Droplet impact on liquid pools

Abstract

Droplet impact on liquid pools can be seen throughout industry and nature: rain-induced icing, spray cooling, raindrops hitting puddles, and agricultural sprays protecting crops - to name just a few examples. However, it is only in the last decade or so that the crucial role of a thin jet of fluid (ejecta sheet) during the first millisecond of impact on the subsequent dynamics has been appreciated. We will discuss some of our recent and ongoing work as part of a joint EPSRC-NSF project in this area, focusing in this talk on fast droplet impact that often leads to splashing.

Initially considering static pools, we will elucidate the effect of pool depth on impact outcomes using a combination of experiments and numerical simulations, including explaining why droplets impacting sufficiently shallow pools always produce a crown reminiscent of Edgerton’s milk drop coronet. However, in many cases, droplets impinge on moving pools, transforming droplet impact into a fully 3D problem. Above a certain threshold, we show that pool movement has a dramatic effect on ejecta sheet dynamics in low viscosity conditions, resulting in different post-impact dynamics arising around the circumference of the droplet. We present a way to parameterise the impact outcome that accurately classifies the post-impact behaviour for a wide range of fluid properties and dynamic conditions, and naturally recovers the transition due to Reynolds number on static pools.