Drop impact: From bumping to irregular splashing

Abstract

More than a century after the early work of Worthington, research on drop splash- ing is still very active, supported by the advances in high-speed imaging and numerical simulations. One of the main challenges is to understand the origin of the smallest droplets ejected during the impact1. High-speed imaging has recently uncovered the early emergence of an ejecta sheet between the impacting drop and the pool at high impact velocities2. Moreover, it was shown that for lower viscosities, only droplets emerge, and no coherent ejecta sheet can be identified. We demonstrate here by high-speed imaging and numerical simulations that this early irregular splashing observed can be explained by the collision of the drop and the ejecta sheet3. The freely available code Gerris4 was used to reach the very high refinements needed for this study. A systematic experimental study allows us to identify clearly the transition region between the smooth emergence of the ejecta sheet (Fig. 1(a)), and the irregular splashing regime (Fig. 1(c)). It shows that it occurs for Reynolds number Re between 2000 and 6000. Direct comparisons show that numerical simulations are able to reproduce the complex dynamics observed. We show that this transition is intimately related to the dynamics of the ejecta sheet. As Re increases, the angle of the ejecta sheet increases faster, leading to the collision of the drop and the ejecta sheet, that we call bumping (Fig. 1(b)). Ultimately, this interaction takes place very early after the ejecta sheet emergence, thus breaking the regular ejecta sheet and splashing micro-droplets.