Oblique bouncing of a droplet from a non-slip boundary: computational realization and application of self-spin droplets

Recent studies have demonstrated the significance of droplet spinning motion on droplet collision outcomes and raised the question of generating spinning droplets in an experiment. The present work proposed the idea of making an initially non-spinning droplet self-spin by its oblique impact on a non-slip boundary. We computationally demonstrated the feasibility of the idea and exploited its application in the binary collision of spinning droplets. Specifically, a parametric study was conducted to investigate the effects of impact velocity, impact angle, and liquid viscosity on the spinning droplet. The results showed that a larger impact velocity or a larger liquid viscosity causes an increased angular speed of the spinning droplet, however increasing impact angle leads to a nonmonotonic variation of the angular speed. The oblique-impact-induced droplet spin is attributed to the asymmetric gas film flow, asymmetric lubrication pressure, and shear stress within the gas film region, leading to the earlier bouncing motion on one side to rotate the droplet. In addition, the synergetic effects of the droplet stretching length and the intensity of asymmetric lubrication pressure account for the variation of droplet spin angular speed and droplet separation. As a preliminary application of the proposed idea, the feasible technical approach of realizing the binary collision of spinning droplets was computationally realized.