- or nano-scopic level, or heterogeneous because of multiple phases, different surface compositions or simple contamination of the surface.19 Most inorganic solids are anisotropic due to multielemental composition and crystalline structure. Such surfaces have at least atomic heterogeneity that can trigger adsorption of constituents from liquid or gas adding to heterogeneity of the surface and its landscape.3 Some of the solids can also react chemically20,21 or interact physically with the probing liquid or surrounding gas and promote surface segregation, reorientation, diffusion or migration.22,23 Dissolution of pharmaceutics, soluble and semisoluble minerals and many inorganic and organic reagents/products makes the

wetting characterization of such materials extremely difficult and sometimes impossible.3 In addition, as spreading is a time-dependent event, the conditions between solid and spreading liquid can vary depending on the time of liquid shape observation during contact angle measurements. Softer materials also deform under the weight of the probing liquid. Another reason for deformation is the unsatisfied normal component of the Young’s equation giving rise to protrusions at the three-phase contact line.24–28 This results in an increase in the force required to slide a drop on the solid surface as can be measured using the centrifugal adhesion balance.3,4,26,29

Mutual saturation between phases or preferential adsorption at interfaces can be very fast, but sometimes, it occurs as a very slow process and thus exceeds the time of contact angle measurements, resulting in data measured under non-equilibrium conditions.30 A good protocol requires contact angle measurements in liquid-saturated gas environment to at least secure equilibrium solid surface conditions.31 The issues of three-phase system saturation and equilibration are typically ignored in modern research laboratories. Although the composition and purity of both the solid substrate and liquid are often well reported, the gas phase, typically air, is almost always ill-defined. Each laboratory has a different air quality, depending on its geographic and urban location, whether it is purified or not, human traffic in the laboratory and so on.32 Trace quantities, even at a level of parts per billion or even parts per trillion, of active organic compounds in the gas phase (mostly exhausted by humans and animals as well as emitted by the industrial activities) that often adsorb on the solid surface and change the wetting characteristics of a solid, especially metals and ceramics.32

Typically, the contact angle varies along the three-phase contact line for a liquid drop resting on a rough and/or heterogeneous solid surface. Local changes in angles of inclination of the rough surface33 and/or variation in chemistry of a heterogeneous surface34 cause the three-phase contact line to contort. Since the contact angles are typically measured macroscopically for liquid drops with a diameter of a few millimeters using low-magnification optical lenses, the local angles are ignored. Instead, the global contact angles are measured and are referred to as apparent contact angles.

A liquid in contact with a rough and/or heterogeneous surface can have more than one apparent contact angle.33,34 The phenomenon of multiple liquid–solid metastable configurations can be analyzed in terms of the Gibbs energy that takes into account the details of surface geometry, topography and local wettability. Figure 5 shows an imaginary correlation between the Gibbs energy and apparent contact angle for a liquid on a rough or heterogeneous solid surface. The Gibbs energy versus contact angle curves have multiple minima, in contrast to the curve for smooth and homogeneous solid surface with only single minimum.35,36 Each energy minimum defines a stable geometry of the liquid, with

SSLV SV SL LV= − −γ γ γ

cosθ γ γγ

Y SV SL

LV

=−

cosθγ

Y SLV

LV

= +1S


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unique equilibrium or quasi-equilibrium contact angle that can be measured experimentally, especially if sufficient times for transition between states and equilibration are allowed. Often external energy needs to be provided to drive transitions between metastable states and overcome the energetic barriers.37,38 Equilibration time can vary from seconds to hours to days to weeks and, unfortunately, such measurements are scarce in the technical literature. It should be recognized however, that only some of these equilibrium contact angles have practical meaning and special protocols need to be followed to measure them.

The most stable equilibrium state is associated with the lowest Gibbs energy value and corresponds to either Wenzel’s state on a rough surface or Cassie’s state on a heterogeneous surface.39 The Wenzel equation is as follows:40

4.

where r is the roughness parameter that expresses the ratio of the true the solid surface to its horizontal projection, and θY is the ideal

(Young’s) contact angle that would be measured on a flat surface of the same solid. This equation describes the most stable contact angle when the liquid completely penetrates into the roughness grooves (‘homogeneous wetting’ regime).

