Plasma Self-organization and its Development on Liquid and Metal Anode Surfaces in Atmospheric Pressure DC Glow

Abstract

The concept of self-organization in nature has been pondered and written about since ancient times. Pattern formation derived from self-organization constitutes a complex phenomenon that is equally fascinating from an observational standpoint. Such phenomena appear in a broad range of systems including biological, chemical, and physical. Their occurrences have been associated with instability, symmetry breaking, bifurcation, and to the formation of dissipative structures. Recently, self-organized pattern structures studied using atmospheric pressure plasma sources are becoming an increasingly important topical area because of their applications ranging from environmental remediation to health care and material processing. A typical class of these weakly ionized plasma discharges is the 1 atm DC glow. To utilize the properties of this type of discharge requires demonstration of control which in turn requires some understanding of physical mechanisms underlying it occurrence. However, the origin of plasma self-organization is still poorly understood. To extend the understanding on the mechanisms of self-organization formation in DC atmospheric pressure glows, the exploration of the parameter space of occurrence was carried out. Here the overarching objective was to understand the control parameters and associated responses thereby allowing for the establishment of a phenomenological model of this self-organization—which heretofore remains elusive. Such plasma self-organization arises on both solid and liquid anodes in pin to plane electrode geometries. An array of self-organization patterns cataloged along with their dependencies on parameters such as discharge current, electrode separation, gas effects, and electrode conditions have been studied. Previously unreported threshold parameters for the pattern formations were recorded, as well as the unusual pattern morphologies. Optical emission spectroscopy (OES) was used to spatially resolve the plasma characteristics in the plasma column and near anode region. It also facilitated estimation of the local electric field at the anode. Particular to the liquid anodes, the species distributions, plasma density, gas temperature, and local electric field were determined. The importance of water vapor in pattern formation was elucidated thus highlighting the role of local evaporation. In this study, it was found that the onset of pattern formation followed a dramatic increase in water vapor in the gas phase. Additionally, it was found that pattern morphology was dependent on the type of electrolyte used. The observation of droplets in the gas phase was also discovered. Under the self-organization condition, it was observed that luminous high-speed particles were emitted from the plasma liquid interface near the center of the plasma attachment point. Apparently droplets are accelerating into the gas phase containing solution nanoparticles. The size and composition of these particles in the droplets was determined using the combination of a theoretical model, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis. Secondly, the plasma attachment on a tungsten anode was found to produce nanostructures on the surface giving rise to tungsten fuzz morphologies as typically observed in fusion wall simulators. This suggests that similar physics might be present in both scenarios. Overall this dissertation not only improves our fundamental understanding on the mechanisms of plasma self-organization, but it also provides an important basis for technological innovations that exploit self-organization in an array of arenas such as materials nanotechnology, chemical synthesis, waste water treatment, and medicine. This work also serves to extend our understanding of key controlling parameters for self-organization and forms a basis for any future model development and validation.