High Hysteresis Suspended Wetting State: A Wetting Regime for Controlled Trapping of Drops on Micro‐Trench Covered Surfaces

Abstract

AbstractWetting of simple liquids on rough surfaces is usually described either by the Cassie–Baxter model where drops are suspended over the roughness features with air trapped below, or by the Wenzel model where the drops are impaled on the roughness features. According to the Wenzel model, drops can be trapped on the surfaces, whereas in the Cassie–Baxter model, the drops can be easily released upon adding a small tilt. Surfaces with very regular surface pattern, such as closely spaced micro‐trenches, can exhibit the so‐called High Hysteresis Suspended State (HySS). In this state the drops assume a spherical shape with air trapped below them, a characteristic feature of the Cassie–Baxter state, but show a very high contact angle hysteresis (up to >70°), typical for the Wenzel regime. The HySS regime is studied as a function of the surface geometry, applied pressure and the mass of the drop. Through roll‐off measurements it is shown that surfaces with controlled trapping conditions can be generated, where drops can be captured, however, upon addition of a small amount of energy (e.g., induced by tilt) can be released. The parameters that can be used to control the “depth” of the trap are elucidated.