Modeling of Interactions Between Low- and High-Pressure Plasmas with Materials
Open AccessThe focus of this dissertation is the development of numerical models used to investigate physical processes that occur during plasma interactions with materials such as soft tissue, liquid, and solid electrodes. Specifically, four separate projects are discussed: three involving atmospheric plasma discharges generated by devices built in the biomedical field, and one relating to metal electrode sublimation in plasmas generated in near atmospheric-pressure conditions. The first project investigates the interaction of a helium atmospheric plasma jet impinging onto liquid and metal targets using experimental and numerical techniques. The primary motivation of this study is to understand the effect of liquid and metal targets on the propagation of a plasma jet, and to numerically characterize chemical and electrical behavior at the plasma-water interface. Experimental measurements are made to obtain ionization wave propagation, average electron density, and optical emission spectrum of the plasma near the interface for cases with copper and water targets and a freely expanding jet. The induced acidity on the plasma-treated water is measured by pH balance. The numerical model uses parameters from plasma-liquid experiments to simulate reactive oxygen and nitrogen, acidic, and charged species in interacting gaseous and aqueous layers by employing a transient advection-diffusion-reaction-solvation equation. The interacting layers are connected by a two-way coupling governed by solvation through Henry’s law. Accumulation of charges in the aqueous layer results from varying diffusion and mobility time scales of charged species. Electric potential on the aqueous layer is characterized and resulting electric fields and Maxwell stresses on the aqueous layer are proposed to affect the movement of the plasma contact spot on the gaseous layer. Energy equations are solved in both phases for neutrals, ions and electrons by considering the effect of the Joule heating term. Current density through the gaseous-aqueous interface is calculated. The second project develops a numerical model to examine the deformation of biological cell-like structures by applying electric fields operating at or near cell resonant frequencies. This research is of particular interest to plasma cancer therapy, where electric fields from atmospheric plasma jets act to stress cells under consideration. In the simulation, the cell is modeled as a solid suspended in liquid media. To investigate cell deformation at resonant frequencies, one mode of natural cell oscillation is considered in which the cell membrane is made to radially move about its eigenfrequency. An electromagnetic wave source interacts with the cell and induces oscillation and viscoelastic response. The source is assumed to carry energy in the form of a distribution function which couples a range of oscillating frequencies with electric field amplitude. The third project relates to atmospheric pressure plasmas and their application in the biomedical field. In this case, a two-dimensional axial discharge column is modeled to investigate plasma expansion from the device nozzle to a target with varying electrical properties. The target with varying properties simulates a petri dish with different cell colonies. The motivation is provided by experimental evidence which suggests that plasma may be biased to some types of cells over others based on their inherent electrical properties. The fourth project simulates plasma and electrode behavior for near- arc discharge during the sublimation of a metal electrode. This process is of particular interest in the synthesis nanoparticles as well as the development of electric thrusters. Parametric studies which vary input current, inter-electrode gap, and erosion model are presented. Energy equation solves a conjugate mass transfer problem by coupling electrode and plasma energy equations.
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Martinez_gwu_0075A_15345.pdf | 2020-12-17 | Open Access |
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