Collision-induced spin-to-orbital angular momentum conversion between two spinning droplets of unequal sizes

This paper focuses on the numerical study on the phenomenon and mechanism of spin-to-orbital angular momentum conversion for the head-on collision between a spinning droplet and a non-spinning droplet of unequal sizes. The droplet deformation process was phenomenologically described, where the gas film suppresses the rotating flow inside the spinning droplet and, in turn, rotates the non-spinning droplet through its shearing effects. The gas film tends to be curved by the size disparity, leading to a more complex gas film flow. The hysteresis mechanism of spin-to-orbital angular momentum conversion was analyzed. The decrease in spin angular momentum and increase in orbital angular momentum have an upper limit as increasing the size ratio, which is attributed to the droplet deformation that is similar to a droplet impacting on a liquid film. In addition, a wider range of size ratio, Weber number, and droplet spin angular speed were discussed, where some numerical findings are significant to the angular momentum conversion coefficient modeling.