Surprising Lack of Influence on Water Droplet Motion by Hydrophilic Microdomains on Checkerboard-like Surfaces with Matched Contact Angle Hysteresis.

Abstract

Chemically and spatially micropatterned surfaces have been successfully prepared for a number of diverse applications, including water/fog harvesting, screen printing, microfluidics, and cell/protein assays. While there have been quite some reports on micropatterned surfaces, less is known about the factors that influence dynamic surface wettability. To that end, smooth checkerboard-like micropatterned hydrophobic/(super)hydrophilic surfaces (2, 5, 10, 20 μm pattern sizes) with regions of matching/mismatching contact angle hysteresis (CAH) were prepared on the basis of a simple chemisorption/photopatterning of monolayers. The effects of regional wettability/CAH and pattern size on the overall dynamic wettability were then examined by measuring the dynamic contact angles (CAs) and substrate tilt angles (θT) of water. It was found that the dynamic wettability on samples with matching regional CAH remained unchanged, even when using hydrophilic regions or changing the pattern size. In contrast, surfaces containing mismatching CAH regions pinned water droplets, leading to overall dynamic wetting properties markedly dependent on pattern size. In addition, the experimental data did not match values predicted by the Cassie equation because dynamic wetting behavior is dominated not by interfacial area but by the interactions of the liquid and solid at the three-phase contact line.