Pinning effect in droplet self-driving and its reduction mechanism by monolayer graphene

The mechanism of pinning effect induced by the surface defects remains unclear. This paper provides a novel understanding of pinning effect on the droplet self-driving at nanoscale, and further reports the pinning-reducing mechanism of monolayer graphene covered on the surface defects. [Display omitted] •The pinning behaviors of liquid droplets on solid surfaces are reassessed at nanoscale in droplet self-driving processes.•This paper provides a pinning-reducing mechanism of monolayer graphene covered on surface defect.•The influences of the shape and size of defects on the pinning-induced damping force of droplet are revealed.•An extra damping force appears after the droplet passes across the defects due to the pinning-induced deformation of a droplet. High-speed and precise self-driving transport of droplet is of great significance in self-cleaning, drug delivery, water harvesting and microfluidic chip. However, the pinning effect induced by surface defects usually restrain the self-driving of droplets on functionalized surfaces. In this paper, the mechanism of pinning effect in droplet self-driving is revealed based on the molecular dynamic simulations for motion behaviors of liquid mercury droplet on copper substrate with different shaped defects. We found that the monolayer graphene covered on the defected solid surface can remarkably reduce the pinning effect in droplet self-driving processes. The results show that a large damping force will appear when the droplet initially approaches and finally moves away from the defects on pure copper (Cu) substrate, whereas a tiny damping force can be observed for the droplet moving across the defects covered by a monolayer graphene. Particularly, a consequent extra-damping force appears because the nano-configuration of the mercury droplet is changed after passing the defects on pure Cu substrate induced by the large deformation in pinning process. The findings will widen the understanding of pinning effect at nanoscale and provide sustainable theoretical support for reducing the pining effect in droplet transport.