Plasma Self-organized Pattern and the Coupling Processes at Plasma-liquid Interface

Abstract

It is known that under certain conditions, the plasma attachment at the surface of the liquid in a DC one atmospheric pressure glow discharge can self-organize both spatially and temporally. Such self-organization is also observed in DC glows with metal anodes as well, taking the form of organized arrays of dot-like attachments. However, the liquid anode pattern is typically more complex. Its formation is also subject to more complicated processes including liquid phase evaporation, fluid dynamics, sophisticated chemical reactions, species interaction, and local sheath electric field. In the community of plasma-liquid interactions, an understanding of many of these processes is still unclear due to their non-linear nature and the limitations of currently available diagnostics methods. On the other hand, one can assert that the patterns are a consequence of dynamical coupling and feedback between the aforementioned processes. By deconstructing this coupling and understanding the feedback pathways it may be possible to unearth the nature of pattern formation and dynamics. In this work, multiple diagnostics were performed to investigate the coupling processes in an atmospheric pressure DC glow discharge and its plasma-liquid interface. The I-V characteristics of the discharge and the corresponding shape and surface area of the self-organized pattern (SOP) were examined to correlate the changes in bulk plasma discharge properties with pattern dynamics. Various pattern morphology and the associated operating conditions necessary to realize the patterns were documented and it was found that both the oxygen entrainment and liquid properties had the most significant effect on the pattern dynamics. The importance of electronegative gas, including species such as oxygen, is then further examined through the examination of the negative ion hypothesis by laser photodetachment. Using detachment spectroscopy, negative ions in the discharge were detached by laser and the SOP showed no change implying that negative ions are not the primary cause of pattern dynamics. The coupling between the gas phase and plasma-liquid interface was further investigated by spatially resolved OES. The emission map of major species in the plasma is obtained by scanning the plasma with the focusing lens and an imaging spectrometer. Gas temperature and electron density are estimated from the rotational temperature of nitrogen second positive system and Stark broadening of hydrogen beta line, respectively. Next, substantial convective flow at the plane vertical to the plasma-liquid interface was observed under SOP by particle image velocimetry. Analysis indicated that the driving forces are plasma gas heating and water evaporation at the interface. Last, a non-trivial mass transport process at the plasma-liquid interface was observed: droplet generation was investigated via a fast camera and OES. The droplet generation from the liquid anode was proved due to gas bubbles bursting at the interface. These droplets can function as vehicles to enhance the mass transport of liquid-phase species toward the gas phase. From the aforementioned diagnostics, the couplings of pattern dynamics with gas phase operation, heat, and chemical dynamics were examined and found to provide useful insights into the SOP mechanism. At its core, the SOP problem is a manifestation of an interfacial process driven far from equilibrium-nonequilibrium thermodynamics. It is hoped that the data acquired in this thesis not only advances our understanding of SOP formation but also provides modelers with the parameters necessary to model this very complex system.