Superhydrophobic surfaces are surfaced which repel water to a high degree. This is characterized by a high contact angle. Like many areas of science, superhydophobic surfaces were inspired by naturally occurring structures in biological systems. The lotus leaf, for example, has been studied extensively because it exhibits extreme hydrophobicity due to the hierarchical wax crystalloid surface features on the micro- and nanometer scales. This unique roughness of the surface of the leaves forces water droplets into spherical drops, minimizing contact with the surface. The droplets, because of their spherical shape, roll off easily and carry dirt away.
The lotus leaf’s self cleaning, superhydrophobic surface could also be extended to other applications. For example, it could have extremely important medical applications where a lot of time and energy goes in to the sterilization of medical instruments. Many methods, such as autoclaves, require a large amount of energy to heat the instruments to kill bacteria. Here the lotus effect could be utilized for sterilization. Due to relative surface energies, the water droplets remove dirt and bacteria from a surgace for easy rinsing. Self-cleaning properties of superhydrophobic surfaces could also potentially be applied to windows on skyscrapers or automobiles, or the surfaces of touch screens devices. Also, superhydrophobic, antisticking surfaces could be used for antibiofouling coatings for boats. Reducing barnacle growth can help cut the cost of shipping.
- 1 History
- 2 Theory
- 3 Examples
- 4 Methods of Fabrication
- 5 Characterizing Superhydrophobic Surfaces
- 6 Applications
- 7 Summary and Conclusion
The modification of a surface to make it extremely repellent to water (superhydrophobic, Fig 1) was started in 1907 when Ollivier noticed that water droplets exhibit high contact angles on surfaces coated with soot (arsenic trioxide and lycopodium powder). Later in 1923, it was observed that water forms a high contact on rough surfaces of stearic acid-coated galena. This peculiar behavior was described separately in 1936-1944 by Wenzel and Cassie and Baxter with wetting models dependent on surface energy and rouchness. Finally, in 1997, contemporary excitement in this field was instigated when Neinhuis and Barthlott described the lotus effect. Water repulsion is greatly enhanced, when compared to regular hydrophobic surfaces, due to surface roughness. Additionally, models that describe surface wetting properties have been developed based on experimental observations.
The theory to describe and thus predict superhydrophobicity is interesting. Although this theory is not fully developed, it has the potential to enable novel and practical design of superhydrophobic surfaces. It is known that two or three phase interfaces with water and air arise due to interactions with the chemical structure and morphology of the surface of a substrate. However, the exact roles and proportions that surface roughness and chemical hydrophobicity play in creating a superhydrophobic surface are not well defined. The object of the following section is to relate the progress in describing and these interactions. The relative interface energy of a surface is typically characterized by its contact angle with water, θ, as described by Young (1).
A superhydrophobic surface is characterized as having an apparent contact angle of 150° or more where γSV, γSL, and γLV are the interfacial surface tensions with the surface (S), liquid (L), and gas (V) interfaces. While this model accurately describes thermodynamic equilibrium of the interface, it is much easier to measure the contact angle than to design to it; it is hard to design surface tensions which account both for roughness and chemical repulsion. The two classic models which describe complex surface interactions either account for roughness or account for interface energy differences across multiple phases. The first model, described by Wenzel in 1936, assumes that water placed on a rough surface penetrates into the space between surface features. The apparent contact angle, θw, is a roughness factor, r, of Young’s contact angle according to (2).
Wenzel’s model holds for surface areas which are up to 1.7 times that of a flat surface, after which a new model is needed to describe the contact angle. The second classical model, developed by Cassie and Baxter, assumes that the water droplet sits on top of the rough surface features, leaving air in between. Cassie and Baxter’s apparent contact angle, θc, is therefore a function of the surface phase fraction, f, and Young’s contact angles of the surface and air according to (3) (assuming that θair = 180° and fair = 1-f).
Both models are good at qualitatively predicting the wettability of a surface, but neither envelope the entire field, since they apply only to particular cases (suspended or penetrating), and neither provide quantitative design insight. Many groups have created model surfaces, with well defined surface features, and investigated the potential of applying the Wenzel, Cassie-Baxter, or creating new models to design superhydrophobic surfaces, but have found contradicting results. For instance, Bico et al. have shown, by creating many different textured surfaces, that the contact angle was independent of the surface roughness and was described by the Cassie-Baxter model. Contrary to this, Patankar found that high roughness (embodied by high height to width ratios of square pillars) brought about high contact angles. Neither of these models seem to take into account the whole of the interactions which participate in repelling a water drop, and by changing some factors, one model or another may fit better to describe a particular system. Finally, Extrand et al. has described the wettability of a surface in a modified version of the Cassie-Baxter model where a critical contact line density, determined by the asperity perimeter per unit area which could suspend a water drop, determines whether or not a drop will be suspended or will wet into surface features. This model uses many factors to determine the wettability; a roughness factor, the slopes of the surface features, liquid density, and surface tension all have to be considered in designing a superhydrophobic surface. The way that a drop wets a surface also depends on how it is applied; drops in metastable, with full penetration of the surface features, wetting states can transition to a suspended (over an air-substrate composite) state (Cassie-Baxter) under the correct stimuli (such as vibrations).3 Zheng et al. have created a model which describes which state (wetted or composite/suspended) is more stable according to the slenderness ratio, η, which depends on height, H, perimeter, L, and area, A, of a pillar (4).
