Molecular Dynamics Study Of Droplet Coalescence Under Electrostatic Field

In the process of electrostatic coalescence,the polarization deformation and coalescence of droplets is the most basic physical process involving multiple disciplines.At present,it is difficult for experiments and macro-simulation studies to reveal the complex interactions between particles in the droplets from the molecular level,and the mechanism of electrostatic coalescence has not been deeply revealed.In this paper,the process of electrostatic coalescence of micro-nano droplets was simulated by Gromacs software,and the movement trajectory of microscopic particles in the system was obtained.The interaction between particles was analyzed,and the coalescence characteristics and influence rules of micro-nano droplets under the action of electric field were analyzed from the microscopic point of view,providing a theoretical basis for the in-depth study of electrostatic coalescence mechanism.On the one hand,the polarization deformation model of single droplet was established by using molecular dynamics method,and the polarization deformation,breaking behavior and criterion of single droplet were studied.By changing the electric field intensity,droplet size,inorganic salt and other parameters,the changing trend of droplet deformation degree was studied,and the changing rule of the critical electric field parameters of droplet breaking was obtained.The results show that the strong attraction in the weak molecular interaction has the most significant effect on the droplet fragmentation.The ion pair effect and ion hydration have a great influence on the deformation and breaking behavior of saline droplets.The critical electric field intensity decreases monotonically with the increase of droplet radius and ion concentration within a certain range.Under low electric field,droplet deformation increases with the increase of electric field intensity and droplet radius.Under high electric field,the droplet appears different breaking patterns,including one end breaking,two ends breaking and the center breaking.When droplet radius is 3nm,electric field intensity is 1.0 V?nm^-1,and droplet concentration is 0.675 mol?L^-1,the crushing time is the shortest.The reason is that the number of free ions in the droplet is the largest at this time.However,as the ion concentration continues to increase,the number of ion pairs increases,resulting in the decrease of free ions and the increase of the droplet breakage time.On the other hand,the molecular dynamics method was used to establish the double droplet coalescence model,and the coalescence behavior and coalescence criterion of the two droplet were studied.By changing the electric field intensity,droplet size,droplet spacing,ion concentration and other parameters,the particle interaction and charge distribution were analyzed to obtain the droplet coalescing mechanism and the optimal parameters were summarized.The results show that the direct driving force of droplet coalescence is the applied electric field,and increasing the electric field intensity can promote the efficiency of droplet coalescence.The reason why the presence of ions promotes the electrostatic coalescence of droplets is that under the action of electric field,ions in the adsorption layer drive the movement of ions in the diffusion layer,which strengthens the polarization of droplets.The efficiency of droplet coalescence can be improved by appropriately increasing the electric field intensity,the concentration of inorganic salts and decreasing the droplet Angle.The droplet center distance should be selected according to the droplet center distance ratio.When the droplet radius is2.0nm,the electric field intensity is 0.50 V?nm^-1,the droplet Angle is 0°,the droplet center distance is 8nm,and the sodium chloride concentration is 1.25 mol?L^-1,the polymerization time of the two droplets is the shortest.