Droplet Impact at Roughness Intersections in Microgravity

Drop tower experiments have been performed to explore outcomes of droplet impact at the intersection of surface roughness types. In these experiments, a droplet jumps from a concave hydrophobic lens when subjected to a sudden change from terrestrial gravity to microgravity. This transition occurs as the experiment is dropped and enters free fall. An aluminum plate, situated above the droplet, with half of its surface polished, and the other half laser etched with grooves perpendicular to the interface. Roughness coefficient of the grooved surface is characterized. The droplet impacts at the rough-to-smooth interface and movement is quantified by tracking of centroid, and leading and trailing edges of droplet via high-speed imaging. Relationship between centroid location relative to surface interface and subsequent droplet movement is explored. Movement is shown to be driven by capillary dynamics and contact angle differential between surfaces. Droplet centroid relative to interface is shown to affect velocity of droplet migration onto smooth surface, as well as its total displacement. Fluid-surface interactions are dependent on hydrophobic or hydrophilic surface properties. While there has been some study on this subject done in terrestrial environments, here a novel parameter space is investigated about these interactions in microgravity. The understanding of these dynamics have value in aerospace design, pharmaceutical manufacture, fluid system management in space as well as other areas.