The Numerical Simulation-based Characterization Method And The Scale Dependence Of The Wettability Of Nanoscale Solid Materials

The wetting is a common physical phenomenon.The materials with the super-hydrophilic or super-hydrophobic surface have been widely applied in the biological,chemical and mechanical fields.The contact angle is measured by experiments for characterizing the wetting properties of materials.In recent,the micro-or nano-scale low dimensional materials,such as graphene,have attracted extensive attention of scholars at home and abroad due to the outstanding optical,mechanical and electrical properties.The wettability is very important to these materials which are applied in the batteries,semiconductors and micro/nano electromechanical systems(MEMS/NEMS).However,the contact angles of droplet on the micro-or nano-scale material is hardly obtained by experiments due to the limitation of small scale.Meanwhile,owing to the non-negligible line tension,the contact angle has a linear relation with contact radius,which leads to the difference between the macroscopic wettability and macroscopic wettability.Hence,the researches on the high-accuracy characterization method and the scale dependence of the wettability of nanoscale solid materials could provide the scientific significance and application value.In this paper,a numerical method is proposed to calculate the contact angle by fitting the ellipsoid with the spatial data based on molecular dynamic simulations,and the corresponding computational steps are summarized.In order to demonstrate the accuracy of the proposed method,the sphere and ellipsoid droplets with the specified contact angles are constructed.Meanwhile,the variation trends of the contact angle with the height of solid-liquid interface used to validate the stability of the proposed method.The scale effect of the wettability of the graphene and the relationship between the initial configuration and macroscopic contact angle are investigated by molecular dynamic simulations.In the calculation of the contact angle of graphene,the density range of the liquidvapor interface and the height of the solid-liquid interface are obtained by the droplets with different sizes and the equilibrium configuration of the water confined in the graphene channel,respectively.The numerical simulations of the wetting on graphene are carried out by the molecular dynamics method.The computational results indicate that the contact angle decreases with the increase in the contact radius and the macroscopic contact angles is obtained by the modified Young's equation.Moreover,the contact angle has a negligible dependence on the initial configuration.The present work proposes a computational method for the contact angle by fitting the ellipsoid with the spatial data.The density range of the liquid-vapor interface and the height of the solid-liquid interface in this method are obtained by molecular dynamic simulations.The accuracy and stability of this method are demonstrated.The scale effect of the wettability of graphene and the relationship between the initial configuration and the macroscopic contact angle are obtained by the proposed method.The proposed method provides an effective and reliable way to characterize the wetting properties of materials at micro-and nano-scale and establishes a good foundation for nanoscale device design by means of the wetting properties of materials.