Impact of Tip Angle on the Divergence and Efficiency of Electrospray Thrusters

Electrospray propulsion has appeared as a promising technology for space applications, particularly in response to the growing demand for small spacecrafts. Its high efficiency, low power consumption, and thrust control make it an attractive option for miniaturized propulsion systems. However, challenges remain in optimizing performance and controlling emission characteristics. Here we hypothesize that extremely sharp microneedles in externally wetted emitter devices, despite offering lower onset voltages and higher currents, can produce undesirable off-axis emission due to the propellant ejection perpendicular to the surface before arriving at the microneedle tip, thereby generating broad angular emission patterns. Through a combination of simulations and experimental evaluations, we analyzed the impact of emitter sharpness on the beam divergence and angular efficiency. It is shown that very sharp emitters (20° tip half-angle) exhibit clear off-axis emission with toroidal shape angular patterns in the plume even at moderate applied voltages, indicating emission of particles from the propellant before arriving at the tip and following trajectories nearly perpendicular to the emitter surface due to the generated electric field. Such off-axis emission significantly decreases the propulsive efficiency and may result in a decreased lifetime. This effect has been mitigated by increasing the tip semiangle to 30°, recovering a beam distribution with a typical Gaussian profile. Moreover, it has been observed that increasing the fluidic impedance of the nanostructured surface can slightly narrow the angular emission distribution to improve the propulsive efficiency. These findings underscore the importance of a precise geometric design to maximize the performance of electrospray thrusters, providing valuable insights into the development of advanced, high-efficiency propulsion systems for small satellites.