A theoretical analysis was performed for the head-on collision of two identical droplets in a gaseous environment, with the attendant bouncing and coalescence outcomes, for situations in which the extent of droplet deformation upon collision is comparable to the original droplet radius, corresponding to O(1)-O(10) of the droplet Weber number. The model embodies the essential physics that describes the substantial amount of droplet deformation, the viscous loss through droplet internal motion induced by the deformation, the dynamics and rarefied nature of the gas film between the interfaces of the colliding droplets, and the potential destruction and thereby merging of these interfaces due to the van der Waals attraction force. The theoretical model was applied to investigate collisions involving hydrocarbon and water droplets at sub- and superatmospheric pressures. The results agree well with previous experimental observations in that as the Weber number increases in the range of O(1)-O(10), collision of hydrocarbon droplets at one atmospheric pressure results in the nonmonotonic coalescence-bouncing-coalescence transition, that while bouncing is absent for water droplets at atmospheric pressure, it occurs at higher pressures, and that while bouncing is observed for hydrocarbon droplets at atmospheric pressure, it is absent at lower pressures.
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