Velocity-Switched Droplet Rebound Direction on Anisotropic Superhydrophobic Surfaces

Droplet well-controlled directional motion being an essential function has attracted much interest in academic and industrial applications, such as self-cleaning, micro-/nano-electro-mechanical systems, drug delivery, and heat-transferring. Conventional understanding has it that a droplet impacted on an anisotropic surface tends to bounce along the microstructural direction, which is mainly dictated by surface properties rather than initial conditions. In contrast to previous findings, it demonstrates that the direction of a droplet's rebound on an anisotropic surface can be switched by designing the initial impacting velocity. With an increase in impacting height from 2 to 10 cm, the droplet successively shows a backward, vertical, and forward motion on anisotropic surfaces. Theoretical demonstrations establish that the transition of droplet bouncing on the anisotropic surface is related to its dynamic wettability during impacting process. Characterized by the liquid-solid interaction, it is demonstrated that the contact state at small and large impacting heights induces an opposite resultant force in microstructures. Furthermore, energy balance analysis reveals that the energy conversion efficiency of backward motion is almost three times as that of traditional bouncing. This work, including experiments, theoretical models, and energy balance analysis provides insight view in droplet motions on the anisotropic surfaces and opens a new way for the droplet transport.