The Cassie equation for a chemically heterogeneous (two-component) surfaces is as follows41:

5.

where f is the fractional area of a component of the solid surface; f

1 + f

2 = 1; and θ

1Y and θ

2Y are Young’s contact angles

corresponding to the two components. Both the Wenzel and Cassie equations apply to surfaces whose protrusions and/or heterogeneities are small in comparison with the size of liquid/vapor interface.36,42

If liquid does not penetrate into surface protrusions and is present on the air pockets or placed over a porous material such as fabric,

cos cosWθ θ= r Y

cos cos cosθ θ θC Y Y f f1 1 2 2

Figure 4. (a) A sessile drop on an ideal solid surface. Vectors represent three interfacial tensions and θ is the contact angle. (b) A shape of a liquid

drop on a horizontal and tilted solid surface (α represent the tilting angle of the solid surface). θmax and θmin are the maximum and minimum

contact angles, respectively. (c) Three cases of a liquid behavior on a solid surface: during non-wetting, complete wetting and partial wetting. (d)

Liquid drops on surface of different wetting characteristics

(a)

(b) (d)

(c)

Vapor

Liquid

Solid

γLV

γSV

γSL

θ

No spreading

Partial spreading

Complete spreading

θ

α

θ

min

θmax

γSV < γSL θ > 90°

θ

γSV = γSL θ = 90°

γSV > γSL θ < 90°

θ

θ


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screen, membrane and so on (see Section 1.3), then the Cassie equation is modified to become the Cassie-Baxter equation5:

6.

where fs is the fraction of the liquid base in contact with solid

surface (fs < 1), and (1 − f

s) is the fraction of the liquid base in

contact with air pockets and rl is the roughness ratio of the solid

that is in contact with the liquid. Air is not wetted by water and, therefore, the water/air contact angle is equal to 180°. The cosine of 180° is equal to −1, leading to the minus sign in the second term of Equation 6.

This equation predicts an apparent contact angle for any geometry and structure of the surface, the topography of which is not fully penetrated by the liquid, also known as ‘heterogeneous wetting’. A practical challenge for such systems is always to define and quantify the value of f

s. In addition, the portion of a liquid in contact with the

solid is often contorted due to the solid’s topography that adds to the complexity of contact angle analysis.

Going back to the Gibbs energy curve in Figure 5, the Wenzel or Cassie-Baxter contact angle is associated with the total minimum, respectively.43,44 Whether the system remains in the Cassie-Baxter or Wenzel wetting state depends on which of them has the lower energy, as well as on the way the liquid is introduced into contact with the solid surface (deposition, immersion, spreading or vapor condensation). Transitions between these two states can

be triggered by external stimuli including pressure, mechanical vibrations and temperature gradient. Recent experimental and computer simulation studies suggest that a transition from the Wenzel state to the stable and robust Cassie–Baxter state is only possible on hierarchical superhydrophobic surfaces.45

Della Volpe demonstrated that equilibrium or quasi-equilibrium (metastable) contact angles can be measured for almost any imperfect solid.38 Such measurements are still not very popular because the conventional instruments require considerable modification to accommodate speakers or other vibration-inducing devices.38,46 Nevertheless, the advancing contact angle and receding contact angle are easily measureable on solids.31 These two angles are well defined and should always be reported. Advancing contact angle is the maximum metastable contact angle measured for the liquid that advances or recently advanced over unwetted solid surface.31 On the Gibbs energy against contact angle curve in Figure 5, it corresponds to the metastable state located at the far right side of the curve (as long as the energy barrier preventing the system from moving to the second metastable state is larger than the intrinsic thermal energy). Receding contact angle is the minimum metastable contact angle measured for the liquid receding or recently retreated from the wetted solid. It corresponds to the contact angle associated with the first metastable state in Figure 5. The experimental protocol for measurements of advancing and receding (static) contact angles was recently published.47

The difference between advancing and receding contact angles is called contact angle hysteresis, and its value provides insights into quality of the solid surface and strength of adhesion to liquid.48 The wetting characterization and, consequently, understanding of solid surfaces are limited if only advancing contact angles are measured. Unfortunately, receding contact angle measurements have not been commonly reported.

