Self-driving Behaviors Of Liquid Droplets On Single-layer Graphene Coating

With the rapid development of micro-nanofluidic devices,the research on droplets being rectified to achieve excellent functions has made extensive progress.the principle of the directional driving of droplets can be applied to the development of droplet rectifiers.making the droplet rectifier generate enough driving force,and realize the directional transport of droplets without external energy.In this paper,Self-driving behaviors of liquid droplets on graphene coating composite substrate was simulated by molecular dynamics method.A few Several substrate models that could drive droplets was constructed,the self-driving behaviors of liquid droplets on graphene coating was studied in the vacuum and gas environment.And explored how the the surface morphology of the substrate and the change of the gas environment influences the self-driving effect of droplets,studied the mechanism of self-driving of droplets,The research results will provide theoretical guidance for the directional fluid transportations based on graphene-coated surfaces and nanochannels.Firstly,the self-driving behaviors of water droplets on graphene-coated surfaces with Wedge-shaped hydrophilic and hydrophobic region was studied.the results showed that water droplets can move to the direction of the width of the hydrophilic region gradually increasing spontaneously.when the wedge angle of the hydrophilic area gradually increases,the maximun velocity of the water droplet during the acceleration phase is greater,but the speed of water droplet drops faster during the deceleration phase.water droplets will also enter the surface of the hydrophilic area completely in a shorter displacement place.the maximum displacement that can be driven also gradually decreases.the maximum displacement of water droplet drives also gradually decreases.when the diameter of the water droplet gradually increases,the maximun velocity of the water droplet during the acceleration phase gradually decreases,but water droplets will also enter the surface of the hydrophilic area completely in a wider width of hydrophilic area.the maximum displacement of water droplet drives also gradually increases.during the whole motion process.theinteraction potential energy between water droplets and the entire composite substrate gradually decreases.the self-driving process of water droplets on graphene-coated surfaces with Wedge-shaped hydrophilic and hydrophobic region is a gradual conversion process from high potential energy to low potential energy.Secondly,self-driving behavior of water droplets on the graphene coating surface with wettability gradient was studied.the results showed the water droplet on the graphene coating surface with wettability gradient spontaneous move to the direction for wettability gradient increases by the driving force due to the difference in wettability between the front and back surfaces.the speed of water droplets fluctuates greatly during the movement.when the water drops move to the edge of the composite substrate.the speed of the water droplet decreases quickly due to the boundary effect.the interaction potential energy between the water droplet and the composite substrate continues to decrease during the driving process,the self-driving process of water droplets on the graphene coating composite substrate is gradual conversion process from high potential energy to low potential energy.Finally,Self-driving behaviors of liquid mercury droplet on graphene-copper composite substrate in gas environment was studied,the results showed the existence of nitrogen can hinder the self-driving of liquid droplet inherently attributed to the change of the pressures inside the droplet at the gas environment.Both the maximum speed and displacement of the droplet are gradually decreased with increasing the density of nitrogen.The interaction force between the droplets and monolayer graphene increases with the channel width,which plays the dominant role in driving the liquid droplets along the groove.The interaction force between mercury droplets and nitrogen molecules hinders the movement of droplet.the interfacial the potential energy among liquid droplet,gas nitrogen and graphene-copper substrate are considered to interpret the self-driving mechanism of mercury droplets in gas environment.The solid-liquid interfacial energy decreases with the displacement until the droplet is adsorbed into the groove to be sufficient contacting state.The gas-liquid interfacial energy turn when the velocity of water droplet decreases sharply.The acceleration and deceleration of mercury droplet in the gas environment is mainlydetermined by the competition between the change in the potential energy of liquid droplet and the interfacial energy of solid-liquid and gas-liquid.The solid-gas interfacial energy has little effect on the motion of droplet.