Numerical study of the sliding dynamics of two coalescing droplets on inclined surfaces

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In this paper, we investigate the sliding dynamics of droplets on an inclined surface using two-fluid flow simulations. Droplet motion is resolved using a front-tracking method, with wettability at the contact line modeled by the generalized Navier boundary condition. The effects of droplet coalescence on sliding dynamics are examined by varying the volume difference between two droplets, their lateral offset, and the substrate wettability. The migration behavior is characterized in terms of sliding velocity, interface evolution, and energy conversion. During droplet coalescence, both surface energy and gravitational potential energy decrease, and the released energy is converted into kinetic energy and viscous dissipation. For a single droplet with the same total volume, the motion is driven solely by gravitational potential energy. The additional release of surface energy during coalescence leads to a higher migration velocity of the coalesced droplet. Smaller volume differences promote more symmetric interface reconstruction and more efficient surface energy release, resulting in higher post-coalescence velocities. Lateral offsets induce complex interfacial interactions, reduce energy transfer efficiency, and lead to curved migration trajectories. On hydrophobic substrates, reduced contact-line resistance enables more efficient energy conversion and higher post-coalescence migration velocities than on hydrophilic substrates.

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