Criterion for droplet collision behavior based on qualitative analysis and experiments

Binary collisions of equal-sized water droplets were systematically investigated to elucidate the transition between rebound and coalescence. The effects of impact velocity, droplet diameter, and eccentricity ratio on collision outcomes were examined experimentally using high-speed imaging, and a theoretical model was developed to predict the critical transition velocity. The results show that, for head-on collisions at a fixed droplet size, increasing impact velocity promotes coalescence. Increasing droplet diameter reduces the critical velocity separating rebound from coalescence. For eccentric collisions, the velocity required for coalescence increases with eccentricity ratio. The theoretical predictions agree well with the experimental results, demonstrating that the proposed model can effectively predict collision outcomes. These findings provide a useful basis for controlling droplet collision behavior in droplet manipulation, sensing, and microreactor applications.