Droplet impact onto sub-millimeter microstructures fully and barely submerged in a liquid film

We study silicone oil B3 droplet impacts onto films of the same fluid with submerged cubic structures with side lengths, heights, and separation distances of 0.1, 0.2, and 0.5 mm. The structures are either barely submerged or submerged in a film that rises to 0.2, 0.3, or 0.5 mm above the pillar tops. The impact Weber numbers range between 270 and 1 100. We use high-speed imaging from four perspectives simultaneously to record the experiments. Experimental results are compared with numerical simulations. We show that the underlying structure size can affect the droplet impact process significantly and that the droplet impact morphology is affected differently from either a film on a smooth surface or dry surface structures. Streaks are observed in the side view, which mainly result from azimuthal variations of the crown thicknesses due to differing flow resistance in the crown crater. For barely submerged structures, this causes symmetric crown detachment. Furthermore, we show that the crown height and lifetime depend on the film height above the pillar tops. Finally, a correlation to predict the maximum crown height for fully and barely submerged structures is presented.