Dynamic Wetting On Lyophilic Structured Surface And Effects Of External Electric Field On Wetting

Dynamic wetting on solid substrates occurs in numberous natural and industrial processes,in which the wetting kinetics is very significant in understanding and controlling these availiable applications.Studies have shown that the wetting kinetics can be efficiently modified through fabricating mico/nano structures on solid surface or applying an external physical field.This study presents the effects of lyophilic mico/nano structures on dynamic wetting as well as the effects of the external electric field on wetting kinetics.The dynamic spreading of droplets on lyophilic micropillar-arrayed surfaces was studied through the droplet-spreading method.The effects of the lyophilic microstructures on early spreading were investigated,and the results show that the early spreading of droplets was only affected by the solid fraction.A semi-theoretical model based on the global energetic equation was provided to model the early spreading on lyophilic micropillar-arrayed surfaces,in which the energy input and energy dissipation were discussed.The results predicted by the semi-theoretical model agreed well with that of the experimental results.The lyophilic nanopillar-arrayed surfaces were then used to investigate the nano-sacle effects on the droplet spreading using molecular dynamic simulations.The results show that the droplet spreading is hindered by the lyophilic nanopillars.By tracing the molecular trajectories during the spreading,we found that the molecules in fringe film were mainly provided by the liquid in the vincity of contact line,thus hindered the droplet spreading.Besides,the energy barrier introduced by the interaction between water molecules and nanopillars increases with the enhanced solid-liquid interaction,resulting in the hinder effects on the propagation of the fringe film.The wettability of smooth and nanopillar-arrayed surfaces with an external electric field was then studied using molecular dynamic simulations.The method for calculating system's pressure with external electric field was provided.The method was then applied to analyze the effects of the external electric field on the interfacal tensions.Beside of the interfacial tensions,due to the electro force,the contact angle is also affected by the solid-liquid interaction.Finally,the effects of the applied electric fields on the viscosity of water,a common working fluid in daily life and industry,were examined.We found that the viscosity becomes anisotropic in an electric field.Based on Eyring theory,a microscopic model was also provided to describe the liquid viscosity with external electric field.In this model,the effects of van der Waals interaction,dipole-dipole interaction,and hydrogen bond interaction were analyzed to reveal the microscopic mechanism of electric field on liquid viscosity.