Two-Color LIF investigation of mixing during droplet impact onto a thin liquid film

TL;DR

This paper introduces a two-color laser-induced fluorescence technique to study droplet impact and mixing in thin liquid films, providing detailed insights into early-time scalar transport and mixing dynamics.

Contribution

The paper presents a novel 2C-LIF method for simultaneous measurement of film thickness and concentration during droplet impact, including a quantitative analysis of mixing transition and an empirical correlation.

Findings

Identified transition from convection to diffusion in mixing.

Developed an empirical correlation for scalar mixing evolution.

Demonstrated method applicability to ethanol-water films.

Abstract

A two-color laser-induced fluorescence (2C-LIF) technique is presented for investigating droplet impact on thin liquid films, enabling simultaneous, spatially and temporally resolved measurements of film thickness and scalar concentration. The method is applied to water droplets impacting thin liquid films over a range of Reynolds numbers, Weber numbers and dimensionless film thicknesses, providing direct access to early-time mixing processes during impact. To quantify scalar transport within the liquid film, the reconstructed concentration fields are evaluated using a coefficient-of-variaton (CV) approach, providing a quantitative measure of mixture homogeneity. This enables identification of the transition from inertia-dominated convective transport to diffusion-controlled mixing. Based on this analysis, an empirical correlation describing the evolution of CV as a function of Reynolds number and film thickness is formulated. Finally, the applicability of the 2C-LIF method is demonstrated for binary ethanol-water films, where additional transport mechanisms influence and modify the mixing dynamics.