Design and fabrication of an integrated MEMS-based colloid micropropulsion system

The design, microfabrication and initial performance results of a prototype electrospray thruster with integrated individual extractor electrodes are reported in this paper. The aim was to demonstrate increased thrust with an array of emitters spraying simultaneously. Micromachining technologies were employed to achieve large emitter density per surface area, simple integration and good structural repeatability. The novelty of this work lies in the combination of high aspect ratio capillary emitters (height of 70μm, inner diameter of 20μm) with individual extractor electrodes having diameters from 80μm upwards and a spacing as low as 25μm from the capillary tips. The individual integrated electrodes, as opposed to the standard approach of one common electrode, allow for greater uniformity in critical voltages between capillaries and more finely modulated thrust control. Tests with the newly developed thrusters using the ionic liquid EMI-BF4 show that starting voltages below 700V and currents around 300nA per emitter can be achieved. Nomenclature d = distance between emitter tip and extractor plane Ea = electrical field at the apex of the cone m = mass of the emitted species pa = pressure at the tip of the capillary q = charge of the emitted particles ra = radius at the apex of the Taylor cone Rc = radius at the tip of the capillary va = liquid speed at the apex of the cone Vcap = voltage applied at the capillary emitters Voc = critical voltage at which spraying initiates α = scaling factor between voltage and electrical field γ = surface tension of the liquid used as fuel 0 = permittivity of free space ρ = density of the liquid ∗PhD Student, School of Engineering, [email protected] †Master Student, School of Engineering, [email protected] ‡Assistant Professor, School of Engineering, [email protected] §Postdoctoral Research Assistant, Engineering, [email protected] ¶Professor of Aerospace Engineering, Engineering, [email protected] 1 The 30th International Electric Propulsion Conference, Florence, Italy September 17-20, 2007