Mixing dynamics of droplets impacted on liquid films*

Droplet impact is essential for many industrial and natural phenomena, including 2D/3D inkjet printing, spray coating, fuel injection in engines, and rain droplets falling on the ocean surface. In many of these systems, post-coalescence mixing of impacted droplets with the liquid film controls the heat-mass transfer and chemical reactions; hence, it is critical for many of these processes. Recognizing such criticality, we present an experimental investigation of mixing dynamics during droplet impacts on liquid films. Experimental diagnostics used for this study include a combination of shadowgraphy, particle image velocimetry (PIV), and planar laser-induced fluorescence (PLIF). Simultaneous PIV and PLIF characterize the vorticity fields and scalar mixing processes. PIV data shows how toroidal vortices are generated after the droplet coalesces with the impacted film surface and translates through the film. From PLIF images, we will quantify the concentration distribution of the droplet liquid and the mixing dynamics using statistical measures. Additionally, simultaneous shadowgraphy and PLIF images will show how the cavity geometry strongly affects the vortex dynamics. We will primarily discuss the evolution and correlation between the vortical structure and the mixing dynamics.