Influence of electrostatics upon electrospray with the intention of application to colloid thrusters

Abstract

A thorough experimental and theoretical characterization of the effect of electrostatics on the electrospray process, focusing particularly on the flow rate sensitivity to the applied potential difference (voltage), has been completed. The flow rate and current increase linearly with the applied voltage within cone-jet mode. The effect of geometry on the flow rate to voltage relationship is sensitive to two parameters – the hydraulic resistance and the variation of the electric pressure sensitivity to external geometry. A theoretical and FEM model based on the calculation of the electric field provides an explanation of the geometry variation. This allows for an estimation of the change of flow rate with voltage, under any geometrical circumstance. For the first time the effect of voltage on flow rate across enhanced dripping, pulsed and multi-jet electrospray regimes are outlined. With the exception of enhanced dripping, a linear increase is noticed within most regimes, and is geometrically sensitive. Also at the onset of cone-jet mode a drop in flow rate occurred. The variation of flow rate with voltage can be applied to colloid thrusters to vary the performance. Using the theory outlined in this thesis, an estimate of the flow rate change for a colloid thruster is described, along with its associated performance variation. The effect of voltage on current in cone-jet mode electrospray is detailed, with a similar geometric dependence as the flow rate to voltage relationship established. It is also sensitive to various other parameters, including nominal flow rate. The stability island of cone-jet mode electrospray is explored, and its relationship to the variation of electric field with voltage is outlined. The effect of emitter and electrode geometry on cone-jet onset voltage and cone-jet voltage range is outlined.