3. Some controversies and recent developments

The contact angle measurements using the sessile-drop and captive-bubble techniques are among the simplest surface characterization methods.31 Researchers with diverse backgrounds representing almost all fields of science and engineering use these methods, not always with sufficient training in preparation of surfaces and measurements of advancing, receding and equilibrium contact angles, and their interpretation. As a result, the modern literature is flooded with contact angles that are in between advancing and receding and are not equilibrium contact angles. Reproduction of these experimental ‘intermediate’ contact angles, if pursued, becomes a challenge to other laboratories. The problem of questionable contact angle measurement methodologies was raised in an earlier publication, and recommendations on sample preparation

cos cosθ θCBl S S= − −( )r f f1

Figure 5. The Gibbs energy for a liquid on a rough or heterogeneous

solid surface

Receding CAAdvancing CA

∗ Metastable states

Energy barrier

Most stable CA

Apparent contact angle

Gib

bs

ener

gy


217


and contact angle measurements protocols were made;31 these will not be repeated here. In short, the question, ‘What does measured contact angle represent?’ (addressed in Section 2) cannot be answered without proper measurements of contact angles and understanding three-phase conditions during these measurements. Simply, many experimental contact angle data cannot be analyzed using the Young, Wenzel, Cassie or Cassie-Baxter thermodynamic equations.

Validity of the Wenzel, Cassie and Cassie-Baxter equations has been questioned in recent years in many publications, as discussed in more detail in the previous review.3 For example, Gao and McCarthy49 doubted the validity of both the Wenzel and Cassie-Baxter approaches and argued that contact line, and not the contact area, is important in interpretation of the contact angles. A similar conclusion was also drawn earlier for chemically heterogeneous surfaces.50 Already in early reports,19,51 the importance of understanding surface characteristics in the vicinity of the three-phase contact line to explain the experimental contact angles was discussed. It needs to be recognized, however, that thermodynamic relationships such as Young, Wenzel, Cassie and Cassie-Baxter equations, discussed in Section 2, were derived for equilibrium contact angles at non-deformable and non-reactive solids based on the assumption of reproducible surface characteristics at submillimeter/microscopic dimensions. Derivations of these equations also rely on the assumption of small displacements made by a liquid. These displacements must be much larger than dimensions of surface pattern or topography variation; in practice, it translates to the size of a liquid drop being orders of magnitude larger than any heterogeneity or asperity. For large surface chemical heterogeneities19 or roughness features,52 there is a need to analyze the shape and contortion of the three-phase contact line instead of measuring one global contact angle. In fact, local considerations of the shape and length of the contact line and global considerations involving interfacial area within the contact line do not contradict but complement each other.53

Measurement of meaningful contact angles is especially challenging on rough and structured surfaces. Experimentally measured advancing and receding contact angles are generally not representative of what thermodynamic relationships predict, a fact that is often not well recognized by the critics of contact angles and their validities in Wenzel and Cassie-Baxter regimes. Here, additional effects such as line pinning, capillarity, porosity pressure buildup, surface curvature and so on, might contribute to the observed contact angles, which are not always accounted for by the thermodynamic relationships. The pinning of the contact line on surface defects such as edges of asperities is a well known phenomenon that causes departure from the Wenzel assumptions – whether in terms of surface area or contact line length.54,55 Both the shape and sharpness of roughness features and their edges affect pinning of the contact line.56 A small curvature of the fiber or post can also change wetting behavior

of the liquid.32 Pressure buildup in porosity or in between short-distanced posts can prevent the liquid from penetrating into the porous structure.57–59

Furthermore, the contact angles are often measured to access indirectly either adhesion or solid surface energy, important quantities in formulation of coatings, painting, printing, de-icing, biocompatibility and so on. The correlations between the advancing and receding contact angles and solid–liquid adhesion for solids with imperfect surfaces (having a certain degree of roughness and/or heterogeneity) are not straightforward, and at present are poorly understood. Measured advancing and receding contact angles represent a combination of the effects associated with not only the solid surface tension but also sample geometry, size, shape and distribution of the roughness/heterogeneity features. For this reason, some recent attempts concentrated on direct measurements of liquid–solid interactions, initiated by a design of two apparatuses. The centrifugal adhesion balance, introduced in 2009, uses centrifugal and gravitational forces to induce different normal and lateral force combinations for direct adhesion measurements between a liquid drop and a solid surface.29 Then in 2011, Samuel et al.60 introduced a microbalance with a specially designed hydrophobic loop to hold a liquid drop capable of measuring the liquid–solid surface interactions on drop-surface approach (snap-in force) and pull-off force (adhesion) during liquid drop detachment. The authors found for a large number of samples having various surface characteristics that advancing contact angles correlate better with the snap-in force whereas receding contact angle with the pull-off force. Extension of this work to equilibrium contact angles could probably help to justify thermodynamic relationships and pinpoint to their limitations for particular surface structures.

