Nonaxisymmetric flow characteristics in head-on collision of spinning droplets

The effects of spinning motion on the bouncing and coalescence between a spinning droplet and a nonspinning droplet undergoing head-on collision were numerically studied by using a volume-of-fluid method. A prominent discovery is that the spinning droplet can induce significant nonaxisymmetric flow features for the head-on collision of equal-size droplets composed of the same liquid. Specifically, a nonaxisymmetric bouncing was observed, and it is caused by the conversion of the spinning angular momentum into the orbital angular momentum. This process is accompanied by the rotational kinetic energy loss due to the interaction between the rotational and radial flows of the droplets. A nonaxisymmetric internal flow and a delayed separation after temporary coalescence were also observed, and they are caused by the enhanced interface oscillation and internal-flow-induced viscous dissipation. The spinning motion can also promote the mass interminglement of droplets, because the locally nonuniform mass exchange occurs at the early collision stage by nonaxisymmetric flow and is further stretched along the filament at later collision stages. In addition, it is found that the nonaxisymmetric flow features increase with increasing the orthogonality of the initial translational motion and the spinning motion of droplets.