Momentum, Heat, and Neutral Mass Transport in Convective Atmospheric Pressure Plasma-Liquid Systems and Implications for Aqueous Targets

TL;DR

This study models momentum, heat, and neutral species transfer in plasma-liquid systems, revealing steep reactive species gradients at interfaces and the importance of temperature effects on reaction kinetics for biomedical applications.

Contribution

It provides a detailed finite element model of plasma-liquid interactions, highlighting reactive species penetration limits and the impact of convective cooling on chemical reactions.

Findings

Reactive species concentrations drop sharply near the interface.

A 10 K temperature drop affects reaction kinetics significantly.

Limited reactive species penetration influences therapeutic mechanisms.

Abstract

There is a growing interest in the study of plasma-liquid interactions with application to biomedicine, chemical disinfection, agriculture, and other fields. This work models the momentum, heat, and neutral species mass transfer between gas and aqueous phases in the context of a streamer discharge; the qualitative conclusions are generally applicable to plasma-liquid systems. The problem domain is discretized using the finite element method. The most interesting and relevant model result for application purposes is the steep gradients in reactive species at the interface. At the center of where the reactive gas stream impinges on the water surface, the aqueous concentrations of OH and ONOOH decrease by roughly 9 and 4 orders of magnitude respectively within 50 $\mu$m of the interface. Recognizing the limited penetration of reactive plasma species into the aqueous phase is critical to discussions about the therapeutic mechanisms for direct plasma treatment of biological solutions. Other interesting results from this study include the presence of a 10 K temperature drop in the gas boundary layer adjacent to the interface that arises from convective cooling and water evaporation. Accounting for the resulting difference between gas and liquid bulk temperatures has a significant impact on reaction kinetics; factor of two changes in terminal aqueous species concentrations like H$_2$O$_2$, NO$_2^-$, and NO$_3^-$ are observed if the effect of evaporative cooling is not included.