Modeling & simulation of nanostructures for superhydrophobic coatings

Materials with surfaces that are difficult to wet with water are called hydrophobic. Hydrophobic molecules tend to be nonpolar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules in water often cluster together. Superhydrophobic surfaces are generally made by additionally controlling the surface chemistry and surface roughness of various hydrophobic materials. Superhydrophobic surfaces can be caused by protrusions (or papillae) present on a hydrophobic surface Generally, two types of wetting are usually considered for rough surfaces, the drop suspended on the papillae (Cassie-Baxter) or resting on both the papillae and surface between protrusions (Wenzel case). Similar microstructures can be artificially produced using a number of processing techniques, including physical vapor deposition and physical wet chemistry. These techniques can control the papillae size, particle size distribution as well as the papillae shape. These parameters can also be altered, in conjunction with the molecular properties of the surface or coating matrix material, to produce other physical properties (transparency over a certain wavelength band, strength, stiffness, coefficient of expansion) to meet specific application requirements. Recently there has been great interest in the development of superhydrophobic surfaces for a wide variety of applications, such as minimizing corrosion. Some methods for the production of superhydrophobic surfaces have been based on the formation of densely packed high-aspect-ratio structures such as polymer nanofibers, aligned carbon nanotubes, Si pillars fabricated by photolithography and plasma etching, and Si nano-rod arrays fabricated by Si vapor deposition techniques. The surface morphology is, in general, very sensitive their mechanical properties and directly related to the type of fabrication process and materials used to create the superhydrophobic surface. We have investigated superhydrophobic surfaces comprised of randomly distributed roughness versus those produced via ordered deposition of particles or microposts. We have modeled the contact angle, the equilibrium angle of contact of a liquid on a rigid surface where liquid, solid and gas phases meet, in order to determine the advantages of ordered versus random superhydrophobic surface roughness and the correlation to superior surface mechanical properties as a function of coating fabrication approach.