Unidirectional wetting and spreading on stooped polymer nanohairs

We report on unidirectional wetting and spreading of a water droplet on stooped polymer nanohairs fabricated by replica molding and oblique electron beam irradiation. When a droplet is placed on the surface with high aspect-ratio (AR similar to 10) directionally bent nanohairs, it wets and spontaneously spreads over the surface in a unidirectional manner along the stooped direction with one edge being pinned to the substrate. To evaluate such anisotropic wetting property, the liquid velocity is measured along and against the stooped direction inside a simple Y-shaped microchannel with a syringe pump after channel bonding. It is found that the velocity is about six times faster along the stooped direction than that against the stooped direction, suggesting that this simple strategy would find uses in microfluidics and other applications involving unidirectional liquid manipulation.