Investigation Of Wetting States On Microstructures After Liquid Drops Contact Groove Base

Abstract

Wetting properties of roughness surfaces have attracted a considerable attention because of both the scientific insight and the practical applications. One of the most intriguing topics of wetting is the so-called Cassie-Baxter and Wenzel transition. Recently, theoretical and experimental works have been done to explore this transition on microstructure-formed surfaces. One of the criteria that most commonly used to judge such transition is whether a liquid drop contacts the base of a roughness groove. It is expected that, after the contact, liquid immediately fills the roughness groove, and subsequently the wetting state is changed from Cassie-Baxter state to that of Wenzel. However, this transition criterion does not always hold true in the case of microchannels. Through theoretical and experimental investigations, we show that, when an angle criterion is satisfied, there may exist an intermediate wetting state inside a microchannel after a water drop contacts the bottom of the microchannel. In this wetting state, water does not completely fill the microchannel, and air pockets still exist in its bottom corners. Also, the wetting state is stable in the sense that its energy state is lower than that of the Wenzel model. In addition to microchannels, the aforementioned intermediate state may also exist on a single corner, when the angle criterion is met. Moreover, we demonstrate that the angle criterion also applies to the case of micropillars with circular or polygonal cross-sections. Finally, based on the results achieved in the cases of microchannels and micropillars, we further investigate the applicability of the angle criterion to the cases of microballs and lotus leaf surfaces.