Low speed drop impact onto moving pools: the bouncing to coalescence transition

Normal droplet impacts on a deep fluid bath have been studied in depth using a range of experimental, theoretical, and computational techniques. In particular, the well-defined transitions from bouncing to coalescence, as well as coalescence to splashing, have been extensively mapped due to their relevance in application. However, comparatively less is known about these transitions in the more general scenario where the droplet and bath have a relative horizontal velocity. In this work, we study the bouncing to coalescence transition for a droplet impinging on a deep layer of fluid moving with a constant speed. We achieve a steady tangential bath velocity by using a large diameter annular fluid container spun up to achieve rigid-body rotation with minimal surface disturbances. We demonstrate that the normal impact velocity at which the bouncing to coalescence transition occurs is modified by the presence of a tangential pool velocity, and study its dependence on pool velocity, droplet size, and fluid viscosity. Experimental measurements are compared to direct numerical simulations which resolve the gas layer dynamics, and ultimately allow us to rationalize the observed change in threshold.