Superhydrophobic Surfaces are surfaces that repel water to a very high degree. These surfaces, like many areas of science, 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 pick up dirt and bacteria 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.1 Also, superhydrophobic, antisticking surfaces could be used for antibiofouling coatings for boats; reducing barnacle growth can
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).2 Later in 1923, it was observed that water forms a high contact on rough surfaces of stearic acid-coated galena.2 This peculiar behavior was described separately in 1936-1944 by Wenzel and Cassie and Baxter with wetting models dependent on surface energy and rouchness.4 Finally, in 1997, contemporary excitement in this field was instigated when Neinhuis and Barthlott described the lotus effect.2 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.3 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.3, The apparent contact angle, θw, is a roughness factor, r, of Young’s contact angle according to (2).3,4
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.3 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).3,4
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.3 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.3
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).3
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.3
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.3
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°. The chemical structures of these molecules are depicted in Figure 2. 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°.
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 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° (comparison illustrated in Figure 3). Something as simple as morphology has a large effect on the properties of the resulting material.
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°.9
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.