Molecular Dynamics Study On The Effect Of Surface Wettability On Line Tension Of Nano-droplets

Surface wetting is a common physical phenomenon.However,the wetting behavior of droplets may be affected by the shape and roughness of the solid surface at the nanoscale,which may produce a completely different variation law from that at the macroscopic scale.At the same time,as a key parameter affecting the surface wettability at nanometer scale,there is no unified understanding of the size,direction and change rule of line tension.Therefore,it is of great significance for study on surface wetting behavior and line tension effect at micro and nano scale to the development of micro nano science and technology,such as micro nano electromechanical system,biochip,bionic medical devices,etc.In this paper,molecular dynamics method is used to simulate the wetting state of nano droplets on convex and concave solid surfaces.The effects of different solid surface curvature,solid-liquid interaction and droplet size on the wettability of droplets on solid surfaces are studied.The results show that on the convex surface,the contact angles of droplets with different sizes and solid-liquid interactions increase with the increase of radius ratio coefficient k,and the curvature radius of three-phase contact line decreases with the increase of k.On the concave surface,with the increase of radius ratio coefficient k,the contact angle of droplets with different sizes increases first and then decreases,and the surface morphology of droplets gradually changes from convex to concave.However,the contact angles decrease with the increase of droplet size.Based on the force analysis of the three-phase contact line of the drops,the modified Young equation with the line tension term on the solid surface of different shapes is derived,and the numerical value of the line tension is calculated by combining the simulation results.The surface curvature and the interaction between the solid and the liquid will affect the line tension.On the convex surface,the line tension can be divided into two stages:when the droplet size is far smaller than the solid surface size,the line tension is less affected by the curvature of the solid surface,and basically remains unchanged.When the droplet size is similar to the solid surface size,the line tension increases with the increase of k on the hydrophilic surface,decreases with the increase of k on the neutral surface,and remains constant on the hydrophobic surface.On the concave surface,when the solid surface is hydrophilic,the line tension increases first and then keeps constant with the increase of k.In order to investigate the mechanism of the line tension effect at nanoscale,the number density distribution and the radial distribution function of the liquid atom near the wall were calculated.On the convex surface,the number of liquid atoms near the wall increases with the increase of radius ratio coefficient k and potential energy parameter ?,but the atomic arrangement structure is basically unchanged.On the concave surface,when the interaction between the solid and the liquid is strong,the number of liquid atoms near the wall increases with the increase of radius ratio coefficient k,and the arrangement structure of liquid atoms is also affected by the curvature of the solid surface.On the solid surface of different shapes,it is shown the linear relationship growth between the value of line tension and the first peak value of numerical density near the wall.That is to say,the more the absolute number of liquid atoms near the wall,the greater the value of line tension.