Plasma Interactions at Interfaces

Interactions of Streamer Discharges with Aqueous Solutions (Carly Anderson)

Plasmas generated in air (or other gases at ambient pressure) are gaining increasing attention for medical applications where the delivery of reactive nitrogen and oxygen species (RONS) is desired. These plasmas produce a complex mixture of reactive oxygen and nitrogen species (RONS), including hydrogen peroxide, nitric oxide, hydroxyl radicals, superoxide, and ozone, among others.

Since most interactions between plasma and cells/tissues will involve a moist or aqueous phase, a better understanding of the interactions between plasmas and an aqueous phase is needed. In my current work, I generate a streamer corona discharge (pulsed, DC plasma) over a target solution with submerged electrode (cathode) and study the effects. 

This form of plasma discharge has the advantages of allowing us to study transport across the gas-liquid interface and species evolution at the liquid-metal interface with a single device. The plasma streamer also induces an ionic wind, allowing us to study the effect of convective forces on mass transfer. Alex Lindsay has done some excellent work modeling this system (see above); our goal is to eventually to “match” our experimental and computational results.

Modeling of Momentum, Heat, and Neutral Species Mass Transport in Plasma-Liquid Systems (Alex Lindsay)

Some very interesting physical and chemical interactions arise at the interface between partially ionized gases and liquids. These interactions can be explored through both experimental and computational methods. Currently, we are using finite element methods (currently implemented in COMSOL) to study the momentum, heat, and neutral species mass transport between gas and liquid phases. A presentation on the topic, presented Fall 2014 at the Gaseous Electronics Conference, can be found in the attached files. So far our models have yielded some interesting qualitative results: sharp temperature gradients at the interface that arise from evaporative cooling which itself is a function of gas convection and sharp reactive species gradients (several logs reduction over microns and tens of microns on the liquid phase side of the interface). Application of traditional chemical engineering concepts like coupled heat-mass transfer is still relatively novel within the plasma community. 
Figure above: Temperature profile for a streamer or jet-like discharge over water. Note that whereas the impinging gas temperature is 300 K, the bulk liquid temperature is ~10 K lower because of evaporative cooling. The problem is modeled using 2-D axisymmetry.

Extending Modeling of Streamer Discharge Physics to Couple with Electrolytes (Alex Lindsay)

A coupled plasma and related fluid model is built on the software developed by the Multiscale Dynamics group at CWI for modeling streamer discharges. For the case of streamer discharge modeling, the CWI source code can be found at their site or at my gitHub site along with the commands I used to build the executable. We now couple the electrodynamic behavior of an aqueous electrolyte with the gaseous streamer to gain insightcomplexities occurring at the plasma-liquid interface; the role of liquid ionic strength on discharge IV characteristics; and the influence of electrochemical reactions occurring in solution and on the submerged ground electrode on the system's physical and chemical characteristics. In this way, we hope to gain fundamental insight into a very complex system using some a priori models.