Intense deformation and fragmentation of two droplet collision at high Weber numbers

Droplet collision is common in nature and industrial processes, while understanding of the phenomena at high Weber numbers is not as comprehensive as the one at low Weber numbers. We experimentally investigate the collision dynamics of two droplets at high We ranging from 200 to 2000 by adopting pure water and pure ethanol droplets. Four distinct droplet collision regimes in the order of increasing deformation and fragmentation are reflection separation, finger separation, stretching separation and splashing. Due to the larger inertia force and lower viscous force, water droplet collision is easier to form finger separation and splashing regimes than ethanol droplets under the same We. The reinforcement of viscous force and surface tension weakens droplet deformation and fragmentation, and further changes droplet collision regime. Due to the viscous dissipation ratio of droplet at the maximum spread moment as a function of Ohnesorge number ( Oh ), the model of the maximum spread factor of coalesced droplet is different under various Oh. For a low Oh, a new formula of the maximum spread factor is proposed by optimizing the maximum spread time in the viscous dissipation model based on energy conservation, which agrees well with the experiment. For a high Oh, the formula of the maximum spread factor is developed by using the viscous dissipation ratio, and it is consistent with the experiment as the viscous dissipation ratio is 0.8.