Super-hydrophilic surfaces by photo-induced micro-folding

A two-step UV curing process of thin acrylate layers was employed to prepare micro-rough top-coats on polymer film. The concept of the process (known as “photonic micro-folding”) is to apply a thin acrylate layer on a substrate and cure the layer by subsequent exposures to VUV and broad band UV radiation. The first curing step leads to curing of the skin of the acrylate layer alone, which induces shrinkage and folding. This structure is fixed in the second curing step which affects the bulk of the acrylate layer. The process is easily applied to any substrate and large areas. By using hydrophilic hydroxypropylacrylate and polyethylenglycolmonoacrylate as the main components of the applied acrylate, perfectly wetting – super-hydrophilic – surfaces were obtained. This basically is in accordance with the concept of Wenzelʹs equation which relates the apparent contact angle to a roughness factor r given by the ratio of true and projected surface area. The analysis of the data of this work, however, shows that the spreading of a droplet on surfaces with r > 1.2 is governed by geometric effects such as blockage by the surface features and cannot by described by Wenzelʹs equation.