Evaluation of Short-Lived Reactive Species Decay Using High-Speed Water Flow in Contact with Atmospheric Pressure Plasma

In recent years, novel applications of non-equilibrium atmospheric pressure plasmas (APPs) in a liquid or in contact with a liquid have been found in the plasma life science field. These applications often employ APP as a source to deliver the reactive oxygen and nitrogen species (RONS) to the liquid phase. However, much of the RONS chemistry at the plasma-liquid interface has not been fully understood. The RONS chemistry at the plasma-liquid interface is an unsteady and nonuniform reaction network include charged particle transport, photochemistry, radical fluxes into the interface, and free radical reaction. Recently, Z. Liu <i>et al</i>,. succeeded in experimental measuring the spatial and temporal distribution of some stable RONS (H₂O₂, NO₂^-, NO₃^-)^[1]. Their findings can contribute greatly to an understanding of millimeter- and second-scale reaction network at the interface. But distribution of short-lived RONS (e.g. OH, HO₂, NO) are hardly characterized in experiments due to their high reactivity and non-uniformity. Because the predominant reactions by APP can occur near the plasma-liquid interface, the experimental fact regarding spatial and temporal distribution of short-lived RONS, such as OH, can be critical discussion in the RONS chemistry.<br/>To experimentally observe the behavior of the liquid phase OH, we built an APP system with a high-speed water jet through APP, which enables us to detect the advection of the liquid phase OH^[2]. In order to detect the liquid phase OH originating from APP exposure, terephthalic acid (TA), which reacts with OH to form 2-hydroxyterephthalate ion (HTA) as a highly fluorescent material, was used. The TA reagent flow is perpendicularly aligned to collide with the water jet experienced plasma exposure. The distance between the termination of plasma plume exposure and the collisional mixing point of the TA flow with water jet (d_g) is precisely controlled with at least 1 mm resolution. Owing to a high-speed water jet (over 10 m/s), this spatial resolution corresponds to temporal resolution with approximately 0.08 ms.<br/>The obtained OH decay using advection system rapidly decreased from 88 nM to 2 nM within 1.2 ms. The N₂ addition to the feed gas tends to accelerate the OH decay even though no significant decrease in H₂O₂ concentration by the N₂ addition. This can be explained by not decrease in OH generation but OH loss enhancement by co-dissolved RONS from N₂ addition. To explain the fast decay of liquid phase OH, we assumed the two distribution models; One is a uniform cross-sectional distribution of OH and the other is highly surface-localized distribution of OH. In both models, OH is consumed only by the reaction with OH, H₂O₂, and NO₂^-. The temporal and spatial distribution of OH in the water flow calculated from the uniform model was not consist with experimental OH decay. Because the rate of the second-order reaction with OH strongly depends on the OH concentration, this discrepancy can be due to the oversimplification of the OH distribution. On the other hand, OH decay calculated using the non-uniform model was much accelerated and explained the experimentally-obtained decay better. However, the OH decay calculated using an experimentally-obtained concentration of NO₂^- overestimated and was much faster than the experimentally-obtained decay. This overestimation of OH decay implies that not NO₂^- but NO₂^- precursor exists in the water jet immediately after the APP exposure.<br/>In the presentation, we would like to discuss the detail comparison between experimental and simulation results.<br/>Reference<br/>[1] Z. Liu, C. Zhou, D. Liu, T. He, L. Guo, D. Xu, and M.G. Kong, AIP Adv. 9, 15014 (2019).<br/>[2] K. Takeda, S. Sasaki, W. Luo, K. Takashima, and T. Kaneko, Appl. Phys. Express 14, 56001 (2021).