Convection velocities in droplets before and after their collisions

High-speed video recording was used together with particle image velocimetry with tracer particles of titanium dioxide to study the characteristics of shifting and mixing of liquid layers in the coalescence, disruption, and separation of droplets. Convection velocities (Uc) were determined in droplets before and after their collisions. Vortex contours of different sizes and in different cross sections of droplets were derived. The average values of Uc were calculated. The effect of a group of factors on Uc was investigated. These factors include the relative velocity before the collision Urel, the ratio of droplet sizes Δ, the Weber number, the impact angle, and the rheological properties of liquid. Uc changed most significantly (more than threefold) from variations in Δ, Urel, and We. When varying the dimensionless linear interaction parameter B, Uc changed by 20%–40%. In disruption, Uc increased more than eightfold. In separation and coalescence, they increased by a factor of 10 and 11, respectively. The convection velocity was maximum after the collision. Then, 0.29–0.37 s after the collision, it fell to the values corresponding to a free-falling droplet. An increase in the convection velocity was compared for different droplet sizes and velocities before and after their interaction. Mathematical equations were obtained to predict the convection velocities affected by several investigated factors, taken separately or in combination. For the first time, the ranges of Uc were found, and the effect of a wide group of parameters (geometric sizes and velocities of droplets, rheological characteristics) on the velocities of convective flows was identified.