Non-frontal coalescence jumping of a rolling and a static droplet

The possibility of coalescence between rolling and static droplets is greater than that of static droplets. Due to the randomness of the velocity direction of rolling droplets and the position of static droplets, rolling and static droplets are more prone to non-frontal coalescence. Most studies focused on static droplets or head-on coalescence jumping, while non-frontal coalescence is poorly understood. In this work, we experimentally and numerically study the head-on and non-frontal coalescence of a rolling droplet and a static one. Furthermore, to measure the jumping characteristics of merged droplets, a “single camera defocusing imaging measurement” (SCDIM) method was proposed to obtain a 3D trajectory of the jumping droplet. For non-frontal coalescence, the morphological evolution of the merged droplet rotates counterclockwise during the coalescence process. The direction of horizontal jumping velocity deviates towards the rolling droplet. The non-frontal deviation and initial velocity determine whether rotation can occur and its strength. The higher initial velocity has a negative impact on the jumping velocity in the Z direction with the increase of non-frontal deviation, while the deviation angle could be negligible. As the non-frontal deviation ratio increases, the dominant mechanism of coalescence jumping is gradually transferred from the impact effect to the contraction of the merged droplets. Different from jumping velocity in the Z direction, the horizontal jumping velocity is approximately proportional to the dimensionless initial velocity.