Impact of Droplets on Inclined Flowing Liquid Films

TL;DR

This study experimentally investigates how droplets impact inclined flowing liquid films, revealing complex phenomena influenced by flow conditions, with implications for understanding droplet behavior in dynamic liquid environments.

Contribution

It provides a detailed experimental analysis of droplet impact phenomena on inclined flowing films, including the creation of an impact regime map and insights into lubrication forces.

Findings

Flow rate increases lubrication force and droplet bouncing.

Droplet impact phenomena are more complex on flowing films.

An impact regime map categorizes different impact outcomes.

Abstract

The impact of droplets on an inclined falling liquid film is studied experimentally using high-speed imaging. The falling film is created on a flat substrate with controllable thicknesses and flow rates. Droplets with different sizes and speeds are used to study the impact process under various Ohnesorge and Weber numbers, and film Reynolds numbers. A number of phenomena associated with droplet impact are identified and analysed, such as bouncing, partial coalescence, total coalescence, and splashing. The effects of droplet size, speed, as well the film flow rate are studied culminating in the generation of an impact regime map. The analysis of the lubrication force acted on the droplet via the gas layer shows that a higher flow rate in the liquid film produces a larger lubrication force, slows down the drainage process, and increases the probability of droplet bouncing. Our results demonstrate that the flowing film has a profound effect on the droplet impact process and associated phenomena, which are markedly more complex than those accompanying impact on initially quiescent films.