Simulating the collision of a moving droplet against a moving particle: impact of Bond number, wettability, size ratio, and eccentricity

This paper presents a direct numerical simulation for the collision of a moving droplet against a moving particle under gravity, based on the pseudopotential lattice Boltzmann model. The effects of Bond number (Bo), particle surface wettability, particle-droplet size ratio (α), and eccentricity ratio (B) on the collision processes are investigated comprehensively. Six findings are reported and analyzed for the first time: (1) an agglomeration process is observed for the collision with a very small Bond number. During the agglomeration process, the vertical velocity of the particle will experience a deceleration, and the deceleration will become weak against the increase in the Bond number. (2) The wettability will influence the variation of the vertical velocity of the moving particle remarkably. The vertical velocity of the neutral particle is nearly linearly accelerated, but the lyophilic particle experiences an obvious deceleration. In addition, the velocity history of the lyophobic particle shows a nonlinear acceleration. (3) The increase in the particle-droplet size ratio will postpone the emergence of the deceleration process. Therefore, the appearance of the peak vertical velocity is delayed against the increase in the particle-droplet size ratio. (4) For different eccentricity ratios, the differences of the velocities (e.g., the horizontal, vertical, and angular velocity) are very small in the beginning of collision, while a big difference appears with time elapses. Besides, the variation of velocities becomes very obvious. (5) There is a critical value for B, where the horizontal velocity, vertical velocity, and angular velocity of the particle investigated in the work all will reach their maximum values. (6) A rebound regime is observed when a moving droplet collides vertically against a moving particle. In the available literature, a rebound regime was observed only when a droplet colliding against a fixed particle, but never for a vertically moving particle. The present research reveals when a rebound process will appear. The finding here may shed some light on the mechanism of the collision of a moving droplet against a moving particle.