Electron kinetics, reactive species generation and transport in atmospheric pressure plasma jets and plasma-liquid interactions

Abstract

Atmospheric pressure plasma jets have received a lot of attention in the last decade due to the generation of large spectrum of reactive species such as electrons, ions, radicals and photons, which enable many potential applications in the field of biomedical engineering, water treatment and nanoparticle synthesis. Nonetheless, the generation, transport and decay mechanism are not well understood to date, which limits the control of reactive species and severely complicates the optimization of the plasma source for applications. This dissertation describes the development and implementation of in-situ spectroscopic diagnostics for detecting reactive species to understand fundamental plasma chemistry and electron kinetics from free plasma jets to plasma-liquid interactions. Specifically, atomic hydrogen (H) generation in an atmospheric pressure radio frequency (RF) plasma jet was investigated by 1-D two-photon absorption laser induced fluorescence in a He–H2 mixture. The measured time and spatially resolved H density profiles enable a detailed analysis of the species generation, transport and recombination along the axis of symmetry of the jet. The H density distribution for a continuous RF plasma jet is well represented by a Pseudo-1D plug flow model. In addition, RF power modulation has a strong effect on the H density in the active discharge region but has a negligible effect on the H density in the far effluent. A further study combining a measurement of both OH and H radicals are performed in a nanosecond pulsed plasma jets operating in humid He by 1D OH laser induced fluorescence (LIF) and H two-photon laser induced fluorescence (TALIF). It shows that H and OH are mainly generated between the electrodes in the APPJ rather than by the guided streamer. The produced H and OH inside the jet are convectively transported to the jet effluent and determine the H and OH densities in jet effluent. The dominant production and destruction mechanisms of H and OH are obtained from a 0D model. It shows that ion hydration and electron impact dissociation reactions are responsible for OH and H production and self-recombination and recombination with OH are responsible for the OH and H destruction, respectively. The different production mechanisms of H and OH can explain the different memory effects observed for OH and H for varying pulse repetition rates of the plasma generation. As many applications involve a scenario of plasma-liquid interactions, the impact of the strong coupling between plasma and liquid water on plasma properties and processes need to be thoroughly investigated. Thus, we studied the impact of the applied voltage, pulse width and liquid conductivity on the plasma morphology and the OH generation for a positive pulsed DC atmospheric pressure plasma jet with He-0.1% H2O mixture interacting with a liquid cathode. By adopting diagnostic techniques of fast imaging, 2D OH-LIF and Thomson scattering spectroscopy, it is shown that plasma instabilities and enhanced evaporation occur and have a significant impact on the OH radical generation. At elevated plasma energies, the plasma contracts radially due to a thermal instability through Ohmic heating and the contraction coincides with a depletion in the OH density in the core due to electron impact dissociation. For lower plasma energies, the instability is suppressed/delayed by the equivalent series resistor of the liquid electrode. An estimation of the energy flux from the plasma to the liquid shows that the energy flux of the ions released into the liquid by positive ion hydration is dominant, and sufficiently larger than the energy needed to evaporate enough amount of water to account for the measured H2O concentration increase near the plasma-liquid interface. To further study the role of electrons injected into the liquid and the induced chemistry, the electron kinetics in a negatively pulsed plasma interacting with liquid anode is studied by the Thomson scattering spectroscopy. A radial plasma contraction is found for a longer pulse width, which is similar to the plasma-cathode study but does not lead to a run-away behavior of the current. Although the gas temperature keeps increasing during the pulse, the development of thermal instability is prevented by the enhanced evaporation near the liquid surface. Plasma heating leads to a significant N2 mixing which enhanced the ionization in the core and is responsible to the observed radial contraction in emission. N2 mixing also results in additional challenges to analyze the Thomson scattering spectrum through a superposition of resonant LIF transitions of excited N2 on the spectrum leading to an unsymmetric Thomson profile. In addition, a spatial measurement shows an increased electron temperature and a drop in electron density near the liquid surface likely due to electron attachment to H2O molecules. The increase in electron temperature indicates a flux of hot electron injection into the liquid and suggests the possibility of a more complex non-equilibrium solution chemistry near the plasma-liquid interface. To understand the plasma induced liquid chemistry, it is necessary to quantify the dominant species flux from gas phase plasma to the liquid surface. In this thesis, the flux of OH and electrons in the plasma at the liquid anode were measured by laser induced fluorescence spectroscopy and current measurements to investigate the role of OH and electrons in plasma-enabled redox chemistry in solution. The impact of the voltage pulse width, voltage amplitude, liquid temperature and conductivity on the OH density distribution was also investigated. We observed a significant OH density near the liquid surface, which showed a transition from a ring-shaped structure to a more uniform structure with increasing plasma power. This transition coincided with a similar transition in the plasma emission intensity and electron density profile. A rotational Raman scattering indicated that this transition can be attributed to an enhanced N2 mixing at larger plasma-dissipated powers. Besides, a time-resolved measurement showed that the OH density segregates radially in the afterglow at velocities exceeding the gas velocity at room temperature due to enhanced gas convection resulting from the gas heating. While the OH flux was of the order of ~1021 m-2s-1 approximately two orders of magnitude lower than e flux, significant reduction of ferricyanide in the solution occurs during the pulse as well as slow oxidation occurs in the afterglow due to the much longer lifetime of OH compared to electrons. The Faradaic efficiency of the liquid redox chemistry was evaluated with H cell measurements and showed a good agreement with a 1D liquid phase chemical model with the measured electron and OH fluxes as the input. This result shows the capability to quantitatively describe the plasma-driven solution electrochemistry for a model redox couple based on OH and electron driven reactions. Overall, this thesis characterized reactive species generation, transport and decay mechanism and electron kinetics from free plasma jet to jet-liquid interactions by various in-situ spectroscopic methods. The gas phase kinetics are also related to the liquid phase chemistry and suggests an importance and complex role of the gas-liquid interface. The obtained results in this dissertation work benefit the plasma community by strengthening the understanding of fundamental plasma chemistry and plasma-liquid interactions, which will facilitate the development and implementation of novel applications.