Droplet impact and flow into a gap between parallel plates

Abstract

Droplet impact on irregular, rough, or porous substrates can lead to inertia driven liquid penetration during droplet spread, air entrapment in voids, and triple-phase contact line pinning. This work focuses on inertia driven flow inside of a model long, narrow pore. We impacted 2 mm diameter water drops on long narrow gaps, photographing the impact and penetration with a high-speed camera, to understand how the flow develops and behaves inside these gaps. The experimental conditions varied were the velocity of impact (0.06–1.5 m/s) and the gap spacing between the plates (50–150 µm). The influence of inertia on the flow between the plates is negligible for impact velocities less than 0.5 m/s and can be predicted using a simple analytical model. Drops flow into larger gaps faster than smaller gaps at all impact velocities. Drops flow faster into gaps as impact velocity increases, but this has diminishing returns: at sufficiently high impact velocity the drop will cleave, which prevents a significant portion of the drop from flowing into the gap. Analytical models are presented to predict conditions under which the droplet will cleave and the rate of liquid penetration into the gap due to capillary forces.