Molcecular Dynamics Simulation Of Wetting And Evaporation Of Nanomeniscus System

With the rapid development of micro-electro-mechanical systems(MEMS),their feature sizes decrease and the level of device integration becomes higher.Therefore,heat transfer efficiency of micro/nano scale gets worse and worse.The efficient cooling technology for electron devices becomes research hotspot.Micro-spray cooling technology is one of the most effective and common methods of cooling the MEMS.During progress of micro spray cooling,wetting and evaporation activities of meniscus system in micro/nano scale have a great impact on cooling efficiency.The researches of wetting and evaporation of meniscus in micro/nano scale have a guiding significance for improving the heat transfer efficiency of micro-electro-mechanical systems.The requirements to equipment and accuracy of measurements are comparatively high for studying the nanoscale droplet and fluid wetting and evaporation by experiments.Along with the progress of computer technique,molecular dynamics simulation becomes the most convenient link between micro and macro.In this research,molecular dynamics simulations are applied to investigate wetting and evaporation of nanoscale meniscus systems(nanodroplets and nano-confined fluids).The effects of interaction between solid walls and fluids,and the roughness of solid walls,the interaction between nanoparticles and fluids and the volume fraction of nanopartiles on wetting behavior,interfaces properties and evaporation progress of nanoscale meniscus systems.For confined argon fluids nanomeniscus in nanoscale channel,effects of interation between solid walls and argon fluids as well as the roughness of solid walls on wetting acticities and interface properties are obtained.The results show that the energy factor of solid wall has a great influence on property of liquid-vapor interface,the density of fluids and the wetting activity of nanomeniscus.With energy factor of solid wall decreasing,the lyophilic channels change to lyophobic and the shape of the liquid-vapor interface change from concave to convex.When the energy factor is larger,there is layering effects and more-dense liquid-bridge.While,less-dense vapor tunnel and special solid-vapor-liquid interface appear if the energy factor becomes smaller.It can be found that the roughness changed the contact angles and the density distributions near the substrates.Besides,the contact modes are Wenzel and Cassie contact mode for liquid confined in lyphilic and lyphobic channels respectively.For sessile argon nanodroplets nanomeniscus,the relationships between the energy factor of solid wall,roughness and contact angle as well as evaporation have been investigated.As the energy factor of solid wall increases,the contact angle decreases and the total evaporation increases.The local evaporation varies with the lyophilicity of changing.For nandroplets on lyophilic substrates,the largest evaporation region locates near the triple-phase-line region.However,for nandroplets on lyophobic and neutral substrates,the largest evaporation region is in the middle and the top of the nanodroplets respectively.For rough substrates,the variations of contact angle and contact radius during evaporation progress are much more complex than droplets on smooth substrates.The roughness of substrates enhances droplet evpoartion,so the total evaporation is much larger than that on smooth substrates.For nanofluid droplets containing nanopartilces,the impacts of energy factor and volume fraction of nanoparticles on wetting and evaporation have been inquired.As the energy factor of nanoparticles increases,contact angles of nanofluids droplets increase and evpoartion decreases.The effects of volume fractions of nanoparticles on wetting and evaporation depend on the wetting property of solid wall.For lyophilic wall,nanoparticles impede evaporation.And the impeding increases as the volume fraction of nanoparticles increasing.However,volume fraction of nanoparticles has little effect on contact angles for lyophobic and neutral wall.The nanoparticles' strengthening effect on evaporation is the best if the volume fraction of nanoparticles is 1%.And the strengthening effect decreases as the volume fraction of nanoparticles increases.For confined liquid bridge nanomeniscus in MgO nanochannel,the evaporation activities of pure water,water-ethanol mixture have been investigated.The difference between contact angles of water bridge confined in nanoscale and macroscale.With the restriction of spatial scale,MgO wall has great effect on density and temperature distribution of water bridge.There is solid-like substance near the wall.The evaporation progress of pure water film is the same as the macro drying progress which can be divided into three phases:increasing rate,constant rate and falling rate stage.Ethanol promotes the evaporation progress which means that the evaporation time is shorten gradually with the volume fraction of ethanol increasing.When the volume fraction is 90%,evaporation time is reduced up to 66.7%.