Session FR1: Plasmas on or Contacting Liquids

Understanding the Formation of Stable and Unstable Cathode Spots in Plasma-Liquid Interactions

Interactions of plasmas with liquid cathode electrodes have gained significant attention in recent years due to their potential applications in various fields, including environmental remediation, biomedicine, and materials synthesis. Cathode spots, localized regions of intense plasma discharge near the cathode surface, play a crucial role in these interactions. Herein, we have investigated the formation of stable and unstable cathode spots on the surface of liquid cathode. Plasma is generated by flowing helium (He) through a tungsten tube enclosed inside a tee tubing & observations of cathode spot is made through the variation of liquid conductivity, temperature, gap distance, applied voltage & He flow rate. At a gap distance of 5 mm and He flow rate of 0.8 liters per minute, the unstable cathode spot transforms to stable one as the conductivity and temperature reach critical values of ~65.4µS and ~92.3°F respectively. Increasing the gap distance or the addition of electronegative gas inhibits the formation of the stable cathode spot. Further investigations showed that at increasing values of applied voltage, the stable shaped pattern transforms to a cone shaped pattern. Increment of He flow induces liquid instability & results in the unstable shape of the cathode spot. The characterization of the cathode spot using optical emission spectroscopy and time-resolved imaging will be presented.

This work was supported by the NSF EPSCoR RII-Track-1 Cooperative Agreement OIA-2148653 and DOE DE-SC0021391. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.   

Probing the negative ions and its role in self-organized pattern of a 1 atm DC Glow Discharge with Liquid Anode

Plasma self-organization is a poorly understood process. Many hypotheses assume a reaction-diffusion system where electrons serve as the core activator and its diffusion, recombination, attachment are the inhibitor [1]. It is then crucial to understand the electron distribution throughout the discharge plasma under self-organization conditions. Research has shown the importance of the presence of oxygen gas [2] in pattern formation; and we have verified that self-organized pattern cannot be formed in a low oxygen (<5%) environment. It is conjectured that the reduction of oxygen partial pressure decreases electron losses due to the absence of processes such as dissociative electron attachment.

In this work, the role of attachment processes in pattern formation is explored. Rather than simply alter the partial pressure of oxygen, here we use photodetachment to affect the local electron density. A tunable pulsed laser is used to perform photodetachment in the threshold of some negative ion species, specifically O^- and O₂^- to determine whether the dissociative electron attachment loss is the major inhibitor. This limited scope activity aims to provide insight into mechanisms that provide a clearer understanding into how patterns form on the surface.

References:

[1] Rumbach, P., Lindsay, A. E., & Go, D. B. (2019). Turing patterns on a plasma-liquid interface. Plasma Sources Science and Technology, 28(10), 105014.

[2] Shirai, N., Uchida, S., & Tochikubo, F. (2014). Influence of oxygen gas on characteristics of self-organized luminous pattern formation observed in an atmospheric dc glow discharge using a liquid electrode. Plasma Sources Science and Technology, 23(5), 054010.   

Influence of electrical conductivity and permittivity on the streamer dynamics at water surface

Streamer propagation in gaseous medium, including air, is rather well understood, but it quickly becomes a complex phenomenon as soon as it approaches a solid or a liquid surface. Indeed, the properties of the surface, such as its electrical conductivity (σ) and dielectric permittivity (ε), strongly influence the streamer dynamics. Although it is a fundamental subject, understanding streamer-surface interaction remains a cornerstone in the context of applications. Initiated by electronic avalanches, streamers take place if the number of produced electrons is higher than 108, i.e. Meek's criterion. In these conditions, the E-field produced from electrons-ions separation becomes relatively high and controls the following steps of streamer propagation, mainly by initiating secondary avalanches close to streamer's head. It is stated that as the streamer approaches the surface, the production rate of photoelectrons decreases significantly leading to a cease of its vertical propagation. Meanwhile, charges accumulate at the surface and produce a radial E-field. If this latter is strong enough, radial avalanches ignite and lead to the formation of radial streamers at or near the surface. In the case of liquid surfaces, streamer propagation is strongly influenced by the liquid properties, mainly σ and ε. The former can be controlled by adjusting the concentration of ions in solution, while the latter can be controlled by choosing liquids with different permittivity. Herein, the influence of σ, from 5 to 1000 μS/cm, and ε, from 25 to 80, on the streamer propagation at water surface is presented. The discharges are produced by single shot nanosecond high voltage, and they are characterized electrically as well as by time-resolved imaging with a 1-ns-temporal resolution. σ influences the movement of ions present in solution, and they reorganize in response to the E-field. Such reorganization can significantly influence the E-field near or at the solution's surface and, therefore, the streamer's propagation dynamics. ε influences the E-field distribution and charge accumulation at the liquid surface, thus, the streamer's propagation dynamics.   

Plasma Characterization on Acoustically Structured and Turbulent Plasma-Liquid Interfaces

Enhancing mass transport at the plasma-liquid interface is an important consideration for the scaling of atmospheric pressure plasma water treatment systems to real world applications. Perturbing the plasma-liquid interface, either in a controlled manner via acoustic excitation of standing waves, or by promoting turbulence, enhances the gas-liquid contact surface area. Such perturbations, along with forced diffusion and mixing, improve the transport of reactive oxygen and nitrogen species (RONS) vital to the mineralization of organic contaminants. Coupled with these hydrodynamic enhancements are plasma chemistry and morphological effects owing to the complex structure of the liquid dielectric. Localized, reduced field (E/N) changes also affect gas phase chemistry and charged particle energetics at the interface. This work experimentally characterizes the effects of these complex surfaces on plasma properties in a pulsed discharge configuration, such as gas temperature and plasma induced optical emission, all of which are functions of E/N. Fast imaging is utilized to resolve length scales induced in turbulent flow relevant to plasma surface attachments. Spatially resolved optical emission spectroscopy is used to infer local E/N changes with interfacial spatial complexity. Finally, a 2D fluid/Monte Carlo model of this system is validated, and used to further generalize experimental results.   

Modelling of Acoustically Structured Plasma-Liquid Interfaces

Plasmas interacting with liquid surfaces produce a complex interfacial layer where the local chemistry in the liquid is driven by fluxes from the gas phase of electrons, ions, photons, and neutral radicals. Typically, the liquid surface has a mild curvature with the fluxes of impinging plasma species and applied electric field being nominally normal to the surface. With liquids such as water having high dielectric constant, structuring of the liquid surface by producing a wavy surface enables local electric field enhancement due to polarization of the liquid, as well as producing regions of higher and lower advective gas flow across the surface. This structuring (or waviness) can naturally occur or can be achieved by mechanical agitation such as with acoustic transducers. Electric field enhancement at the peaks of the waves produce local increases in sources of reactive species and incident plasma fluxes. In this paper, results are discussed from a computational investigation of pulsed atmospheric pressure plasma jets and pulsed plasmas in air onto structured liquid water surfaces consisting of standing wave patterns having wavelength and wave depth of hundreds of microns to 1 mm. The potential of structured liquid surfaces to enhance nanoparticle synthesis in the context of plasma driven solution electrochemical applications will be discussed.