Date of Award

2018

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Lyon King

Committee Member 1

Jeffrey S. Allen

Committee Member 2

Chang K. Choi

Committee Member 3

Durdu O. Guney

Abstract

This dissertation presents three studies on the electrospray of ionic liquid ferrofluid. Ionic liquid ferrofluids are electrically conductive super-paramagnetic fluids which respond strongly in the presence of electric and magnetic fields. When a small reservoir of ionic liquid ferrofluid is positioned within a magnetic field, magnetic stresses will deform the fluid interface into a peak. The addition of a strong electric field will further stress the fluid interface until a threshold stress is reached at which point the surface tension cannot contain the combined stresses and a spray of fluid or ions results at the apex. This process is termed electrospray, albeit a less understood form of electrospray owing to the addition of magnetic stresses which are not present in traditional electrospray.

The first study included in this dissertation presents a computational fluid dynamics model of the combined electro-magnetic instability critical for electrospray. The developed model utilized the static formulation of the Maxwell equations to calculate the Maxwell stress tensor for an ionic liquid ferrofluid. When combined with the Stokes stress tensor, the duo of equations capture the fluid stresses present within the instability. The model was first employed to study the influence of a magnetic field on the onset potential of a capillary needle electrospray source. The simulation predicted onset potential agreed well with the experimentally captured onset under matching field conditions. The numerical tool was then utilized to study the dynamics of sessile ionic liquid ferrofluid droplets. The computational results were verified against laboratory images of sessile drops obtained under matching field conditions. The simulation performed exceptionally up until about 85% of the onset potential at which point the simulation began to over predict the apex height of the combined instability.

The second portion of this dissertation consisted of long duration emission studies of an ionic liquid ferrofluid normal-field source. An operational procedure was developed which permitted a source consisting of a single emitter to operate with constant extraction potential for spans in extent of 15 hours. Time-lapse imagery of source enabled the mass flow rate to be approximated, permitting derived propulsion performance parameters to be obtained. Three different magnetic field strengths were investigated, and it was found that the magnetic field strength has no identifiable impact on propulsion performance. On average, the mass flow rate of the source was 28 ng/s (15.5 pL/s), with a specific impulse of 1385 s and a thrust of 0.380 µN per emitter. During the telemetry, the sensitivity of the source was analyzed and it was found that for moderate changes in extraction potential the source remained stable, but for increases on the order of 25-30% of the baseline voltage secondary emission sites were observed to form.

The final set of studies included in this dissertation focuses investigated the angular divergence of ferrofluid electrospray emitting via the normal-field instability. The angular current density was measured through the use of a segmented Faraday probe and quantified in terms of an angular power utilization efficiency factor. For the source, the average power efficiency was found to be 94%. A strong correlation was found between increased emission current and increased mass flow rate and decreased power efficiency. Finally, a very small difference in efficiency was resolved between the positive and negative emission polarities.

The last chapter of this dissertation models the magnitude of the Kelvin and Lorentz forces in the emission plume to determine their potential to influence particle trajectories. It was found that in the apex region, the Coulomb force dominates the Kelvin force by several orders of magnitude – indicating that the Kelvin force is unlikely to affect the trajectories of emitted magnetic particles. It was also found that the magnitude of the Lorentz force in the apex region was too small to influence particle trajectories for even the lightest ions expected.

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