AVS 61st International Symposium & Exhibition
    Plasma Science and Technology Thursday Sessions
       Session PS+SE-ThA

Invited Paper PS+SE-ThA8
Diagnostics of an Atmospheric-Pressure dc Glow Plasma in Contact with Solution: Insight into Plasma-Liquid Interaction

Thursday, November 13, 2014, 4:40 pm, Room 308

Session: Atmospheric Pressure Plasma Processing; Fundamental and Applications
Presenter: Koichi Sasaki, Hokkaido University, Japan
Correspondent: Click to Email
The interaction between an atmospheric-pressure plasma and a liquid attracts much attention in conjunction with plasma-aided medical treatments and plasma-assisted agriculture. Major interests are the production and the transport of OH radicals in plasmas in contact with liquids. In this work, we examined the spatial distribution of the OH radical density in the gas phase of a dc glow plasma produced between a stainless-steel nozzle as the anode and electrolyte of NaCl solution as the cathode by laser-induced fluorescence (LIF) imaging spectroscopy. We were careful about the influence of collisional quenching and the rotational temperature in deducing the spatial distribution of the OH radical density from the LIF images.

The maximum rotational temperature was approximately 3000 K, which was observed in the plasma column in contact with the electrolyte surface. The frequency of collisional quenching was also high in the contacting region to the electrolyte surface, suggesting that the dominant quencher of the excited state of OH was water vapor. The absolute density of water vapor was evaluated from the quenching frequency and the rate coefficient of collisional quenching. It was found that the partial pressure of water vapor was more than five times higher than the saturated vapor pressure of water at room temperature.

The OH radical density was high in the neighboring region to the electrolyte surface, suggesting that water vapor produced from the electrolyte is the parent of OH radical. However, the peak of the OH radical density was separated from the electrolyte surface, and also the OH radical density on the electrolyte surface was not zero. It is noted that, if OH radicals are not reactive on the electrolyte surface, the decrease in the OH density toward the electrolyte surface would not be observed. On the other hand, if OH radicals are too reactive in the gas phase, the OH radical density on the electrolyte surface would become zero (Note that the production of OH in the cathode sheath is negligible), and in this case we cannot expect the interaction of OH radicals with the electrolyte. The axial distribution of the OH density observed in this work indicates that the electrolyte surface interacts with OH radicals as their sink. In other words, OH radicals are lost on the electrolyte surface. A possibility of the loss process is recombination to form hydrogen peroxide and water. In addition, we can also expect the transport of OH radicals into the inside of the electrolyte.