Ultrafast spontaneous driving of water droplets on monolayer graphene-covered gradient nanopillared surfaces

[Display omitted] •A novel design of gradient nanopillared surface covered by a monolayer graphene is proposed to realize ultrafast self-driving of water droplets.•The droplet moves spontaneously at a ultrahigh speed of 75.7 m/s (272.52 km/h) along the graphene-pillared texture.•The monolayer graphene can significantly reduce the pinning effect in the self-driving process.•A wetting transition from hydrophobic to hydrophilic leads to an opposite driving direction attributed to the different adsorbed configuration of monolayer graphene. Spontaneous driving of liquid droplets on textured substrate is of great importance in understanding some biological processes and designing numerous functional surfaces. However, the pinning force usually reduces the velocity and moving stability of liquid droplets in self-driving process. In this paper, a novel design of nanopillared surface with a gradient density of structural pillar matrix covered by a monolayer graphene is proposed to realize ultrafast self-driving of water droplets. Classical molecular dynamics results provide a deep insight into the enhanced spontaneous driving behaviors, and reveal the underlying mechanism of the decrease in pinning force by a monolayer graphene. The droplet can move spontaneously at a ultrahigh speed of 75.7 m/s (272.52 km/h) from sparsest to densest regions of pillars while a wettability gradient is created by the gradient distribution density of pillar matrix relying on the wetting transparency of monolayer graphene. In particular, the gradient short pillared texture triggers an opposite self-driving regularity in which the water droplet moves from densest to sparsest regions of pillars, intrinsically because the gradient short-pillared surface leads to a wetting transition from hydrophobic to hydrophilic since the monolayer graphene can be adsorbed into the sparsest short-pillared texture. Furthermore, the influence mechanism of the pillar shape and pillar matrix on the self-driving behaviors of the water nanodroplet is clarified. The findings will theoretically direct the design of graphene-covered functional surfaces for spontaneous driving of liquid droplets.