Recently,the micro nano hydrodynamics team of the Institute of mechanics,Chinese Academy of Sciences,used molecular simulation to investigate the phenomenon of ion ejection induced by electric field at the ionic liquid vacuum interface.This work provides guidance for the selection of appropriate ion jet molecular simulation strategies,and lays a foundation for the subsequent study of more complex electrojet phenomena.Relevant research results were published in physics of fluids and selected by editors.
Ionic liquid electrospray is very important for nanofabrication,space propulsion,mass spectrometry and other applications.In order to grasp the law of ion ejection,some scholars have studied the phenomenon of ion ejection at the ionic liquid vacuum interface induced by electric field.Although progress has been made in the theory and experiment of ion jet,there are relatively few studies on the simulation of nanoscale ion jet in the academic community.How to simulate ion jet molecules more realistically remains to be explored.
The team calculated the variation of ion jet current density with liquid surface electric field,compared the influence of Langevin temperature controller,dissipative particle dynamics(DPD)temperature controller and berendsen temperature controller on the simulation results,and found that DPD temperature controller was more suitable for simulating ion jet phenomenon.Compared with Langevin and berendsen temperature controllers,DPD temperature controllers do not change the movement speed of ions in vacuum,and do not apply friction resistance and random force to ions in vacuum,so they are more suitable for practical applications.
Furthermore,the three methods of applying liquid surface electric field,constant potential,constant charge and constant electric field,were studied and compared.It was found that the current density liquid surface electric field curves obtained by the three methods were basically the same.In the past,the constant potential method was used in ion jet molecular simulation,but only a few specific molecular simulation software can realize the constant potential simulation in the world.In contrast,constant charge and constant electric field methods can be directly used in commonly used open source molecular simulation software.The results show that for the parallel electrode planar liquid film system,the constant charge/constant electric field method can better simulate the ion jet phenomenon.
The team further analyzed the change rule of electric field in vacuum with time and found that due to the electrostatic interaction between ions in vacuum and liquid-phase ionic liquid,ions in vacuum will change the electric field strength of liquid surface.The higher the ion jet current density,the higher the fluctuation frequency of the liquid surface electric field.Based on the classical theory of electrostatics,if periodic boundary conditions are applied(not applied),the induced electric field between the ejected ions and the ionic liquid is independent(related)to the position of the ions in the vacuum.Therefore,the effect of periodic boundary conditions on the liquid surface electric field and ion ejection should be considered in the molecular simulation of electrojet.
The results provide support for the molecular simulation of ion jet at the liquid vacuum interface,and lay a foundation for the follow-up exploration of more complex nano scale electrojet phenomena.The research work was supported by the youth talent reservoir project of the Chinese society of mechanics.
(a)Screenshot of cation injection simulation system;(b)Screenshot of anion injection simulation system
Schematic diagram of simulation system for electric field induced ion ejection at ionic liquid vacuum interface
