Session EW4: Plasma Liquid Interaction

Plasma reactive species production and delivery into liquids

The field of plasma medicine explores the unique therapeutic potential of plasma-activated solutions for various applications. Understanding plasma-water interactions and the penetration depth of reactive species is crucial for optimising plasma-based treatments. From an application perspective ultimately, the species within the liquid phase are those interacting with the various biological systems and are those mediating the biological signalling mechanisms and outcome. Shorter-lived radicals like hydroxyl radicals (OH) can be generated close to the plasma-liquid interface and diffuse into the liquid, while longer-lived more stable reactive species such as hydrogen peroxide (H 2 O 2 ) can be produced within the plasma phase and mass transport from the gas-phase into the liquid [1]. The combination of experiments and simulations play a pivotal role in unravelling the complex dynamics of plasma-water interactions, offering valuable insights into the chemical processes occurring at the plasma-liquid interface. This presentation will focus on recent advancements in benchmarking simulated plasma-water interactions with experimental measurement. Additionally, details of the depth of penetration of different radicals and reactive species into liquids will be assessed.[1] Y. Gorbanev, D. O'Connell, V. Chechik, Chem. Eur. J. 2016, 22, 3496.   

Interactions at the Interface between Atmospheric Pressure Pasmas and Liquids

Low temperature atmospheric pressure plasmas (APPs) interfacing with liquids have been extensively investigated in the context of plasma-aided decomposition of recalcitrant organic pollutants in water, plasma medicine and material synthesis. These applications leverage the unique ability of APPS to deliver large fluxes of highly reactive plasma species to liquids. Nonetheless, the strong non-linear coupling between the plasma and liquid phase leads to complex interfacial interactions which remain not well understood and have a major impact on the plasma properties and species fluxes impinging on the liquid. For example, liquid evaporation can lead to significant changes in the plasma composition, and sheaths and boundary layers have length scales of 100 μm or less leading to significant diagnostics challenges. Furthermore, atmospheric pressure plasmas are prone to instabilities introducing high spatial and temporal variations in plasma properties and dynamics.

 

In this presentation, we will report time resolved measurements of the development of plasma instabilities in pulsed discharges with a liquid electrode and describe the underpinning mechanisms that can trigger or prevent the formation of such instabilities in the presence of a liquid electrode. We will discuss the impact of plasma-induced liquid evaporation on the discharge morphology and reactive species formation, and show that with a detailed knowledge of the gas phase OH and electron densities near the liquid interface, we can quantitatively explain plasma-induced redox reactions with probe molecules in the solution phase for specific cases. Several examples of reactions enabled by plasma-induced solution electrochemistry with significant promise for chemical synthesis will be shown and a few case studies of unexplored liquid phase reactions will be presented.   

Complexity visualization and numerical simulation of reacting chemistry in plasma-treated liquid

Low-temperature plasmas have been widely studied in the fields of biomedicine, agriculture, sustainable energy conversion. In these applications, plasma-treated water is gaining increasing attention because they can produce abundant reactive species, whereas the aqueous chemistry is complex and its mechanism is not fully understood. We still have some challenges to reach comprehensive understanding of the nature of plasma-treated liquid chemistry. This study proposes mathematical/numerical approaches to tackle the issue. The complex network analysis based on the graph-theory, one of the information mathematics, enable us to reveal the hidden feature of liquid chemistry through the visualization and centrality-based identification of the reacting network [1,2]. A newly-developed one-dimensional reaction-diffusion model with hundreds of reaction processes of air-saturated water can simulate the influence of the irradiation of plasma-induced reactive species on the liquid chemistry by quantifying how various plasma species permeate into the liquid and what reactions are triggered.[1] T. Murakami and O. Sakai, Plasma Sources Sci. Technol. 29 (2020) 115018 [2] O. Sakai, S. Kawaguchi and T. Murakami, Jpn. J. Appl. Phys. 61 (2022) 070101   

