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An Angle Inequality for Judging the Transition from Cassie-Baxter to Wenzel States when a Water Drop Contacts Bottoms of Grooves between Microstructures

Cheng Luo (The University of Texas at Arl)

SES Medal Symposium in honor of D.J. Steigmann

Wed 10:45 - 12:15

MacMillan 115

It is well-known that, when a water drop is placed on a rough surface, there are two possible wetting states: Wenzel or Cassie-Baxter. The Wenzel state favors, for example, the creation of a super-wetting surface, while the Cassie-Baxter state is preferred when a liquid-repellent surface is desired. It is considered that, after a water drop contacts the base of a roughness groove, water should immediately fill this roughness groove. Subsequently, Cassie-Baxter wetting state is transitioned to that of Wenzel. Accordingly, one of the criteria used to judge the transition from Cassie-Baxter to Wenzel states is whether a water drop has contact with the base of a roughness groove. In this work, through theoretical and experimental investigations, we show that this transition criterion does not always hold true in the cases of microchannels, micropillars and microparticles, which are three microstructures commonly used to enhance surface hydrophobicity. We first theoretically prove that, when an angle inequality is satisfied, there may exist an intermediate wetting state inside roughness grooves after a water drop contacts the groove bottoms. In this wetting state, water does not completely fill the roughness grooves, and air pockets still exist in the bottom corners of the grooves. Also, the wetting state is stable in the sense that its energy state is lower than that of the Wenzel model. According to the angle inequality, such intermediate states may exist, for example, in microchannels with vertical sidewalls, when contact angles on the inner surfaces of these microchannels are larger than 1350. On the other hand, once the angle inequality is violated, water completely fills the corresponding roughness grooves. These theoretical results are then validated by experimental tests on microchannel-, micropillar- and microparticle-structured surfaces, as well as on leaf surfaces of three different lotuses.