Fluid modeling of solvated electrons and reactive species chemistry at direct current plasma-water interfaces

Shane Keniley, Lam Research

One of the most promising emerging fields in plasmas is that of plasma-water interactions. Electrons in the plasma facilitate the production of both short- and long-lived reactive oxygen and nitrogen species, which have been shown to disinfect surfaces, induce cancerous cell death, and open up reaction pathways for high value chemical synthesis. A plasma in direct electrical contact with water will induce all of these effects in both the gas and liquid phases. This work is focused on the multiscale modeling of a direct-current (DC) plasma incident on liquid water. A new open source plasma chemistry software, Crane, was developed in the MOOSE finite element framework in order to study the detailed chemical reactions that exist in plasma-water systems. Crane was coupled to a MOOSE application dedicated to plasma transport, Zapdos, which was upgraded to facilitate the modeling of multispecies plasmas. A fully coupled plasma-water interface model was developed using the combined software, including electron transport across the interface, neutral species solvation and evaporation, and tightly coupled chemical reaction networks in both plasma and liquid phases. 

The coupled plasma transport and chemistry models allow for the analysis of the electronic and chemical structure of the plasma-water interface. Results show that the chemical composition of the water is dramatically affected by the polarity of the driven electrode in the plasma phase. Simulations were supported by optical emission spectroscopy and chemical probe measurements in an equivalent electrochemical cell. The numerical model predicted that the disparate H2O2(aq) concentrations seen between anodic and cathodic plasma operation are the result of solvated electrons degrading any H2O2(aq) before it is allowed to accumulate. Experiments carried out in support of this prediction demonstrated that the H2O2(aq) concentration increased as solvated electron scavengers were added, showing strong agreement with simulation results.