Single etch fabrication and characterization of robust nanoparticle tipped bi-level superhydrophobic surfaces

Though hierarchical roughness gives the best anti-wetting surfaces, their performance degrades quickly as nanostructures fail even under small mechanical stresses. Using spin coated alumina nanoparticles as an etch mask we report a single-etch based wafer-scale fabrication of robust nanoparticle tipped superhydrophobic surfaces with dual-level roughness. The top-level structures in the dual-level roughness provide mechanical robustness and the surface maintains its liquid repellency even when damaged due to mechanical shear. This complex dual-level structure leads to interesting droplet bouncing dynamics which was studied for several fluids. Though the normalized spread diameter showed good agreement with previous reports, we observed a dependence of contact time on both surface wettability and impact velocity. By breaking the impact event into spreading, recoil and detachment we show that the variation in contact time is mostly in the detachment phase. Contact time variation with impact velocity is attributed to partial impalement of the top-level nanostructures which increases the contact line stiction. For highest impact velocity while water droplets rebound completely, xanthum gum droplets having a similar surface tension and hysteresis leave residual droplets on surfaces with a higher solid fraction which is contrary to the current understanding. A large range of shear-rate dependent viscosity in conjunction with partial impalement explains this new observation.