Molecular Dynamics Simulation On Wetting Characteristics Of Nanodroplets

With the frequent occurrence of oil spills,the separation of oil and water has become very urgent.Due to the distinct opposite affinities for oil and water,special wettable surfaces have been successfully used for selective oil-water separation and have shown attractive application prospect.Therefore,the study of the wettability of water or oil on a solid surface is extremely important.In this paper,the wetting characteristics of water nanodroplets and water-oil mixed nanodroplets on smooth and rough surfaces were studied by molecular dynamics simulation.The wetting processes of water nanodroplets on polytetrafluoroethylene(PTFE)surfaces were explored,and the rough surface was optimized and simulated.For smooth surface,the contact angle(CA)of water nanodroplet decreases with the increase of the energy coefficients.When the surface is hydrophobic,the CA increases with the increase of temperature.When the surface is neutral,the CA is hardly affected by the temperature.When the surface is hydrophilic,the CA decreases with increasing temperature.The number of water molecules has little effect on the CA.In addition,the thickness of the vapor-liquid interface increases with increasing temperature,which is basically independent of the number of water molecules and wall energy coefficients.For rough surface,when the surface is hydrophobic,the CA of water nanodroplets increases with the increase of the column height;as the phase area fraction increases,the CA of nanodroplets firstly increases and then decreases,and the wetting state gradually changes from the Wenzel state to the Cassie state.When the surface is neutral or hydrophilic,the CA of water nanodroplets shows no remarkable change when column height or phase area fraction changes,and the wetting state is Wenzel state.For smooth PTFE surface,the CA of water nanodroplets increases with increasing temperatures,and it is basically independent of the initial nanodroplet diameters.For fence form PTFE surface,the CA of the water nanodroplet increases first and then remains unchanged with the increasing roughness factor and it increases with the decrease of the phase area fraction.By optimizing the simulation conditions,the CA of water nanodroplets on PTFE surface reached 145.6°,which is close to the super hydrophobic state.With the increase of temperature and volume fraction of n-hexadecane,CA of water-oil mixed nanodroplet shows an increasing trend overall.For lyophobic surface,the greater the column height,the larger the CA.With the decrease of the groove width,the CA of mixed nanodroplet first increases and then decreases.For lyophilic surface,the CA generally increases with the increase of the column height;the CA shows no remarkable change when groove width changes.