Molecular Dynamics Investigation On Properties Of Interfacial Transport And Heat Transfer For Droplets Under Electric Field

Interfacial transport and heat transfer of droplet under the electric field play an important role in electrospray, electrospinning, electrospray deposition, electrospray ionization and electric dehydration of crude oil and other advanced technologies. In these techniques, evaporation, break-up, coalescence or heat transfer behavior of the droplets under electric field is the basic physical process. Complex interaction of the particles (water molecule, ions and polymer), formation of special structures, and directional movement of the charged particles in multicomponent droplet under electric field, result in many new issues and new phenomena in the interfacial transport and heat transfer of the droplet under the electric field. Currently, it is difficult to reveal the interaction between the particles from the micro-perspective by the experimental and the theoretical researches, leading to that the issues and phenomena are not yet clear, it has become the bottleneck for further optimization and development of advanced technologies.In this paper, the interfacial transport and heat transfer behaviors of droplets under the electric field are studied by molecular dynamics simulations, the contents include three parts:(1) Evaporation and break-up of single charged droplet (water droplet with dissolved salts) under electric field; (2) Electro-coalescece of two charged droplet; (3) heat transfer of droplets on the charged substrate with high temprature. The study aims to reveal new mechanisms contained in the new phenomena in the interfacial transport and heat transfer of the droplet, and propose effective methods to controlling interface transport and heat transfer behaviors of droplets. The study will provide a solid theoretical basis for optimization and development of advanced technologies.Simulation results of droplet evaporation and break-up show that adding salts into the droplet under the electric field, the droplet evaporation and break-up can be significantly enhanced, and morphology of the molecular chain in the droplet can be optimized. In addition, adjusting the concentration of salts in the droplet can be effectively regulating droplet evaporation and break-up behaviors. Observing trajectories of particles in the droplet under the electric field, there are two states of ions in the droplet, one can move freely (named free ions) and another form the "ion pair". The droplet evaporation and break-up behaviors depend on the number of free ions in the droplet, high-speed motion of free ions under the strong electric field can lead to that free ions escape droplet and ejection behavior of daughter droplets occurs, which enhance the evaporation and break-up of the droplet. In addition, the coordination structure can be form between ion and molecular chains, and molecular chain can move by driven of free ions, which can optimize the morphology of the molecular chain. Thus, the quality and performance of the product can be regulated by adjusting the salt concentration in the electrospinning, electrospray deposition and electrospray ionization.Coalescence and non-coalescence of adjacent charged droplets can occur under the electric field. When the applied electric field is below the critical field strength, the two droplets fully coalescence after the droplets contact; however, non-coalescence of charged droplet droplets occurs when the electric field strength is above the critical value. This work firstly observes the ions transfer process between the droplets after the droplets contact, and reveals the charge transfer is the mechanism of the droplets non-coalescence from the micro-perspective. The simulation results also found that the smaller value of the critical field strength is obtaind for the larger droplets. Applying a pulsed electric field, the critical field strength can be improved, and the electro-coalescence efficiency can be enhanced, which is propitious to the electric dehydration of crude oil.When the temperature of substrate is much higher than the saturation temperature of the liquid droplet, leidenfrost phenomenon appear for the droplet on substrate. Vapor layer between the droplet and the substrate appears, which suppresses heat transfer between the droplet and the substrate. The simulation results shows that the heat transfer are stronger for the droplet on the substate with stronger wettability, and the leidenfrost phenomenon of droplet is more likely to occur. When the substrate surface carries sufficient charge, the strong electric field will be formed, which can significantly enhance the attraction between the droplet and the substrate, and improve the number density of water molecules at solid-liquid interface. Thus, the substrate with sufficient charge can effectively suppress the occurrence of leidenfrost droplet, and enhance the heat transfer between the droplet and the substrate.