High-speed drop impact onto moving pools: ejecta sheets and splashing*

Droplets often impinge at high-speed onto moving liquid surfaces, transforming droplet impact into a fully 3D problem, up from the typical axisymmetric configuration of normal impact on a static pool. Examples include rain drops landing on oceans and vehicles, in addition to inkjet printing and additive manufacturing. We investigate fast (>2 m/s) droplet impact onto slower-moving (<0.5 m/s) deep liquid pools, whilst also varying the droplet impact velocity and diameter, in addition to fluid properties. 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, and altered splashing behaviour. 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 well-known transition due to Reynolds number on static pools. We use this parameterisation alongside observations from higher viscosity conditions to offer insights into the physical mechanisms underpinning impact outcomes on both moving and static deep pools.