Water drop impacts on regular micropillar arrays: Asymmetric spreading

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