Coupling of a microfluidic device with a reference cold plasma jet

With liquids being ubiquitous in living organisms, plasma-liquid interaction is of high relevance for applications of plasma science such as water decontamination, agriculture and medicine. A promising diagnostic tool for plasma-liquid interaction lies in microfluidic devices. With the objective of using Lab on a Chip technologies for studying the treatment of cancer by plasma, we developed a platform that enables the coupling of microfluidic devices with an atmospheric pressure plasma jet. The plasma device is based on the μAPPJ. It uses a capacitively coupled discharge excited at 13.56 MHz with two stainless steel electrodes clamped between a glass plate and a microfluidic device. The plasma channel has a volume of 30x1x1 mm³. The effluent of the same dimensions is confined by dielectric walls. The feed gas is helium with variable admixture of O₂, N₂ and H₂O. The 3D-printed microfluidic channel allows high control on the position of the plasma-liquid interface and on the velocity of the liquid. The microfluidic device can be operated with a continuous liquid multiphase flow for increased reactive species transfer.The developed plasma-microfluidic platform is to our knowledge the first demonstration of the coupling of a reference biomedical application-focused plasma jet with a microfluidic device. The platform provides high control over the delivery of plasma-produced reactive oxygen and nitrogen species and is a very powerful diagnostic tool for tailoring plasma reactivity in biomedical applications.   

Analysis of short-lived reactive species in gas-liquid interfacial plasma with high-speed liquid column flow

Gas-liquid interfacial plasmas (GLIPs) have been of great interest and have been widely applied. For medical and agricultural applications, it is necessary to control the reactive species produced by GLIPs, and it is especially important to understand the behavior of short-lived reactive species. In this study, we have developed a GLIP system with a high-speed liquid column flow to measure the spatio-temporal distribution of short-lived reactive species in the order of msec [1]. Using this system, we have succeeded in experimentally measuring the fast decay of OH radicals and analyzing their decay times using a numerical model that takes surface localization into account. Furthermore, we are trying to measure not only OH radicals but also short-lived reactive nitrogen species. We have successfully measured the time decay of precursors of reactive nitrogen species by using the reagent p-HPA (p-hydroxyphenylacetic acid), which is a scavenger of nitrite and nitrate precursors. The results, such as the fact that only the precursor of nitrite decayed with time, led to the conclusion that the precursor of the reactive nitrogen species detected in the present study was N₂O₃.   

Spatial and temporal dynamics of single nanosecond discharges in air with water droplets

Discharges in the presence of liquids are applied in many fields, such as water processing (depollution, agriculture, etc.), material treatment (synthesis, functionalization, machining, etc.), and health care (virus deactivation, wound healing, etc.). Regardless of the targeted application, a profound understanding of plasma–liquid interactions is needed to optimize the process. Therefore, extensive research efforts have been put towards analyzing the physical and chemical phenomena occurring at the plasma– liquid interface, particularly during the last decade. Recently, important progress has been made in studying the dynamics of a discharge generated in gaseous medium with millimeter-scale droplets. Specifically, the influence of discharge processing on the properties of the liquid droplets has been assessed by analyzing the hydrodynamic, physical, and chemical phenomena (e.g. internal flows, plasma capillary phenomena, droplets drying under the effect of discharge, and residues left on substrates) occurring in the droplets. This study investigates the electrical characteristics and the spatial-temporal dynamics of nanosecond discharges in air containing one or two millimetric droplets of deionized water. Analysis of the effects of voltage amplitude (V_a) and pulse width on the discharge mode shows that at low V_a, the discharges are run in streamer mode; however, at high V_a, a streamer-to-spark transition is observed. Although we found that the droplet size (diameter between 2 and 4 mm) does not significantly influence the discharge dynamics, its position with respect to the gap (on- or off-axis) has a strong effect. Time-resolved imaging of three droplet configurations (one on-axis droplet, one off-axis droplet, and two on-axis droplets) was used to unveil the ignition and propagation dynamics of streamers and sparks at nanosecond time scale. We also investigated the role of droplet electrical conductivity on the discharge behavior. On the other hand, a simplified fluid model is developped, and the initial results show a great agreement with the experimental results. Therefore, the model can be utilised to determine the role of different physical parameters, such as dielectric permitivity, electrical conductivity, and droplet shape.