Furcated droplet motility on crystalline surfaces

Abstract

Directed liquid motion has been conventionally mediated by functionalizing chemical inhomogeneity or texturing topological anisotropy on target surfaces. Here we show the self-propulsion of droplets that furcated in well-defined directions on piezoelectric single crystals in the absence of any apparent asymmetry or external force. By selecting the crystal plane to interface with the droplets, the thermoelastic–piezoelectric interplay yields intricate electric potential profiles, enabling various forms of self-propulsion including unidirectional, bifurcated and trifurcated. This effect originates from an anisotropic crystalline structure that generates contrasting macroscopic liquid behaviours and is observed with cold/hot and volatile droplets. Intrinsically oriented liquid motions have broad applicability in processes ranging from soft matter engineering, autonomous material delivery and thermal management to biochemical analysis. A droplet falling on a non-wetting plane is expected to randomly roll. Tang et al. uncover that by interfacing piezoelectric crystal plane, droplets self-propel in a furcated direction, a motility fuelled by cross-scale thermo-piezoelectric coupling.