Experimental study of miscibility of liquids in binary droplet collisions

Abstract

The paper presents experimental research findings on binary collisions of droplets of water, glycerol and surfactant solutions containing fluorophore and a dye to record the position and form of the droplet-droplet interface at the moment of collision. The fluorophore used in the first droplet was Rhodamine B with a concentration of 1,000 μg/l. The gel dye concentration in the second droplet was 0.5 vol%. The integral characteristics of droplet miscibility were investigated by analyzing changes in the fluorescence intensity of liquid when recording laser-induced fluorescence effects. Droplets were illuminated by a system of spotlights and a continuous wave laser. Droplet collisions were captured by a high-speed video camera. The conditions were recorded for the formation of a droplet-droplet interface with different geometry while varying the dimensions, velocities, trajectories and impact angles. After processing the video frames, we plotted collision regime maps, calculated the ratios of free surface areas of liquid before and after droplet breakup, and determined the characteristics of secondary fragments. In particular, fragments of the target and projectile, as well as mixed fragments were distinguished. The fluorescence of two initial droplets and the droplet formed from their coalescence was recorded. We established the characteristics of the near-surface layer deformation of two colliding droplets and rates of liquid miscibility, as well as times of complete miscibility of droplets during coalescence.