Fast droplet impact onto slowly moving deep pools

TL;DR

This study investigates how a slowly moving deep pool influences droplet impact dynamics, revealing complex three-dimensional behaviors and a universal parameterization for impact outcomes across various fluids and conditions.

Contribution

It introduces a length-scale invariant parameterization for impact transitions considering pool movement, extending understanding beyond static pool models.

Findings

Pool movement alters ejecta sheet evolution and impact outcomes.

A universal parameterization for impact transition applicable to diverse conditions.

Numerical simulations confirm similar dynamics in oblique impacts, reducing boundary layer significance.

Abstract

When a fast droplet impacts a pool, the resulting ejecta sheet dynamics determine the final impact outcome. At low Capillary numbers, the ejecta sheet remains separate from a deep static pool, whilst at higher viscosities it develops into a lamella. Here, we show that the common natural scenario of a slowly moving deep pool can change the upstream impact outcome, creating highly three-dimensional dynamics no longer characterised by a single descriptor. By considering how pool movement constrains the evolution of the ejecta sheet angle, we reach a length-scale invariant parameterisation for the upstream transition that holds for a wide range of fluids and impact conditions. Direct numerical simulations show similar dynamics for an equivalent oblique impact, indicating that the pool boundary layer does not play a decisive role for low pool-droplet speed ratios. Our results also provide insight into the physical mechanism that underpins pool impact outcomes more generally.