Two-liquid electroosmotic thrusters for micro propulsion applications

We investigate analytically the thruster performances and power consumption rates of a two-liquid electroosmotic thruster based on slit microchannels with hydrodynamic slip walls. The two electrolytes are considered to have different material properties and are arranged in the configuration of a core liquid layer surrounded by immiscible outer liquid layers with the outer layers in contact with the microchannel solid walls, thus forming electrical double layers at the solid-liquid interface. Interfacial potential jumps and surface charge densities are included to model the liquid-liquid interfacial double layers. Results reveal that, with the properties of both liquids being identical, nonzero liquid-liquid interfacial electrostatics only slightly increase the thrust but noticeably reduce the thruster efficiency and thrust-to-power ratio due to the enhanced Joule heating and viscous dissipation caused by the increased charge distributions and distorted velocity profiles. Moreover, the thrust and efficiency can be substantially increased as the dynamic viscosity ratio is decreased with the density ratio fixed at one, whereas the thrust, efficiency, and thrust-to-power ratio are all significantly enhanced by increasing the dynamic viscosity ratio when the kinematic viscosity ratio equals to one. The bulk electrolyte concentration/conductivity ratio is identified as a key parameter capable of simultaneously maximizing one or more thruster performances. While improving upon the performances of the single-liquid electroosmotic thruster previously reported, the two-liquid results and modeling presented herein may likely relax the limitations on the choice of electroosmotic propellants, increase the operational flexibility of electrokinetic thrusters, and be further applied in space or underwater micropropulsion applications. Published under license by AIP Publishing.