Droplet impact: compressibility and wetting effects

Abstract

High-speed impacts of droplets are well-known to cause serious erosion to surfaces. One reason are high pressures occurring on impact. The importance of shock waves for the generation of extreme pressure peaks will be explained based on a classical shock polars approach for the case of liquid-liquid impacts. After impact, expansion waves propagating within the droplet may result in the formation of cavitation bubbles whose collapse can also contribute to erosion. At low velocities droplets impinging on a dry solid surface can spread on the surface or splash. It is shown that critical conditions of splashing inception, the so-called splashing threshold, have often been obtained under slightly different experimental conditions and are partly contradictory. Furthermore, no general agreement on the basic physical mechanism causing splashing does exist. After considering those parameters that are likely to govern splashing in a dimensional analysis, limiting conditions of splashing are re-evaluated. Information drawn from these considerations is used to distinguish between different splashing thresholds. For smooth surfaces there is evidence that the wetting behaviour of the liquid plays an important role for the inception of splashing. A mechanism causing the onset of splashing on smooth surfaces is proposed that may also explain the well-known fingering of the liquid lamella formed after impact.