Modelling Interactions of Atmospheric Pressure Plasmas with Liquids.

Abstract

Plasmas in and in contact with water are being widely investigated for their ability to produce chemically reactive species, energetic photons and localized high electric field for environmental and biomedical applications. Understanding the interaction of plasma with liquid still remains a challenge due to the complexity induced by multiphases (gas phase and liquid phase) and temporal scales (from nanoseconds to minutes), and the difficulty of quantitative diagnostics on the plasma dynamics and aqueous species. In this thesis, results from a computational investigation of plasmas in contact with liquids are discussed with the goal of improving our fundamental understanding of the interaction of plasma with liquid, and to suggest ways to control outcomes in applications of interest. Plasmas in bubbles in water are investigated and the computational results are compared to experiments. Plasma emissions are found to depend in large part on electron impact dissociative excitation of water vapor, electron impact excitation of dissociation products and excitation transfer from the plasma excited injected bubble gases to water vapor. Variations in the contributions of these processes are responsible for differences in the observed optical spectra and differences in radical production. Atmospheric dielectric barrier discharges (DBDs) in treatment of liquid covered tissue are computationally investigated for biomedical outcomes. When the plasma reaches the liquid, the plasma produced species are solvated and initiate aqueous reactions. Both short-lived radicals, like hydroxyl and superoxide, and long-lived reactive species, like hydrogen peroxide and ozone, are observed and their production pathways are discussed. The fluences of reactive species to the underlying tissue are recorded and found to be sensitive to dissolved oxygen, alkane-like hydrocarbons, UV/VUV photons and the thickness of liquid layer. The alternation of characteristics of the liquid layer is observed after multipulses. The liquid layer is acidified and the power of peroxynitrite is enhanced. The conductivity of the liquid layer is increased and will possibly affect the discharge. The frequency of multipulse DBDs is found to influence the interaction between pulses. Higher frequency enhances the interactions between pulses; and vice versa.