4. Defining hydrophobic and hydrophilic surfaces using contact angle criterion

A variety of different definitions and techniques have been used in the literature to describe/measure the hydrophobic and hydrophilic (homogeneous and smooth) surfaces as discussed in detail in the previous contribution3 (the authors will not repeat the discussion in this article). In general, a hydrophilic surface has strong affinity for water as compared with nonpolar fluid such as air or oil whereas a hydrophobic surface repels water. The term hydrophobicity originates from two ancient Greek words: hydro for water and phobos for fear. The Greek word philic means love, therefore hydrophilicity stands for ‘loving water’. On the basis of the dependence on capillary penetration of the water contact angle, and on the sign of the nominator in Equation 2, it is natural to draw the line between hydrophilicity and hydrophobicity at a Young contact angle of 90° (Figure 4).61 It is expected that a hydrophobic smooth surface has a weak affinity for water, as it interacts with water exclusively through van der Waals forces.62 Although the van der Waals attraction of a hydrophobic solid with water is commonly


218


stronger than with vapor, it is weaker than water–water cohesive forces involving additional polar and hydrogen interactions.63

The definitions of hydrophobicity and hydrophilicity can be generalized to define hygrophobic and hygrophilic surfaces, respectively (i.e. a surface which a liquid fears or likes, as shown in the first row of Table 1).61,64 It is, however, too general for the classification of a variety of different solids having different contact angles, especially those used in technological operations. Therefore, the classifications of hydrophilic and hydrophobic surfaces based on contact angle, work of spreading, free energy of hydration and water adhesion tension were proposed in the past as shown in Table 2 (the reader should return to the original publication3 for the source of the numerical limits used in this classification). Hydrophilic surfaces are those on which water spreads completely, visually ‘zero contact angle’. Partially hydrophilic and hydrophobic solids – classes which encompass a vast majority of materials – called here weakly hydrophilic and weakly hydrophobic, are those on which macroscopic water films are unstable and bead up to lenses with contact angle smaller than 90° (Figure 4). Hydrophobic surfaces are those commonly recognized having water contact angles of at least 90°. Superhydrophilic and superhydrophobic surfaces are also included in Table 2, but they will be discussed Section 5.

Examples of hydrophilic and hydrophobic materials were provided in the earlier publication and will not be repeated here.3 Hydrophilic surfaces according to the above definition are more abundant in nature than hydrophobic ones, but they become contaminated very quickly and surfaces lose their natural strong affinity for water. As of today, only organics that are hydrophobic are known; there is no reported inorganic for which the water contact angle of the clean surface is 90° or larger. Among these organics, fluorine-containing

polymers and molecules are the most hydrophobic, with water contact angle near 120°. It seems that the natural substances have been surpassed by man-made ones in terms of hydrophobicity, as organic fluorine-based molecules have not yet been reported in biological systems and they are more hydrophobic than any natural hydrocarbons.

5. Defining superhydrophobicity and superhydrophilicity

In spite of success in manufacturing hydrophobic (organic) materials and broad availability of hydrophilic materials, many applications benefit from surfaces and coatings that enhance spreading of water (or other liquid) or reduce it, beyond what natural or synthetic materials with smooth surfaces demonstrate. Surface roughening is necessary to enhance or reduce the spreading of liquid on a solid. The principles of this fabrication were founded several decades ago by Wenzel40 and Cassie and Baxter5 who described contact angles and different mechanisms of wetting on rough surfaces. The detailed analysis of these two equations, 4 and 6, is commonly presented in the literature and will not repeated here beyond what was discussed in Section 2.

