Moving Behavior of Nanodroplets on Wedge-Shaped Functional Surfaces

Surface functionalization has important application prospects in many aspects, for example, anti-drag, anti-pollution, anti-icing, self-cleaning, reversible adhesion, water collection, and so forth. Recently, a new technique of functional surface was proposed, in which a droplet can be activated to move from the narrow end of a wedge-shaped hydrophilic region imbedded in a hydrophobic surface to the wide end. Systematic molecular dynamics (MD) simulations and theoretical analysis are carried out in this paper in order to understand the motion mechanism, and more systematic designs for advanced functional surfaces are further proposed. It is found that the potential energy of the droplet decreases continuously as a function of distance when the droplet moves from the narrow end of the wedge-shaped track to the wide end and the corresponding potential energy achieves the minimum when the droplet completely enters the hydrophilic region. Effects of the wedge angle of hydrophilic area and the size of the droplet on the moving behavior are systematically investigated. It is interesting to find that both factors would show significant influence on the moving behavior of droplets, which is well consistent with the experimental observations. Theoretical analysis is further carried out, where driving force of droplet transport is well achieved. All the MD simulation results and experimental observations can be well explained by the nonlinearly changed driving force. Finally, several new designs of functional surfaces for controlling droplet transport, including droplet turning, pinning, accelerating, gathering, and moving path selection, are suggested, the feasibilities of which are further verified and discussed. The results in this paper would be helpful for the design of advanced functional surfaces to manipulate droplets in real applications.