Engulfed small Wenzel droplets on hierarchically structured superhydrophobic surface by large Cassie droplets: Experiments and molecular dynamics simulations

Abstract

Lotus leaves boost the development of superhydrophobic surfaces (SHSs) and water droplets applied for measuring the superhydrophobicity are usual macroscopic. Recent years, it shows that the superhydrophobicity vanishes when nano-size water droplets form on SHSs resulting in the Wenzel state, such as condensation. In fact, SHSs such as lotus leaves always keep superhydrophobicity, and water droplets quickly roll on surfaces. Here, we suppose that small Wenzel water droplets could be engulfed by large Cassie water droplets and verify this hypothesis by experiments and molecular dynamics (MD) simulations. It experimentally shows that the rolling large Cassie droplet engulfs small Wenzel droplets. We fabricate a Cassie droplet on the nanopillared SHS with the same contact angle as the experiment using MD simulation, as well as small Wenzel droplets. It shows that the moving large Cassie droplet engulfs small Wenzel droplets. Moreover, engulfing small Wenzel droplets does not reduce the velocity of large Cassie droplets because the emit energy of merging overcomes the adhesion energy of small Wenzel droplets. Effects of the number, distribution, and size of small Wenzel droplets on motion state and velocity of large Cassie droplets are also investigated. We believe this study would contribute to understanding nanodroplets on SHSs in practical applications.