Molecular Dynamics Simulation Of The Directional Passive Transport Of Droplets On General Axisymmetric Surfaces

The directed and spontaneous propulsion of droplets on specific solid surfaces has great application prospects,including water harvesting,anti-icing,water purification,cooling,bio-analysis and so on.The self-propelled motion of droplets is attributed to the gradient of surface curvature or surface wettability,which generates a driving force.The strategy of using surface curvature gradient(e.g.conical structure)is generally preferred in experiments due to relatively easy fabrication and the capability of rapidly removing macroscopic droplets,compared to that of a wettability gradient.However,using fixed structures to efficiently transport tiny droplets remains a challenge in engineering,and how surface wettability,surface geometry,and droplet size affect the movement of nanodroplets on axisymmetric surfaces remains unclear.In this paper,Molecular Dynamics Simulation is used to study the directional motion of nanodroplets on axisymmetric surfaces with curvature gradient.The main conclusions are as follows:Firstly,the molecular dynamics model of axisymmetric surface and droplet is established to study the droplet directional movement characteristics.The effects of surface geometry,wettability,droplet size and ambient temperature on droplet directional movement are studied.The results show that the whole movement process of water droplets can be divided into two stages:fast movement stage and deceleration stage.When the axisymmetric surface changes from convex to concave,the transport efficiency of droplet is greatly improved,and the average transport speed of droplet is increased by about 14 times.The droplet velocity changes nonlinearly with the change of surface wettability,and the droplet transport velocity reaches the maximum when the contact angle is about 75°.The size of droplet is positively correlated with the transport speed,and the larger the droplet is,the better the stability of the droplet is in motion.The droplet velocity changes nonlinearly with the simulated temperature.When the system temperature is about 350 K,the droplet transport velocity reaches the maximum.The high temperature will promote the evaporation of water droplets and reduce their stability.The low temperature will limit the thermal motion of water molecules and cause the droplet to move slowly.Secondly,by building a theoretical model,the surface free energy of droplets on general axisymmetric surfaces were studied.By calculating the pressure gradient caused by the curvature gradient in the droplet,an analytical formula for the driving force of droplets on general axisymmetric surfaces has been derived,which is confirmed by MD simulations.The results show that the surface free energy of droplet decreases with the axisymmetric surface from high curvature to low curvature,and the surface energy image is agreed with the motion trajectory of droplet.The theoretical value of the formula fits well with the simulated data.Compared with the formulas in other literatures,the driving force derived in this paper can predict the driving force more accurately.Finally,the retentive force of droplet on cylinder surface is deduced,and the onset of directional motion is studied.The results show that there is a critical radius r_s for the droplet on the curved surface on both micro and macro scales.The droplet begins to move until the radius of droplets is larger than r_s.The dimensionless number Π is proposed as a universal theoretical criterion for the onset of directional droplet motion on axisymmetric surfaces,which offer a general picture of how the surface geometry,surface wettability,and droplet size affect the droplet motion.