Time-Dependent Wetting Behavior of PDMS Surfaces with Bio-Inspired, Hierarchical Structures

Wetting of rough surfaces involves time-dependent effects, such as surface deformations, non-uniform filling of surface pores within or outside the contact area, and surface chemistries, but the detailed impact of these phenomena on wetting is not entirely clear. Understanding these effects is crucial for designing coatings for a wide range of applications, such as membrane-based oil-water separation and desalination, waterproof linings/windows for automobiles, aircrafts, and naval vessels, and antibiofouling. Herein, we report on time-dependent contact angles of water droplets on a rough polydimethylsiloxane (PDMS) surface that cannot be completely described by the conventional Cassie-Baxter or Wenzel models or the recently proposed Cassie-impregnated model. Shells of sand dollars (Dendraster excentricus) were used as lithography-free, robust templates to produce rough PDMS surfaces with hierarchical, periodic features ranging from 10-7-10-4 m. Under saturated vapor conditions, we found that in the short-term (