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.

Eye-catching shapes are produced when water drops land vertically and spread on horizontal surfaces with micropillars arranged in regular square arrays. The positions of protrusions and fingers are often determined by the microstructure design and may be produced repeatably, which suggests possible manufacturing and analytical applications. This paper uses high-speed imaging of droplet shapes following impact to record and analyze asymmetries as drop spreading reaches its maximum extent. The range of experimental parameters used produced results varying (often non-monotonically) from symmetric spreading to many fingers. Impact Weber numbers (We) were systematically adjusted between 50 and 250, while surface microstructures featured circular (◯) and square (□) cross-sectional pillars of width d = 20 μm; height h = 15, 22, or 30 μm; and pitch p = 40, 60, or 80 μm. Many observed trends correlate with the extent of the fully wet impact region, including a general increase in asymmetry with We, p, and for □ rather than ◯ pillars. More detailed understanding of asymmetry mechanisms is also developed. For example, protrusions may be nucleated by jetting in directions of high gas flow within 100 μs of impact. A new analysis of gas flow under the drop, which accounts for Laplace pressure, explains anomalous spreading and asymmetry measurements. Reduced spreading velocity is identified as the cause of finger suppression where the microstructure is wet.

Rheology and surface microstructure affect many drop impact processes, including in emerging printing and patterning applications. This study reports on experiments systematically addressing the influence of these parameters on drop...