It follows from Equation 4 that any surface roughening (expressed in term of r roughness factor) will reduce the contact angle on hydrophilic material, and can lead to the complete spreading of water or another liquid. Roughening of a hydrophobic surface, on the other hand, will increase the contact angle. In such a system, a large water drop will remain suspended on the tops of asperities or other roughness features, thus requiring the use of Equation 6 to predict the contact angle. Liquid drops can also remain suspended on many rough and textured surfaces of hydrophilic materials if the three-phase system is trapped in a metastable state (see

Morphology of surfaceWettability

classificationContact angle (θ)

Smooth Hydrophilic θsmooth < 90°

Hygrophilic

Hydrophobic θsmooth ≥ 90°

Rough/porous Parahydrophilic θ < θsmooth < 90°

Parahygrophilic

Parahydrophobic θ ≥ θsmooth ≥ 90°

Very rough/very porous Superhydrophilic θ ~0° < θsmooth

Superhygrophilic

Superhydrophobic θ > θsmooth ≥ 150°, very low hysteresis (a few degrees)

Superhygrophobic θ > θsmooth > 90°, very low hysteresis (a few degrees)

Data adapted from Ref. 61.

Table 1. Terminology for wettability classification


219


Figure 5).64,65 The invasion of liquid can be inhibited on material surfaces through careful selection of the design, geometry, size and contour of surface features and protrusions.64,66–68 An energy barrier unfavorable to liquid wicking may be maximized in this manner.59,69–73 This energetic barrier, if it is larger than the inherent thermal energy7 needs to be overcome by mechanical means such as vibrations,74,75 impact76,77 or load imposed on the drop.73,78

The term of superhydrophobicity was introduced in 1996 by Onda et al.6,7 to describe unusually high water contact angles, observed on fractal hydrophobic coatings, although the foundation to this discipline can be dated back a few decades.79 The now commonly accepted meaning of a superhydrophobic surface is a surface on which the water (advancing) contact angle is at least 150°, and the contact angle hysteresis as well as the sliding (or rolling off) angle (sliding/rolling angle is the minimum angle of sloped solid at which water (liquid) drop rolls off the surface) do not exceed 5–10° (Table 2). Although currently superhydrophobic surfaces are inspired by biological specimens,80–97 the early research was inspired by the practical need to enhance coating repellency of water and snow.98,99 These days, superhydrophobic coatings are manufactured by chemical, physical and/or mechanical modifications of both organic and inorganic materials.69,100–113

The opposite term of superhydrophobic is superhydrophilic, defined in the previous contribution.114 This type of surface is also of great interest at this time, although still some questions regarding its definition remain open.113 It is generally accepted that the first prerequisite for a surface to be superhydrophilic (superwetting) is that its apparent contact angle with water is less than 5°. In the previously published note,114 the authors suggested that the term superhydrophilic (or superwetting) only refers to a textured and/or structured surface (rough and/or porous) possessing roughness factor (r = ratio of real surface area to projected surface area) defined by Wenzel equation40 larger than 1, on which water (liquid) spreads completely (Table 2). Therefore, again, superhydrophilic

(superwetting) surfaces cannot be achieved without manipulating the roughness of hydrophilic materials, on flat surfaces of which water (liquid) droplets do not spread completely and remain as lenses with contact angle smaller than 90°. In terms of a wicking parameter, W:

7.

A minimum roughness of the surface necessary to initiate liquid wicking that results in zero apparent contact angle is commonly predictable through the Wenzel equation:

8.

Obviously, roughening of the surface or change in a coating texture do not necessarily lead to superhydrophobic or superhydrophilic systems. Weak or moderate changes in surface topography can produce surfaces or coatings that do not comply with the definitions of superhydrophobic or superhydrophilic. For that reason, Marmur recently proposed terms of parahydrophobic and parahydrophilic (Table 1).61

6. How superhydrophobic surfaces are designed?

The essence of superhydrophobicity is the easy removal of a drop from the solid surface.115 For achieving this goal, a small external force should be able to get the drop out of equilibrium, and the motion of the drop should be relatively rapid. A small external force could be, for example, the component of the drop weight along the surface when the latter is slightly tilted. The reaction of the drop to this force depends on contact angle hysteresis. When the solid surface is, for example, tilted (Figure 4(b)), the shape of the drop may transform into another equilibrium shape. This will be the case if the new shape may be such that the local contact angles along the contact

W = − = >γ γ γ θsv sl l cos 0

r ≥1

cosθ

Type of surface

Measure of hydrophilicity/hydrophobicity (20°C)

Contact angle: degree

Water adhesion tension: mJ/m2

Work of spreading: mJ/m2

Energy of hydration: mJ/m2

Supehydrophilic (r > 1) ~0a ≥73 ≥0 ≤−146

Hydrophilic ~0 ≥73 ≥0 ≤−146

Weakly hydrophilic (56–65°) > θ > 0 73> τ > (30–40) 0 > Ws > −(32–42) −113 > ΔGsl > −146

