Two jets during the impact of viscous droplets onto a less-viscous liquid pool

TL;DR

This study reveals a novel two-jet phenomenon during viscous droplet impacts on less-viscous liquids, identifying a high-speed surface-climbing jet alongside the traditional Worthington jet, influenced by viscosity, impact speed, and miscibility.

Contribution

It introduces the discovery of a second jet during droplet impact, with detailed analysis of its mechanism and influencing factors, expanding understanding of droplet impact dynamics.

Findings

Identification of a surface-climbing jet during impact.

The surface-climbing jet can reach speeds ten times higher than impact speed.

Impact conditions control the occurrence of the two-jet phenomenon.

Abstract

It is generally accepted that the Worthington jet occurs when a droplet impacts onto a liquid pool. However, in this experimental study of the impact of viscous droplets onto a less-viscous liquid pool, we identify another jet besides the Worthington jet, forming a two-jet phenomenon. The two jets, a surface-climbing jet and the Worthington jet, may appear successively during one impact event. By carefully tuning the impact condition, we find that the two-jet phenomenon is jointly controlled by the droplet-pool viscosity ratio, the droplet Weber number, and the droplet-pool miscibility. The mechanism of the surface-climbing jet is completely different from that of the Worthington jet: the liquid in the pool climbs along the surface of the droplet and forms a liquid layer which converges at the droplet apex and produces the surface-climbing jet. This surface-climbing jet has a very high speed, i.e., an order of magnitude higher than the droplet impact speed. The effects of the impact speed, droplet viscosity, droplet size, and surface tension on the surface-climbing jet are also analysed. This study does not only provide physical insights into the mechanism of droplet and jet dynamics, but also be helpful in the optimisation of the droplet impact process in many relevant applications.