Water drop impacts on regular micropillar arrays: The impact region

A liquid drop impact on to a rough solid typically produces an “impact region,” which is an area of fully wetted surface smaller than or equal to the projected area of the drop. Here, high-speed photography is used to study the size and symmetry of this impact region and microbubbles within it for water drop impacts on regular square arrays of hydrophobic micropillars. Outcomes are most strongly influenced by pillar pitch and impact Weber number (We), and there is an apparent transition from vertical to more horizontal wetting near the edge of the projected area of the falling drop. The impact region size is well described by energetic and pinning transition analyses, but profound asymmetries are observed, indicating the influence and superposition of cross-flows for gas and liquid escape. Zipping of the liquid–air interface between pillars during later stages of drop spreading is also studied. The surfaces have 20 μm wide polydimethylsiloxane pillars of circular or square cross section. Variations in array pitch (40–80 μm) and height (15–30 μm) are systematically investigated using droplets of diameter 2.51 ± 0.04 mm over the range 50 < We < 250. The geometric regularity of these surfaces could give rise to technological applications, but the results are also relevant to the many natural and industrial processes in which liquid drops impact upon dry surfaces with micrometer scale roughness.