When the slenderness ratio exceeds a critical slenderness, ηe, determined by Young’s contact angle and the solid fraction, f, the composite, suspended state is more stable. Wetting and the transition between wetting and suspension are not described well by either the Wenzel or the Cassie-Baxter models. The additions made to these models by other researchers suggest that a more inclusive model, which accounts for factors seen in both, will be necessary to completely describe the phenomena of superhydrophobicity.
Surface from Hydrophobic Polymers
As discussed earlier in this paper, the relationship between surface roughness and water repellency has been effectively theorized. In general, the chemical make-up of a material is not enough to create superhydrophobicity. Fluorinated polymers, for example, are hydrophobic of their own accord. Agarwal and coworkers showed a copolymer of 2,3,4,5,6-pentafluorostyrene (PFS) and styrene simply spin-coated onto a glass slide showed a contact angle of 110°. By modifying the surface roughness, they were able to increase the hydrophobicity to such a high degree that they could not measure the contact angle because it was impossible to keep water droplets on the surface. Because of this, they assume that the contact angle is much higher than 160°.
Surface made from Simple Plastic
Materials that are not intrinsically hydrophobic can also be rendered superhydrophobic through modification of surface roughness. Mert and coworkers  demonstrated the ability to transform a simple plastic into a superhydrophobic surface. This example is particularly interesting because of the low cost materials involved. A smooth polypropylene surface showed a contact angle of approximately 104°. After being dissolved into a solvent mixture of p-xylene and methyl ethyl ketone and subsequently dried in a vacuum oven at 70°C, the polypropylene surface was no longer smooth. The morphology was transformed to a gel-like fibrous network that displayed a contact angle of 160°. Something as simple as morphology has a large effect on the properties of the resulting material.
Surface made from Amphiphilic Polymer
Surface roughness is a strong enough driving force to transform something that is actually hydrophilic into a hydrophobic surface. Zhu and coworkers  displayed the creation of a superhydrophobic surface from an amphiphilic polymer, poly(vinyl alcohol) (PVA). Amphiphilic surfaces display water contact angles of less than 90°. Using a template of PVA precursors on an anodic aluminum oxide membrane, Zhu et. al. were able to grow nanofibers of polymer. PVA contains both hydrophilic –OH groups and hydrophobic hydrocarbon groups. The nanofiber morphology forces the orientation of the polymer such that the hydrophobic groups are on the outside. In this manner, with the incorporation of a sharp, branched, tree-like morphology, superhydrophobicity is induced. The location of the hydrophobic portions of the polymer at the surface decreases the surface energy and causes the contact angle to be approximately 171°.
Influence of Defects
The common factor that all three of these examples have is the morphology. All three required deviation from normal techniques which create a planar coating of polymers on surfaces. From an alternate point of view, superhydrophobic surfaces arise due to control of surface defects. Instead of the naturally formed structure with an equilibrium number of defects, methods for the creation of superhydrophobic surfaces require that the number of defects in greatly increased. A rough surface does not reflect a regular, periodic packing of molecules. In order for a surface to be rendered superhydrophobic, it requires a system that pushes the number of surface defects to a much higher number.
Methods of Fabrication
Many methods are available for fabrication of a superhydrophobic (SHP) surface, including top down, lithography type and bottom-up, self assembly type methods. The goal of most processes is to fabricate controlled, micro-scale, fractal surface features which impart the phobicity properties.
