Molecular Dynamics Study Of Droplets Evaporation On Rough Substrates

Droplet evaporation on rough substrates is ubiquitous in nature and plays a significant role in various industrial technologies.Understanding and controlling the evaporation process of droplets on rough substrates contributes to the production of intelligent devices and the design of functional patterns.To this end,it is necessary to study droplets evaporation on rough substrates by molecular dynamics method.In this paper,the evaporation process of nanodroplets on the heated rough substrates has been studied by molecular dynamics method.In order to study the effect of substrate rough structure and substrate roughness factor on nanodroplet evaporation characteristics,based on Wenzel wetting model,three systems with different roughness factors(r)of 1,1.5and 2,respectively,and three systems with different substrate rough structure,in which the ratios of contact area(RCA)between substrates and nanodroplets are 33.3%,50%,and66.6%,respectively,have been constructed in this paper.On this basis,the effects of the rough structure and the roughness factor of the substrate on the dynamics and heat and mass transfer during nanodroplet evaporation has been discussed.At the same time,the effects of substrate roughness(Ra)on dynamics and heat and mass transfer during droplet evaporation has been discussed experimentally.And the effects of substrate roughness on droplet evaporation characteristics at the macro-scale were compared with those at the nanoscale,so as to explore the influences of the scale effect on droplets evaporation on rough substrates.The main results and conclusions of this paper are as follows:(1)The Molecular dynamics simulation results of nanodroplets evaporation on substrates with different rough structures indicate that the evaporation mode of the nanodroplet in the system with RCA = 33.3% is the mixed mode(Mixed mode),the evaporation mode of the nanodroplet in the system with RCA = 50% is the constant-contact-radius mode(CCR mode),and the evaporation mode of the nanodroplet in the system with RCA = 66.6% is the constant-contact-angle(CCA)mode.Meanwhile,The temperature increase rate and evaporation rate of nanodroplet in the system with RCA=33.3% is significantly slower than that of the other two systems.(2)The Molecular dynamics simulation results of nanodroplets evaporation on substrates with different roughness factors indicate that with the increase of the substrate roughness factor,the contact angle of nanodroplets at equilibrium first rapidly increases and then slightly decreases.During the evaporation process,with the increase of the substrate roughness factor,the evaporation mode of the nanodroplet realizes the transformation from the CCA mode to the CCR mode,and the heating rate and evaporation rate of nanodroplets increase.(3)The experimental results of droplets evaporation on rough substrates indicate that the whole evaporation process of droplets on rough substrates can be divided into five typical stages,namely expansion stage,first stick stage,second stick stage,jump stage and final stage.The evaporation mode does not change significantly with the increase of the substrate roughness in the evaporation process of droplets,all are the mixture of the Mixed mode and the CCR mode.The heat and mass transfer in the evaporation process of droplets is dominated by the non-uniform evaporation effect,and with the increase of the substrate roughness,the heat and mass transfer is promoted.In addition,the scale effect has a significant influence on the dynamics of droplets evaporation on rough substrates.