Molecular dynamics study of ionic liquid electrospray: Revealing the effect of types of electric fields

Abstract

The microscopic behaviors and characteristics of BMIM+BF4− ionic liquid electrospray in the presence of the axial electric field and axial-radial electric field were investigated by molecular dynamics method. The formation and fragmentation of the jet, the structure characteristics of the ions, the interactions between the ions and the energy properties were analyzed. Since ionic liquid is commonly used in spacecraft thruster, their specific impulse and thrust are also evaluated. It was found that the occurrence of ionic liquid electrospray was determined by the axial voltage, not less than 0.8V/nm. The flow rate and radial voltage mainly affect the jet instability and the secondary disintegration. When the jet is broken, it is almost ejected in the form of (BMIM+BF4−)1(BMIM+) and BMIM+, but finally it presents four main clusters, including BF4−, BMIM+, (BMIM+BF4−)1 and (BMIM+BF4−)1(BMIM+). For the unbroken jet at the capillary outlet, BMIM+ is concentrated on the surface of the jet, and BF4− is concentrated inside. The number of clusters will occasionally decrease owing to coalesce. The internal bonds of BMIM+ and BF4− is likely to break, and the bond angle may also vary. In the presence of the axial-radial electric field with an axial electric field intensity of 1.2V/nm, the electrospray ionization effect is the best at the radial electric field intensity of 0.001V/nm, and the specific impulse and thrust can be obtained by adjusting the axial electric field intensity. This work provides a new insight into the microscopic mechanisms of BMIM+BF4− ionic liquid electrospray.