Top Down Methods
Top-down fabrication methods for superhydrophobic surfaces include lithography, etching, micromachining, and templating. These methods seek to have direct control over the surface features. However, in many cases, they are slow and labor intensive, thus inhibiting application on large scales. For instance a templating method uses a highly controlled template to imprint the structure of a soft material; an example of this is using the biological SHP lotus as an inspiration. A poly(dimethylsiloxane) (PDMS) cast of the leaf surface serves as a negative, and, after a mold release agent is added, can be directly cast onto with another layer of PDMS to create a positive replica of the lotus leaf. This replica retains the high contact angle of the original due to the morphology and texturing, even down to the nano scale, which were copied from the lotus. Alternately, a hard surface, such as silicon or aluminum oxide, can be used as a template in a nanoimprinting technique. The template is prepared via a machining technique, such as electron beam lithography, which can directly write out an inverse of the desired structure. Next, the template is applied to a thermoplastic polymer, such as polystyrene, under pressure and heat to force the polymer to take on the shape of the mold.1 After cooling, the mold is removed via dissolution, sublimation, or lifting to reveal a replica of the desired surface. A third type of templating involves using a soft, elastomer based mold and relying on capillary forces to pull a material into the molds features. The mold is placed on a heated or still solvated film, and capillary forces draw the wet or mobile material of the film into the mold.1 Once cooled or once the solvent is evaporated, the mold can be removed revealing a micro-patterned SHP film.
Photolithography techniques involve casting a photoresist, irradiating the resist with light or electrons through a patterned mask, followed by an etching step. The etching step removes substrate material, where the protecting resist was not exposed to radiation, leaving behind a pillar like structure. Furthermore, even without a photoresist, etching, such as plasma etching, can be used to micropattern the surface through inhomogeneous removal of the surface. For instance, Cao et al. have investigated the electroless etching of a silicon substrate, directed by gold nanoclusters in strong acid (HF). The gold clusters acted as a reaction site for the etching, leading to a hierarchical structure of micrometer asperities on a matrix of nanometer pores; a super hydrophobic surface because of the textures.
Bottom Up Methods
Bottom up fabrication methods involve design of materials so that secondary bonding interactions lead to self assembly of the desired morphology. These techniques do not have as great of control over surface morphology as top-down methods, but they are much quicker, and less labor intensive. For instance, chemical vapor deposition can be used to grow nanorods. Alternately colloids, spherical aggregates of materials in solution, can self assemble into stacks of close packed spheres on a surface. After an etching step to enhance the roughness of the colloidal assemblies surface, the material displays SHP properties. Another self assembly method, layer by layer film deposition, relies on repeated dipping of a charged substrate between two solutions of alternately charged molecules. By incorporating nanoparticles in the film, acid treating after deposition, or choosing a highly hydrophobic molecule as a base for the film (such as one based on tetrafluoroethylene), one can enhance the roughness or hydrophobicity of the film.
Characterizing Superhydrophobic Surfaces
One of the toughest challenges in characterizing superhydrophobic materials is determining a common way of comparison. There are many different techniques that have been used in the past to determine the hydrophobicity of a sample, and the different techniques yield different results. These methods range from water contact angle to multiresonance thickness-shear mode sensors.
Static Water Contact Angle
The static contact angle of water is the most commonly used method of characterizing the superhydrophobicity of a material. This method is one of the easiest methods to perform, but it has been shown recently that it can produce inconsistent results. The contact angle is determined by dropping a bead of water (~2µl) on the surface of the material. A goniometer is used to capture an image of the bead of water on the substrate and then a fitting model is used to determine the contact angle. The major issue that has been noted with this technique is that the contact angle can vary greatly depending on the fitting model used.    As seen in Figure 4, the contact angle of the same drop of water can vary from 152°-180° purely based on the fitting model used. Thus when a contact angle is reported, the specific means of characterization and the model used must also be reported.
Contact Angle Hysteresis
Another important property of superhydrophobic surfaces is the contact angle hysteresis. A low hysteresis is what allows a water droplet to easily roll off the surface. But there are still many different methods of determining the contact angle hysteresis of a material.
One of the easiest methods of characterizing the hysteresis is through tilt angle. This is determining the angle between horizontal and the substrate at which the bead of water will roll off the surface. It is interesting to note that this result is similar, although not equal to the difference between the advancing and receding contact angles. The advancing contact angle is the contact angle on the side of the liquid that is advancing across the substrate, whereas the receding is the one receding from the surface. In a low hysteresis material, the advancing and receding contact angles will be close, on the order of a couple degrees. This would result in a low tilt angle measurement. But if there is a larger hysteresis, the tilt angle will generally be larger.  Multiresonance Thickness-Shear Mode sensors have been used as an additional technique in order to further discern a difference between materials that have shown similar contact angles and different hysteresis. This technique employs high frequency sound waves generated by a quartz resonator to characterize surfaces loaded with a liquid by measuring the harmonic frequency shift. It has been shown that the frequency shift is correlated with the amount of penetration of water into the surface. A surface that has a lower contact angle hysteresis will have less penetration of liquid into the valleys of the rough surface than a surface high contact angle hysteresis. Therefore, the materials with less hysteresis will show less harmonic frequency shift.
