Water drop impacts on regular micropillar arrays: Asymmetric spreading

Abstract

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.