Resolving Physics and Chemistry at the Plasma-Liquid Interface
Lai, Janis
2019
Abstract
The interaction of atmospheric pressure plasmas with liquids is currently being investigated for a wide range of environmental remediation applications, such as drinking water and wastewater treatments. Plasmas in contact with water can drive advanced oxidation processes at the gas-liquid interface directly and indirectly in the bulk liquid. These processes produce reactive species, such as hydroxyl radicals, ozone and hydrogen peroxide, that can degrade organic pollutants and microorganisms in water. As such, maximizing the plasma-liquid interfacial contact area can optimize the production and transport of these reactive species into the target liquid. The production of plasma in gas bubbles constitutes one approach to both increase the plasma-liquid contact area and reduce the electric field required for breakdown, thus optimizing the energy efficiency of production of plasma-derived reactive species in liquids. However, bubbles in liquids do not lend itself well to optical diagnostics. The surrounding liquid obscures the bubble volume, which prevents the direct interrogation of gas phase plasma parameters, such as the characterization of plasma species using optical emission spectroscopy. In addition, imaging of spherical bubbles inherently projects 3-D structure inside the bubble onto a 2-D plane, and leads to the loss of inherent spatial features that might provide valuable insight. A 2-D discharge cell was designed and constructed to enable direct simultaneous imaging of the liquid, gas and interface region. A thin layer of liquid is trapped between two clear plates, in which a single bubble is injected using a precision syringe. Plasmas are excited inside the bubble using nanosecond pulsed power supplies. Chemical probes were used in the 2-D cell to visualize the production and transport of plasma-derived species at the interface and in the bulk liquid. We observed the formation of large-scale circulation patterns, which drove the transport of oxidative species, such as hydroxyl radicals, and induced fluid mixing. Absorption spectrophotometry was used with a chemical probe to characterize the production of ozone in water, and led to the observation of plasma-driven capillary waves on the bubble surface. These traveling waves distorted the shape of the air bubble into a star polygon. The resulting shape was found to be dependent on bubble diameter and plasma excitation frequency. The bubble surface deformation led to self-organization of subsequent plasma discharges. This coupling between self-organization of plasma and interfacial capillary waves appeared synergistic. Bulk liquid conditions can also impact discharge conditions and resulting streamer propagation in bubbles. We investigated the effects of liquid conductivity on discharge morphology inside the bubble and plasma-driven fluid flow in the bulk liquid. Fast ICCD imaging showed that high liquid conductivity can increase refraction of electric field lines at the interface and reduce charge relaxation time, which led to the development of intense surface hugging streamers. Energy dissipation scaled positively with liquid conductivity as well, and can lead to increased thermal effects at the interface at high conductivity. Local temperature gradients can induce surface tension gradients in the liquid and drive Marangoni flow. Plasma-driven flow was measured using particle image velocimetry (PIV), and a positive correlation was found between liquid conductivity and flow speeds in the bulk. This result points to the possibility of high contribution of Marangoni effects in plasma-driven fluid flow.Subjects
plasma plasma-liquid interactions plasma chemistry fluid dynamics water treatment
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