Size effect on the transition from coalescence to bouncing regime in binary droplet collision

In the present study, the effect of droplet size on binary droplet collision was investigated experimentally by using decane, dodecane, tetradecane, and hexadecane as working fluids. Two identical droplets were generated individually by piezoelectric droplet generators and the colliding processes of droplets were recorded by a high speed camera. For plotting regime diagrams of droplet collision with different diameters, a Matlab program based on the Hough transform method was applied to calculate the corresponding impact parameter (B) and Weber number (We) for each collision event. The results showed that the transition We (denoted by We_c) between coalescence and reflexive separation regime followed the model derived by a previous study which was based on energy conservation. This implies We_c may decrease as the colliding droplets become larger due to the linear relation with Oh. Furthermore, the regime of stretching separation was found for all alkanes. These results did not reveal any specific effect of droplet size. When the droplet diameter was increased from 0.3 mm to 0.6 mm, however, the bouncing regime was ostensibly enlarged. The enlarging bouncing regime thus shrinks the area of coalescence regime upon all alkanes. To understand the mechanism, the Ohnesorge number (Oh) and Capillary number (Ca) were used for plots of We-Oh and Ca-Oh for further analysis. In the case of head-on collision, the We-Oh plot showed that We_b may decrease as Oh increase, while Ca_b may increase linearly with Oh at fixed droplet size in the Ca-Oh plot. Here We_b and Ca_b denote the transition Weber number and Capillary number, respectively, from bouncing to coalescence regime. These results infer that, in addition to the competition between impact inertia and surface force, viscous force should be considered in order to account for the transition between the coalescence and bouncing regimes.