Molecular Dynamics Simulation of Ion Emission from Nanodroplets of Ionic Liquids

Abstract

The work reported here is motivated by a desire to fully understand the physics of colloidal thruster propulsion technology. Colloidal thruster technology is based on the electrostatic acceleration of small droplets and/or ions that are generated by feeding a conducting fluid through a small capillary and applying a large acceleration potential between the capillary and an extraction electrode. Maximum thrust is obtained when the device is operated in the droplet mode. However, by transitioning to an ion mode, the specific impulse can be greatly increased. Here, we use molecular modeling to study the dynamics of small droplets of ionic liquids (EMIM-BF4) in the presence of large external electric fields. Interatomic and molecular forces are described using a molecular mechanics force field, and the whole ion structure is included. Before an electric field is applied, minimization methods are used to determine the equilibrium structure of the droplet. An electric field is then applied and the resulting motions of droplets and individual ions are observed. In addition, the surface tension of the droplets is calculated from the dynamic simulations. Surface tensions calculated from the simulations agree with macroscopic experimental values within uncertainty. Thus, the results indicate that the Taylor expression for the critical electric field for droplet ionization is order-of-magnitude-correct.