Transition from Cassie–Baxter to Wenzel States on microline-formed PDMS surfaces induced by evaporation or pressing of water droplets

Abstract

In this work, we directly observed the evolution of air/water interfaces suspended between polydimethylsiloxane (PDMS) microlines when water droplets reduced their sizes due to evaporation. The inclined angles of the microline sidewalls were slightly larger than 90°. Two important phenomena were observed regarding the transition from Cassie–Baxter to Wenzel States. First, when a water droplet gradually shrank, an air/water interface between two neighboring microlines increased its deflection but decreased its angle with the vertical direction. In the meanwhile, the two edges of this interface were still at the top corners of the two microlines. Second, once water passed the top corners of these two microlines, it kept moving down and filled the gap. Based on these two phenomena, the equilibrium of a triple line and the uniformity of pressure inside a small water droplet, critical values of droplet sizes and Laplace pressure were derived to predict when the transition would occur on microlines. The derived theoretical relationships indicate that air/water interfaces may be stationary on both top corners and sidewalls of microlines if the inclined angles of the microline sidewalls are less than 90°. Otherwise, the interfaces can only be stationary at the top corners of the microlines. The predicted values of droplet sizes for the case that the inclined angles of these sidewalls are larger than 90° were validated by experimental results on three arrays of PDMS microlines. In addition, we also directly observed the evolution of air/water interfaces on PDMS microlines when a water droplet was slowly pressed using a glass slide. The critical values of the droplet sizes derived in the case of evaporation applied to this pressing case as well, and had a good match with experimental results on the three arrays of PDMS microlines. In addition to the cases of evaporation and pressing, the theoretical relationships derived in this work may also apply to other cases, in which Laplace pressure is gradually increased inside a liquid droplet and half sizes of the droplet are less than the capillary length of the liquid. Finally, based on developed transition criteria, a set of criteria were also proposed to design microlines for reducing the critical droplet size that triggers the transitions from Cassie–Baxter to Wenzel States.