Molecular Dynamics Investigation On Wetting Transition And Heat Transfer Characteristics Of Microstructured Surfaces

The wetting transition and heat transfer phenomena on a microstructured surface is significant to many industrial applications such as self cleaning,anti-icing,electrostatic printing,lubrication drag reduction,electronic device cooling,and condensition as well as heat transfer.When the dynamic wetting and heat transfer phenomena can be controlled by external fields,it further promotes the application of microstructured surfaces.Based on the exprimental and theoretical researches,at the molecular level,it is difficult to reveal the dynamic wetting and heat transfer phenomena on microstructured surfaces subjected to external fields.As a result,the related mechanism at nanoscale is an open question.In this paper,for a nanodroplet on a nanostructured surface subjected to external fields,its dynamic wetting and heat transfer phenomena are investigated by molecular dynamics simulations.The contents include three parts:(1)the electic-field induced reversible wetting transition between Cassie and Wenzel states on nanostructured surfaces;(2)the statics and dynamics of nanodroplets on nanostructured surfaces subjected to electric and/or thermal fields;(3)evaporation and explosive boiling on nanostructured surfaces.The study aims to reveal new mechanisms contained in the dynamic wetting and heat transfer phenomena,and propose effective methods to control reversible wetting transtion and heat transfer behaviors of droplets.The study will provide a solid theoretical basis for optimization and development of advanced technologies.The simulation results of electric-field induced reversible wetting transtion show that there exists an energy barrier on the energy pathway of Cassie-Wenzel wetting transition,this energy barrier hinders the spontaneous Cassie-Wenzel wetting transition.When exerted an electric field,the energy barrier is removed by the electric field,such that the Cassie-Wenzel wetting transition occurs.After the Cassie state translates to the Wenzel state,the electric field is removed.When the energy of Wenzel state exceeds that of Cassie state,the Wenzel-Cassie wetting transition can spontaneously occur.However,when the energy of Cassie state exceeds that of Wenzel state,the Wenzel-Cassie wetting transition cannot spontaneously occur.Based on the spontaneous reversible wetting transiton,the nanoparticle trapped into the asperities of nanostructures can be picked up by the liquid film.When the amphilic Janus nanoparticle is added to the liquid film,it facilitates the pick-up of hydrophobic nanoparticle.With the nanoscale effect,the statics and dynamics of nanodroplets on nanostructured surfaces differ from that of microdroplets on nanostructured surfaccs.When exerted external fields,the phenomeona are more significant.Under an electric field,the static contact angle and spreading exponent change with the direction and strength of electric fields.Under a thermal field,the static contact angle decreases with an increase in substrate temperature.Accordingly,the spreading exponent also increases with the increasing substrate temperature.When the electric and thermal fields simulataneously are applied to a nanodroplet,the sensitivity of the nanodroplet on the electric field are weakened by the evaporation of the nanodroplet.For a nanoscale liquid film on a high temperature nano structured surface,the phase change mode of the nanoscale liquid film changes with initial wetting states,film thicknesses,intrinsic wettabilities and aspect ratios.At a constant substrate temperature,the explosive boiling occurs for the thick liquid film,whereas the evaporation takes place for the thin liquid film.Because the thermal resistance is lower for the thicker liquid film.Moreover,the onset temperature of explosive boiling decreases with the enhanced wettability,especially as the initial wetting transition of liquid films is triggerred.As a result,the explosive boling easily takes place for the film in the Wenzel state.