Bouncing droplets on solid surfaces is of great significance in diversified applications such as anti-icing and self-cleaning. It is important to establish a unified model to predict whether an impacting droplet can rebound from a surface or not. This work focuses on the rebound dynamic of a droplet impacting a hydrophobic surface via theoretical methods. Based on energy conservation, a new theoretical model to predict the rebound behavior of an impacting droplet is established. For an ideal surface, the contact angle hysteresis Δθ can be ignored and the rebound condition is θ ≥ θc,i, where θ is the equilibrium contact angle and θc,i is the critical rebounding contact angle (CRCA) of an ideal surface. For a real surface, Δθ is considered and the rebound condition is θr ≥ θc,r, where θr is the receding contact angle and θc,r is CRCA of a real surface. Especially, when Δθ is not large enough, the rebound condition for a real surface can be expressed as θr ≥ θc,i. This work is the first to establish the theoretical model considering both the energy dissipation throughout the impact process and the contact angle hysteresis, which shows a higher consistency with the previous works.
When a droplet off-center impacts a macro-ridge, the contact time increases with off-center distance [Formula: see text], which are closely related to two mechanisms, i.e., the redistribution of liquid volume and the asymmetry of the liquid film. Therefore, changing the asymmetry of the liquid film may provide fundamental inspiration for the efficient control of the contact time. Using lattice Boltzmann method simulations, the dynamics of a droplet off-center impacting a ridge on a superhydrophobic surface are explored to demonstrate the feasibility of reducing contact time by changing the asymmetry of the liquid film, which is changed by manipulating the inclination of the ridge. For positive off-center impact [Formula: see text], the contact time decreases with the increase in the inclined angle as increasing the inclination can decrease the asymmetry of the liquid film. For negative off-center impact [Formula: see text], tilting the ridge can further reduce the asymmetry of the liquid film to a limit, and its influence can be ignored at [Formula: see text], leading to the contact time decreasing more significantly compared with that for [Formula: see text]. On this basis, a quantitative relationship of contact time for a droplet off-center impacting an inclined ridge is established. This work provides fundamental and practical inspiration for the efficient reduction of contact time for off-center impacts.
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