Weakly hydrophobic 90° > θ > (56–65°) (30–40) > τ > 0 –(32–42) > Ws > −73 −73 > ΔGsl > −113

Hydrophobic 120° > θ ≥ 90° 0 ≥ τ > −36 −73 > Ws > −109 −36 > ΔGsl > –73

Supehydrophobic (r > 1) θ > 150°a τ ≤ −63 Ws ≤ −136 ΔGsl ≥ −10

aApparent contact angle.See Ref. 3 for the source of the numerical limits.

Table 2. Proposed measures of hydrophilicity and hydrophobicity of solid surfaces


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line match both the geometry of the drop required by the Young-Laplace equation and the local Young contact angles. On a tilted solid surface, the drop is not axisymmetric; therefore, for geometric reasons, its local contact angle must vary along the contact line (it is highest at the lower end). Thus, a drop can be at equilibrium only if the hysteresis range is sufficiently wide to account for the geometric requirements of the drop shape. Consequently, easy removal of a drop from a surface requires minimization of hysteresis. In addition, it is reasonable to assume44,116,117 that the drop will quickly react to the removing force if its actual contact area with the solid (the ‘wetted area’) is as small as possible.

Thus, the main design consideration is the minimization of contact angle hysteresis as well as of the wetted area. This can be achieved, as in nature, by making the drop settle at the Cassie-Baxter state. In this state, the contact with the solid is minimal (if properly designed), so most of the drop base is in contact with air. Now, air is very uniform, thus hysteresis is minimized. In addition, air is very hydrophobic, therefore the contact angle in the Cassie-Baxter state is high, a fact that contributes to minimizing the wetted area. It turns out then that a suitable roughness that leads to the drop being in the Cassie-Baxter state answers both the need for minimal wetted area and minimal hysteresis.115

The Cassie-Baxter state may be thermodynamically stable, metastable or unstable in comparison with the Wenzel state. It is stable if the Gibbs energy of the Cassie-Baxter state is smaller than that of the Wenzel state (this is easily indicated by the contact angle: the lower contact angle corresponds to the lower energy). To find out whether the drop is unstable, a ‘feasibility condition’ that depends on the specific roughness details was developed.44 Mathematically, this condition indicates when the Gibbs energy function has a saddle point. The ‘feasibility condition’, for example, implies that convex-type protrusions may allow the Cassie-Baxter state, but concave dents will not. If the drop is neither stable nor unstable, it is in the metastable state. The latter could be practically applied, if necessary, and if the energy barrier between the Cassie-Baxter state and the Wenzel one is high enough.

The main question now is how to optimize the roughness in order to get the best possible superhydrophobic surface? To answer this question, the parameters to be optimized need to be defined. In principle, these are the chemistry of the surface, its morphology, and the cost. The latter is not discussed here, however, it should be remembered that the cost may be detrimental to any suggested superhydrophobic product. As far as the chemistry of the surface is concerned – it simply should be as hydrophobic as possible. The problem of the optimal roughness is more complicated. Some theoretical simulations indicated44,117 that rounded-top protrusions appear to be better than flat-top ones with sharp edges. This seems to be in agreement with

designs by nature itself. Another question is whether fractal or multiscale roughness, which is ubiquitous in nature, is essential for superhydrophobicity. The researchers agree that multiscale roughness is helpful.118 A recent, relatively detailed study44 of three types of roughness geometries with up to four roughness levels showed that the main effect of this type of roughness is in reducing the sizes of the roughness protrusions that are necessary for stable superhydrophobicity. Thus, it may be assumed, until further studied, that multiscale roughness helps in making the surface more stable from a mechanical point of view.

7. Applications of superhydrophobic, oleophobic, superhydrophilic and superwetting surfaces

Hundreds of scientific reports on formulation of superhydrophobic, superhydrophilic, oleophilic and so on, appear every year in the literature since the beginning of the twenty-first century, including several published in the previous issues of this journal.68,119–129 Review of all the literature and possible applications targeted by the authors exceeds the scope of this review. Only a few examples of commercial products that authors are either aware of or found through the Internet are provided.

