Experimental investigation on current modes of ionic liquid electrospray from a coned porous emitter

The ionic liquid electrospray-based propulsion system is one of the most promising candidates for space micro-propulsion, which is in urgent need with the rapid development of micro/nano satellites. The current mode in the electrospray process directly dominates the performance of ionic liquid electric thruster. Hence, in order to study the general current modes of ionic liquid electrospray, a series of experiments have been conducted with a coned porous nickel emitter. With the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate as propellant, both the time-varying voltage-current curves and corresponding optical images of the electrospray process are obtained. Firstly, based on the voltage-current curves and optical images, the classification of current mode was discussed, which showed that the current is a result of basic-mode composition in time and space and the mode transition is dominated by the electric field strength. Secondly, the liquid flow at the tip of coned porous emitter was studied based on the images, which is found to be a combination of porous media flow and liquid film flow influenced by the electric field. The projected area of meniscus can be much larger than the pore size, which indicates that the emission can span several pores in our system. As the electric voltage increases, the size of meniscus decreases and three-site emission is optically captured, which explains the spatial composition of the current mode. Finally, the onset delay is calculated from the time-varying current curves. Based on the observation that the meniscus size is variable, an onset delay model is developed, whose prediction agrees well with the experimental results. Since a single emitter is a basic element for array porous emitters, this work is valuable to the design of efficient ionic liquid electric thruster.