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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.