Experimental study on two consecutive droplets impacting onto an inclined solid surface

The present study is concerned with the experimental impingement of two consecutive droplets on an inclined solid surface. Attention ismainly paid to the effects of impingement timing with various oblique angles (Phi) of the surface on the impact phenomena, which mainly affect the maximum droplet spreading diameter. The investigation considers four impingement scenarios differentiated by impingement timing, namely Case 1: single-droplet impingement; Case 2 of Delta t(1): the moment when the leading droplet starts spreading along the oblique surface; Case 3 of Delta t(2): the moment when the leading droplet reaches its maximum spreading; and Case 4 of Delta t(3): the moment when the leading droplet starts retracting. It is observed that deformation behavior of two successive droplets impacting on the inclined surface experiences a complex asymmetric morphology evolution due to the enhancement of gravity effect and various conditions of the impingement timing. The merged droplet becomes slenderwith increasing oblique surface angle in the final steady shape, causing the decrease in the value of front and back contact angles. The impingement timing has a significant influence on the change of the maximum height of the merged droplet. The coalesced droplet spreads to themaximum dimensionless width diameter at Delta t = Delta t(2) and the oblique angle of Phi = 45 degrees, but reaches themaximum dimensionless height for Delta t = Delta t(2) at Phi = 30 degrees. The front contact angles converge to a fixed value eventually for all conditions of impingement timing, and the values become lower with the increasing surface inclination.