Experimental study on splashing, spreading, and air entrapment dynamics in droplet impacts onto micrometric ramps, pyramids, and staggered cubes

While droplet impacts on micrometric cuboids or cylinders have been studied frequently, the effects of other structure shapes remain less understood. Here, we study water and isopropanol droplet impacts onto microstructured surfaces with surface features shaped as pyramids, ramps, and staggered cubes while varying the inherent surface wettability between superhydrophilicity and hydrophobicity. The surface structures feature characteristic sizes between 20 and 100 μm while the impact Weber numbers of the impacting droplets range between 90 and 1200. We show how the surface structures can affect the droplet impact morphologies asymmetrically. Crown splashing is increased in directions on ramps on the diagonals where fluid can flow “up the ramps,” while pyramids and staggered cubes show more classical behavior. Additionally, the wetting shapes are reported using a total internal reflection view. While the maximum spreading diameters for cases in which a receding motion is observed are still well predicted by existing correlations, the structures can affect the shape of the spread region significantly. Furthermore, the structures can lead to heterogeneous dewetting on pyramids and ramps. Finally, air entrapment characteristics during the droplet impacts are compared and it is found that for ramps, air is mostly entrapped behind the tops of the ramps, while in pyramids gaps in air entrapments are seen in regions where splashing is observed.