Estimation of surface free energy at microstructured surface to investigate intermediate wetting state for partial wetting model

While partial wetting at nano-/microstructured surfaces can be described using the intermediate wetting state between the Cassie-Baxter and Wenzel states, the limitations of the partial wetting model remain unclear. In this study, we performed surface free energy analysis at a microstructured Si-water interface from both theoretical and experimental viewpoints. We experimentally measured the water contact angle on microstructured Si surfaces with square holes and compared the measured values with theoretical predictions. Furthermore, the surface free energy was analyzed using the effective wetting area estimated from the measured contact angle and electrochemical impedance spectroscopy results. We verified the validity of the partial wetting model for fabricated Si surfaces with a hole aperture a less than 230 μm and a hole height h of 12 μm, and for a < 400 μm, h = 40 μm. The model was found to be applicable to microstructured Si surfaces with a/h < 10.