Microscopic Wetting at Microstructured Surface of Porous Silicon

Surface wettability is an important factor for micro/nanoscale thermal fluidic systems and it has attracted much interest for both fundamental research and practical applications. As one of the most attractive materials with controllable wettability, porous silicon is easy to be produced by the electrochemical etching. In this study, the effects of the microstructures of porous silicon on the wetting behavior of a pure water droplet were investigated experimentally. The solid-liquid contacting surface of the porous silicon substrate was prepared by varying both the geometrical microstructure and the chemical composition. The anodic etching was applied to the n type silicon substrate of orientation (100) and the geometrical microstructures of porous silicon were controlled by varying the fabrication conditions of the electrochemical etching. The pores of diameter ranging from 1–6 micrometers and the porosity up to 0.8 were obtained. Also, the surface chemical composition was controlled by coating the SiO₂ layer or the CYTOP fluoropolymer layer directly on the porous silicon surface. The contact angle of the pure water droplet was measured at the prepared porous silicon surface in a room with constant temperature and humidity. The effects of the microstructures on the contact angle were discussed and the results were compared to both the classical theoretical models and a modified model based on the molecular dynamics simulations.