Hydrophobicity, adhesion, and friction properties of nanopatterned polymers and scale dependence for micro- and...

Applications in micro/nanoelectromechanical systems generally require low adhesion and friction values between two materials of interest. By alteration of the material combinations and surface roughness, including nanopatterning, adhesion and friction can be tailored to meet a specific requirement. Surfaces found in nature, such as hydrophobic lotus leaves, provide a good example of this optimization. Recent models of hydrophobic leaf surfaces show a correlation between roughness and hydrophobicity, which can be mimicked by the presence of nanopatterned asperities on a polymer surface. In addition, by introducing nanopatterns on the polymer surface, the real area of contact decreases when another surface comes into contact with the patterned surface, which reduces adhesion and friction. This study explores the effect of nanopatterning on hydrophobicity, adhesion, and friction for two different hydrophilic polymers, poly(methyl methacrylate) (PMMA) and polyurethane acrylate (MINS), with two types of patterned asperities, low aspect ratio and high aspect ratio, investigated by use of an atomic/friction force microscope (AFM/FFM). In addition to the polymers, a hydrophobic coating was deposited on the surface of the patterned PMMA to study the effect of roughness on the contact angle, along with adhesion and friction. Relative contribution due to change in contact angle and real area of contact are explored. Scale dependence on adhesion and friction was also studied using AFM tips of various radii. Since applications of these surfaces will require operation in varying environments, the effect of relative humidity is investigated.