7.1 Self-cleaning (superhydrophobic) paintsA self-cleaning paint has been marketed by the Sto AG company, Germany,130 and is probably the first commercial self-cleaning paint introduced on the market (Figure 6). Polysiloxane emulsion with particles of titanium dioxide and silicon dioxide is used as an exterior paint and can be used on mineral (concrete) and organic (non-elastic such as shielding) substrates. Dirt particles have a reduced adhesion to the painted surface and are easily cleaned off by the rain.

7.2 Self-cleaning (superhydrophobic) product coatings

Several different companies have made self-cleaning superhydro-phobic coatings available on the market, although many of the small spin-off nanotechnology-based companies are still searching for a specific application of their coating product. One of the first companies, if not the first one, working on commercial super-hydrophobic coatings was NTT in Japan, where one of us had the privilege to work with in the middle of 1990s.98,99 Their spray that contained polytetrafluroethylene particles and a binder was specifically designed for antennas and other transmission systems, particularly located at a high elevation, to reduce the accumulation of snow (Figure 7).

Hypho Technology Pte Ltd. (Singapore) introduced the Uri-pel coating specifically designed for the spray coating of toilet urinals to eliminate the need for flushing water (flushless urinals).131 This company is trying to introduce their coating to other products such as shower screens, solar cells and panels, automobile windshields and others.


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7.3 Self-cleaning (superhydrophilic) paints and product coatings

Haruna Singapore Pte Ltd. offers self-cleaning paints (HP Tiocoat),132 which are based on photocatalytic titanium dioxide nanoparticles, patented by the Singapore Institute of Manufacturing Technology, a research institute under the

Singapore Agency for Science, Technology and Research (A*STAR). Titanium dioxide decomposes organic substances including mold, mildew, fungi and other microorganisms through photocatalytic reaction using ultraviolet radiation from sunlight or artificial light sources. This company also offers similar titanium dioxide nanoparticle-based coatings for windows and other glasses. Such coatings can also be used on ceramics in bathrooms and kitchens to fight bacteria attached to the tiles, counters, glass walls or oxidize/remove foul smell from stains in the toilet (e.g. titanium dioxide–coated tile and glass are commercially available – see Section 7.4). Superhydrophilicity accelerates washing dirt and stain with the stream of water or by rainfall. Photoreactivity of titanium dioxide coating is strong enough to attack an organic paint surface, causing the so-called paint-chalking phenomenon. To prevent the direct contact of titanium dioxide particles with organic paint, an inorganic–organic layer is used to prevent substrate-damage from the photocatalytic reaction. Additional benefits of using paints and coatings with anatase, titanium dioxide, include the decomposition of other organic pollutants (e.g. car exhausts NOx, formaldehyde, benzene, volatile orgainc compounds), and protecting the surface from UV damage. It has also been suggested that a titanium dioxide–based coating reduces the energy consumption needed for cooling buildings in the summer.

7.4 Self-cleaning (superhydrophilic) windows/glassAs early as in 2001, self-cleaning windows were introduced on the market by Pilkington Glass under the brand name of Pilkington Activ (Figure 8). Soon after that, several other major glass companies released similar products including PPG Industries (SunClean),133 Cardinal Glass Industries (Neat Glass) and134 Saint-Gobain (Aquaclean and Bioclean).135 The products rely on a similar invention to that discussed in Section 7.3. Glass for windows (typically soda lime silicate float glass) are coated with a thin (from

Figure 6. (a) Lotus leave, (b) self-cleaning of the lotus leave with a water

droplet, (c) Painting with the Lotusan paint and (d) self-cleaning of the

Lotusan coating with water droplets. All images are courtesy of Sto Corp

(b) (d)

(a) (c)

Figure 7. HIREC superhydrophobic and antisnow coating offered

by NTT-AT in Japan. (a) Optical image of a water drop on top of the

coating and (b) picture of the coated and uncoated satellite dishes

during winter. All images are courtesy of NTT-AT

(a) (b)

Uncoatedantenna

Coatedantenna


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less than 10–30 nm, depending on the technology) transparent layer of photocatalytic (hydrophilic) nanocrystalline anatase (TiO

2).

