Moving Water Droplets Over Nanoscaled (Super) hydrophobic Wettability Contrasts: Experimental Test of a Simple Model Describing Driving Forces

Applying advanced nanolithography techniques, various arrays of nanopillars on top of Si‐wafers are fabricated with all geometric parameters on the nanoscale. Additional chemical functionalization together with control over areal pillar density, height, and diameter allows the preparation of superhydrophobic surfaces exhibiting a wide range of contact angles (CA). Further improvement of this approach enables the production of step‐like wettability contrasts involving various CB–CB (Cassie‐Baxter) and CB–S (Smooth substrate)‐transitions. Such samples in combination with a high‐speed camera allow studying under optimized conditions quantitatively additional driving forces acting on a water droplet due to CA gradients. Experimentally it turns out that the maximum driving force on the droplet is well predicted by a simple model assuming circularly‐shaped base lines during the passage of a step‐like gradient of wettability. The provided study permits a comparison between maximum retention forces when tilting the substrate up to a critical angle and the presently determines maximum driving forces acting on a droplet due to a step‐like CA gradient. Both situations can be nicely described by a joint linear relation between normalized forces and CA hysteresis values with a slope close to theoretical values.