Transition From Cassie-Baxter to Wenzel States on Micropillar-Formed Surfaces Induced by Evaporation

As a liquid droplet is placed on a rough surface, the wetting may be either in Wenzel state [1] or in Cassie-Baxter state [2]. In the Wenzel state, the droplet completely penetrates between microstructures (e.g., lines and pillars), while in the Cassie-Baxter state air is trapped between these microstructures and the droplet stays on top of the microstructures and trapped air. The Cassie-Baxter state may be transited to the Wenzel State when Laplace pressure reaches a certain value [3]. To date, three pressure criteria have been reported in the literature to predict when the transition may occur from the Cassie-Baxter state to the Wenzel State. The first criterion is [4] Pl−Pg>−4γlgcosθe2b(1+b2a),(1) where Pl is the pressure in liquid, Pg is the pressure in gas, (Pl − Pg) is so-called Laplace pressure, γlg is surface tension between liquid and gas, θe is the equilibrium contact angle on the flat surface and b is the space between two adjacent pillars. The second criterion has the form Pl−Pg>−4γlgcosθe2b.(2) The third criterion is given by [5] Pl−Pg>−4γlgcosθe2b.(3) Although these pressure criteria have similar forms, there still exist differences in their denominators. It is not clear which criterion gives the best prediction of the transition during the evaporation of a water droplet. Therefore, in this work, we examined these three criteria based on the experimental data obtained through in-situ observation of the transition of a droplet.