7.5 Antifogging (superhydrophilic) mirrorsThe antifog mirrors are now offered in Asia and Pacific Region including Japan, China, Singapore, South Korea and other countries. For example, Shanghai Divas Glass Co. Ltd. offers variety of antifog mirrors for bathrooms.136 The Blackwood-Eddy Pty. Ltd. (Australia) is offering fogless shaving mirrors.137

7.6 Antifogging (superhydrophilic) shields, goggles and eyeglasses/sunglasses

When the temperature changes and the humidity is high or goes up lens fogging can become a major problem because of reduced visibility. A wide variety of antifog safety glasses and antifog safety goggles are offered by manufacturers. Antifog sunglasses and goggles, including swimming goggles, are also broadly spread and offered. Also, motorcycle, safety and surgical face shields are available from several manufacturers. Although details about antifog

Figure 8. Self-cleaning windows produced by Pilkington Glass and

images illustrating their performance. All images are courtesy of

Pilkington United Kingdom Ltd

The window is exposedto daylight

Daylight triggers thePilkington Activcoating

Reaction loosens dirt

Rain water hits windowand sheets down glass

Dirt is washed away byrain

Window is left clean


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coatings are typically not publicized, many of the products most likely rely on either silicone- or titanium dioxide–based coatings.

7.7 Antifogging (superhydrophilic) bags and packaging films

Food bags and packaging films made of polymer with antifog surfaces are offered broadly. Details of this technology are unknown, but, most likely, the surfaces of polymers, such as polyethylene terephthalate (PET), are oxidized and/or roughened to enhance water ability to spread.

7.8 Self-cleaning and stain-resistant (superhydrophobic) fabrics/clothes

Dickson in Germany fabricated Orchestra Max fabric, made of UV-resistant acrylic resin, with superhydrophobic and self-cleaning characteristics, designed primarily for retail, hotels and catering outfits.138

7.9 Other applicationsIt should be mentioned at the end that new products with either superhydrophobic or superhydrophilic, or related coatings, are currently booming on the market and many of them can already be found on the shelves of stores. For example, frying pans used a rough Teflon coating for many years now, although at the time of invention this coating was not called superhydrophobic or superoleophobic but could be grouped into these categories these days.

Quite recently, Apple introduced a fingerprint-resistant oleophobic coating present on the glass screen of Apple iPhone 3GS. It does not eliminate grease from the screen but make its cleaning much easier and more efficient by simply wiping the screen with a soft, lint-free cloth to remove oily fingerprints.139

8. ConclusionThe research on superhydrophobicity and superhydrophilicity dates back to the second half of the 1990s, although the solid foundation for this new subdiscipline was established in eighteenth to middle of twentieth centuries. It has exploded at the beginning of the twenty-first century and will certainly attract attention of many research groups in the years to come. The progress on fabrication and characterization of superhydrophobic and superhydrophilic surfaces and coatings, along with understanding of a liquid spreading and adhesion on such materials, is driven by a broad application of superhydrophobic and superhydrophilic surfaces in products with antifogging screens, windows and lenses, antifouling coatings, microfluidic devices, biocompatible implant devices, coatings for enhanced boiling heat transfer, foils for food packaging and many others. There is already a wide spectrum of products available on market whose design was inspired by the superhydrophobic and superhydrophilic phenomena.

AcknowledgementsThe authors express deep appreciation to Michelle Leader of Sto Corp. (Germany), Julia Berkin of Pilkington United Kingdom Ltd. (Great Britain) and Yoko Katsuyama of NTT-AT (Japan) for providing images for this publication. Jaroslaw Drelich acknowledges Patrick Bowen for his corrections to the manuscript.

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http://www.hyphotech.com/product.html

http://www.hyphotech.com/product.html

http://www.harunapaint.com/home.html

http://www.harunapaint.com/home.html

http://www.ppg.com/corporate/ideascapes/glass/products/sunclean/Pages/suncleanglass.aspx



http://www.cardinalcorp.com/products/neat-glass/

http://in.saint-gobain-glass.com/upload/files/bioclean_b2_apr_06.pdf



http://www.divasglass.com/en/Products/Bathroom-mirror-defogger-list_12.html

http://www.divasglass.com/en/Products/Bathroom-mirror-defogger-list_12.html

http://www.themirrus.com/

http://www.themirrus.com/

http://www.dickson-constant.com/en/EN/3/technical-fabric/5/orchestra-max

http://www.dickson-constant.com/en/EN/3/technical-fabric/5/orchestra-max

http://appleinsider.com/articles/09/06/10/apples_iphone_3g_s_sports_600mhz_chip_oleophobic_coating/



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Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings

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