Impacts of liquid droplets with another stationary droplet resting on a surface are important basic processes in many applications such as agricultural sprays, spray cooling, and inkjet printing. We investigated the head-on collision of unequal-size droplets of the same liquid on wetting surfaces both experimentally and theoretically at different size ratios and low-impact Weber numbers (We). A series of high-speed camera images showing representative sequences of collision processes for greatly different size ratios are analyzed. Different collision outcomes such as coalescence, bouncing, and partial coalescence–partial bouncing are analyzed thoroughly. Four different stages are identified for characterizing the complete bouncing process during the impact of unequal-size droplets on a solid surface. Subsequently, an analytical model based on energy balance is developed to calculate the maximum spread diameter and restitution coefficient of falling droplets, and compared with experimental data, satisfactory qualitative agreements are obtained. Results show that the dimensionless maximum spread diameter of falling droplets depends weakly on We and it is small for a higher size ratio. The restitution coefficient does not change significantly at a higher size ratio at a fixed We despite more viscous dissipation in bigger sessile droplets and it scales with We−1/2.
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