Microscopic Mechanism Of Droplet Wetting Behavior Using Molecular Dynamics Simulation

When the solid surface exposed to the air is in contact with the liquid,the behavior of the liquid replacing the air on the solid surface and adsorbed on the surface by the solids is called wetting,such as dew on the leaves of the morning.As the tangent of the gas-liquid interface at the three-phase junction of solid gas liquid,the contact angle is a measure of wettability.The exploration of traditional researches on contact angle is mainly experimental research methods,and these methods have expounded the regularity of contact angle,but did not explain the wetting behavior from the action mechanism.In this paper,using the method of molecular dynamics simulation,the contact angle is taken as the cut-in point,the formation process of contact angle is understood from microcosmic angle,and the research of wetting behavior is extended to the molecular level on the basis of acquiring molecular structure and motion information.In this paper,the gas-liquid coexistence system composed of argon and water molecule,the liquid droplet on the smooth surface,vibrating surface and the wetting model of the rough surface composed of square columns are established respectively.The formation and influence factors of gas-liquid interface are studied.The influence of contact angle on smooth surface was discussed from four aspects of solid-liquid interaction,water model,solid surface arrangement and temperature.Causes of the morphological difference and hysteresis of contact angle on rough surfaces was researched.Firstly,the gas-liquid interface of argon and water system is simulated by molecular dynamics.The formation mechanism of gas-liquid interface from initial state to equilibrium state is discussed,and the influence of temperature on its interface is compared.The results show that the formation of the gas-liquid interface is mainly caused by the internal anisotropy of the gas-liquid two-phase,which provides a theoretical basis for the influence of the gas-liquid boundary on the wetting behavior of the liquid droplet on the solid surface.The temperature determines the state of the molecule motion and the physical properties of each phase in the molecular system,which leads to the difference of the two phases and is the main factor affecting the gas-liquid interface.Comparing the contact angle of droplet at different temperature,it is found that the contact angle is different from the gas-liquid interface.Secondly,the vibration characteristics of micro-nano droplet in normal mechanical vibration solid smooth surface are studied by means of molecular dynamics simulation.The result shows that the period of force and the movement of centroid are the same as the driving cycle of the forced vibration of the droplet.Both the vibration period and amplitude have effect on the movement of droplet,and the effect is opposite.Liquid droplets in the smooth surface with simple harmonic excitation,will be the energy of vibration input.When the input energy reaches a critical value,the droplet will overcome the threshold of energy on the solid surface and eventually disengage from the solid surface.Finally,the rough surface simulation shows that the roughness and the square column resolution are respectively the main factors that affect the surface contact angle of Wenzel form and the Cassie form.After changing the roughness and the resolution of the square column and seeing the solid surface as an external vibration excitation,it is found that it is more effective to transform the droplet from Wenzel form into Cassie form by increasing the height of the generous column and applying the mechanical vibration of the solid surface.