UCLA

Although electrospray devices traditionally use a capillary for fluid flow and electric field concentration, a sharpened porous medium can be used instead for these purposes. In porous media electrospray emitters, a working fluid flows through the porous medium to an emission surface, where the local electric field and fluid pressure allow for electrostatic emission of droplets or ions. Long lifetime and high-performance operation of these devices requires understanding the flow through the porous medium to emission sites. To investigate the behavior of porous electrospray devices, two complementary models are proposed: a multiplexed emission model and a transient flow model. Multiple experiments were developed and utilized to inform and validate these models; these experiments also provided insight into other electrospray thruster phenomena. Computational analysis was performed where necessary as well: verification of transient flow solutions and electrostatic analysis. In this sense, the research presented in this manuscript combines analytical, experimental, and computational analysis of the porous electrospray emitter in order to thoroughly understand the various aspects of its behavior.

A wedge is a common shape of electrospray emitter which allows for many individual emission sites to form on its sharp end. A sharp edge creates a region where the electric field is strong and nearly uniform. Along this edge, emission sites form due to the combined effects of the subsurface flow and local electric field. An analytical model is developed to examine the behavior and spacing of these emission sites via the pressure variation in the porous fluid flow associated with the flow focusing on each emission site, which is coupled with the local electric field. The solution for site spacing and current is informed by empirical results with support from electric field modeling and investigation of porous media parameters. Emission site currents of up to 500 nA and site spacings of roughly 50 to 300 micron are predicted. Experimental measurements at UCLA of site spacing on a porous wedge confirm analytically predicted trends.

In all applications of electrospray propulsion, dynamic events are unavoidable. Transient flow to an emission site is described through the `sucking-in' of the meniscus at the emitter surfaces where emission does not occur. Pressure in the porous medium is enforced using the Young-Laplace equation and Darcy's law. Even though the flow through porous electrospray emitters is generally low Reynolds number, which is typically associated with instantaneous reaction to pressure changes, the pressure-dependent fluid accumulation at free surface pores can cause transient flow conditions. A diffusion equation for pressure in a porous medium is presented along with flow solutions for the three common emitter geometries of pillars, cones, and wedges. Flow solutions predict that flow to the emission surface initially peaks and decays to a constant value, which corroborates published transient electrospray current traces. Experimental measurement of the transient response of a pillar-shaped electrospray emitter shows qualitative agreement with the diffusion model. Experimental investigation of a porous wedge electrospray emitter requires a more nuanced model to take into account the significant volume required to form the many Taylor cones on the surface of the emitter.

A porous tungsten electrospray thruster (PoWEE) was developed to validate porous flow and electrostatic models. Up to 130 uA of emitted current was demonstrated in vacuum at 4.8 kV emitter potential. The wire probe diagnostic has revealed that tens of emission sites exist per millimeter of emitter length, both on and off the centerplane of the wedge. Transient wire probe measurements confirm that individual sites exhibit delay-overshoot-decay behavior that conforms to published electrospray experimental results, despite the total wedge response differing from typical published data. The porous wedge electrospray was operated with a variety of target voltages in order to understand thruster facility effects. Current measurements, in situ photography, and a novel wet and dry thruster experiment have show that glow and secondary species collection at the emitter can be simultaneously suppressed through adequate target biasing.

An electrospray emission model was developed using the first principles of fluid flow and electrospray emission. The model assumes that emission sites form on the surface of the electrospray emitter due to a distribution of electric fields on the surface of the emitter. Emitted current varies between sites due to the variation in driving pressure to each emission site. The current-voltage response of the porous wedge electrospray device is replicated, albeit at a reduced current throughput. The transient response is predicted by adding the steady response and transient homogeneous response of each emission site, while accounting for the fluid volume required for onset of emission. The experimental and model transient response match following cone volume iteration within a range of physically allowable sizes. The model shows that onset delay, rather than transient flow in the porous emitter, is the dominant mechanism for causing the transient response of porous wedge electrosprays. The corroboration of experimental results and modeling predictions shows that the transient ramp observed with porous wedge emitters is due to the distribution of onset delays of numerous emission sites, which is a direct result of nonuniform electric field on the emission surface. Furthermore, the nonuniform electric field is due to the sharp features present on the emission surface due to the emitter manufacturing process. As a result, this investigation highlights the effect that manufacturing uniformity has on both the transient and steady response of porous wedge electrospray emitters.