Molecular Dynamics Study Of Nanodroplets Evaporation

Droplet evaporation is one of the most common phenomena in nature and indus-try,such as " coffee ring effect" and the mixture evaporation of ethanol-water.These common phenomena are related to the interaction between the droplet and the solid wall,while evaporation is related to wettability,surface tension,contact line movement,phase transformation,the flow inside the droplet and the evaporation dynamics at the liquid gas interface.Therefore,droplet evaporation is a complex and multi-field prob-lem.In this paper,the molecular dynamics method is used to study the evaporation phenomenon of nano-cylindrical droplets.The main research contents and results are as follows:(1)The molecular dynamics method was used to study the pinning phenomenon and the effect of thermal effect on the morphology of the droplets.For convenience,we use the nanometer-sized cylindrical droplet and the Lennard-Jones potential function is used for the interaction between liquid and gas particles.Different het-erogeneity of the surface was achieved through alternating stripes of equal width but two chemical types,which lead to different contact angles.The evaporation induced by the heated substrate instead of the isothermal evaporation is investi-gated.It is found that the whole evaporation process is generally dominated by the nonuniform evaporation effect.However,at the initial moment the volume expansion and local evaporation effects play important roles.From the pointview of nanoscale,the slow movement of the contact line during the pinning process is observed,which is different from the macroscopic stationary pinning.Particular-ly,we found that the speed of the contact line may be not only affected by the intrinsic energy barrier between the two adjacent stripes(u)but also relevant to the evaporation rate.Generally speaking,the larger the intrinsic energy barrier,the slower the movement of the contact line.At a specified temperature,when u is less than a critical energy barrier(u*),the speed of the contact line would increase with the evaporate rate.When u>u*,the speed of the contact line is only determined by u and no longer affected by the evaporation rate at different stage(the first stick and the second stick).(2)The molecular dynamics method was used to study the evaporation of nano-cylindrical droplets on the heterogeneous interface.The effects of stripe width and substrate temperature on contact angle,contact radius,heat and mass trans-fer in droplet evaporation were investigated.It is found that the stripe width mainly affects the change of droplet morphology,while the substrate temperature mainly affects the heat and mass transfer of droplets.When the stripe width is large,the CCA-CCR evaporation mode will transform.CCA evaporation mode represents constant contact angle mode,while CCR evaporation mode represents constant contact radius mode.However,if the stripe width is too small,the e-vaporation behavior of the droplets on the heterogeneous interface will behave like that on the homogenous interface,which means only CCA evaporation mode will appear.For the whole heat and mass transfer,it is found that increasing the substrate temperature can accelerate the evaporation of droplets and improve the mass flux per unit area.The initial evaporation rate of the droplets is approxi-mately proportional to the substrate temperature.For the local heat and mass transfer,it is found that the transition contact angle from edge enhancement to edge weakening is approximately 96°?101°,which deviates from the theoretical value of 90°.This is due to the adsorption layer near the three-phase contact line.(3)The evaporation of a cylindrical binary nanodroplet consisting of Lennard-Jones particles from homogeneous surfaces was studied by molecular dynamics simu-lations.The binary droplets contain a hydrophilic component(type-2 particles)and a hydrophobic component(type-3 particles),which lead to different contact angles.To study the effect of surface tension and substrate wettability on the evaporation,we mainly study two parameters:liquid-liquid interaction intensity(?23)and mass fraction of hydrophilic particle(?).It is found that ?23 and ? have a great influence on the equilibrium contact angle and surface tension which have a tremendous impact on evaporation.Our results show that there are two factors affecting relaxation time during the evaporation process,i.e.,contact angle and the interaction between type-2 and type-3 particles.And the evaporation rate(or the condensation rate)is also affected by two factors,i.e.,contact angle and surface tension.For local mass transfer,the boundary of mass flux enhancement and suppression near the three-phase contact line is approximately 102°?106°,which deviates from the theoretical value of 90°.The discrepancy is attribut-ed to the existence of the adsorption layer near the TPCL.And the small the?23,the greater the deviation.During the whole evaporation process,no pinning phenomenon was observed due to the substrate is ideally smooth and chemical-ly homogeneous.Our simulations show that when ?23?1.0,the evaporation mode of binary droplets is CCA mode.When ?23=0.9,the more ? tends to 0.5,the more similar the evaporation behavior of binary droplet is to CCA mod.Otherwise,the evaporation behavior is more similar to the mixed mode.