There are two other notable techniques that have been used to determine the contact angle hysteresis of a material. One of these techniques has been used by Gao and McCarthy, by lowering the surface onto a supported droplet compressing and releasing the droplet several times. If the surface has a high hysteresis, it will have a higher affinity with the water droplet and will not release as easily. A low hysteresis surface will release the droplet easily. The bouncing drop method is another method that has been used to determine the contact angle hysteresis. This is done by dropping a water droplet on the surface. If the surface is highly hydrophobic, the droplet will bounce. It has been shown that the bouncing ability of the droplet can be correlated to the contact angle hysteresis of the surface.
The idea of creating superhydrophobic surfaces was inspired following the biological model of the highly water repellent lotus leaf. It is then no surprise that many applications of superhydrophobic surfaces have been created by mimicking natural, functional examples. Moreover, the principals that were found to give rise to the natural phenomena have been used to lead to other innovations in applications.
Surface Coatings on Ship Hulls
The key feature of the lotus leaf that was found to give rise to its super hydrophobicity was the presence of microstructures on the surface that kept water droplets physically raised and off of the surface itself. Applications such as surface coatings on floatation devices or ship hulls have been theorized to increase buoyancy because the air pockets in between the microstructures would assist in floatation of a surface on water. This application also follows the biological model of the feet of a water strider, which have feet with these extra buoyant properties.2 Other applications that utilize the same properties are coatings on ship hulls designed to reduce fluid drag. The idea behind this application is that the air pockets between microstructures and the microstructures themselves create a composite interface with the fluid which reduces drag, compared to the amount of fluid drag on a surface with a layer of air or vapor between the fluid and the surface. However, the dynamics of the microstructures themselves only gives a glimpse at the multitudes of applications utilized by these surfaces.
Water and Oil Repellency
A good number of applications of superhydrophobic surfaces are only concerned with the ability of the material to repel water and oils. Many superhydrophobic textiles are made by producing fibers of block copolymers with microstructures that produce the highly repellent behavior when the fabric is woven. The phobicity can be fine tuned in these processes by altering the diameter of these fibers. Another concept involving the general repellence of these surfaces is their use in anti-fouling. The property of super-water or oil repellency prevents the adsorption of the proteins necessary for bio-adhesion. On the surfaces of many boat hulls, superhydrophobic coatings are applied which have the same microstructures that inhibit water adhesion, but they also act to keep barnacles from attaching to the ship hull. The idea behind the superhydrophobic surface is that it prevents “bio-fouling” and is also safer for the environment than chemical coatings, thus preventing drag forces which barnacles would introduce. Many electromagnetic antennae exposed to the outdoors have “humidity proof” coatings to prevent functional inhibition by water or ice adhesion. Microwave antennae, for example, have a polytetrafluoroethylene (PTFE) coating to prevent ice adhesion which maximizes its production. Yet another example of an application of superoleophobic surfaces to prevent adhesion of contaminants or water is in the coatings of solar panels. The coatings on solar panels are interesting because they must be repellent yet highly transparent in order for the light to pass through and be utilized in the devices. One such coating material is polyethylene terephthalate. The key to superoleophobic coatings applied to solar panels is that the microstructure details must be smaller than the wavelength of light in order for the light to pass through without being absorbed. Coatings for solar panels have been very successful in increasing the power output per unit. By repelling the water the power output is increased and the high transparency allows light wavelengths from UV to almost infrared to be absorbed for, again, more output.
Sterilization of Medical Devices
Among the most important applications that exist and are being explored further are the use of anti-fouling in sterilization of medical devices. Much like repelling of barnacles on ship hulls, needles can repel stray bacteria once a highly superoleophobic surface is utilized. And if the bacteria themselves maintain some adhesion a simple spray of water or alcohol should take care of the rest (depicted in Figure 5). Other areas where the anti-fouling attribute of the superoleophobic surfaces is utilized, and in great amount, are the touch screens on electronic devices. Some examples include computer tablet screens, iPod and iPhone screens, and other PDA screens.
Summary and Conclusion
The preceding discussion presents the pertinent ideas for the design and fabrication of superhydrophobic surfaces; the development of novel characterization, fabrication and modeling techniques for superhydrophobic surfaces have been driven by the potential for novel applications. While many materials function only with respect to their defects, superhydrophobicity is a property brought about through control of a defect structure. The disruption of the physical and chemical ordering at the surface of a material relative to the bulk, through roughness and surface coatings, brings about super water repellant properties. Also, both highly ordered and highly defect filled morphologies have been fabricated, and display the desired superhydrophobic properties. Therefore, the role which defects play in super-phobic properties is ambiguous. While, the practical use of superhydrophobic surfaces can be far reaching, the ease of fabrication and theoretical understanding of the chemical, physical, and defect structures of these materials has much potential for further development.
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