Bouncing to coalescence transition for drop impact onto moving liquid pools

When a droplet impacts a bath of the same fluid, it can rebound completely provided that the mediating air layer remains intact during the process. However, above a critical impact velocity, the air layer is forced to drain and coalescence is initiated. While this problem has been studied extensively for the axisymmetric scenario of normal impacts on a still bath, little is known about the more general scenario of oblique impacts or impacts onto a moving liquid layer. In this work, we experimentally demonstrate that the critical normal velocity required to transition from the bouncing to coalescence regime is reduced for impact onto a moving liquid bath. Experimental measurements are compared to results obtained via direct numerical simulation that resolve the gas layer dynamics, and ultimately allow us to identify the physical mechanism responsible for the reduction in threshold, as well as extend our understanding to the case of oblique impacts.