Session MR12: Plasma-liquid Interaction II

Atmospheric pressure plasma jet treatment of empty and water filled microchannels

With increasing interest in plasma activation of liquids, more refined methods of controlling the interaction between atmospheric pressure plasmas and liquids are needed.  In addition to plasma properties, achieving such control requires consistency in the properties of the plasma-liquid interface and exposure time of the liquid to the plasma (the “dose”).  One configuration to obtain this control is water flowing through channels – the area exposed to the plasma and residence time are well known and can be specified.  In this paper, we discuss results from a computational investigation of atmospheric pressure air plasmas interacting with water filled channels.  The work is performed using a 2-D plasma-hydrodynamics model nonPDPSIM [1].  Investigations were performed of dry and water filled rectangular channels (depth of a few 10s to 100s microns).  The dynamics of the surface ionization wave propagating across the channels will be discussed.  We found that the hydrophilicity of the material of the channels plays a role in the production of near-surface liquid species making the surface convex or concave.  The polarization of the water with surfaces having different shapes then affects the electric fields near the water surface.

[1] Seth A Norberg et al 2015 Plasma Sources Sci. Technol. 24 035026.   

Not Participating

Atmospheric plasma in contact with aqueous solutions have been shown to act as gaseous electrodes, capable of promoting electrochemical reactions at the plasma-liquid interface. While the reduction potential in conventional electrochemical systems is controlled by applying a known voltage to a solid electrode via an external circuit, methods of controlling the reduction potential in plasma-liquid systems remain poorly understood. The hypothesis is that the plasma-liquid interface imposes a boundary condition in the solution, wherein the reduction potential is constrained by the plasma parameters. In this work, a RF-driven Ar jet impinging upon an aqueous solution was characterized using laser Thomson scattering and electrochemical measurements. The experimental findings will be discussed and an analysis establishing a link between the plasma parameters in the gas phase to the reduction potential in the liquid phase will be presented.   

Exploring chemical modifications caused by the COST-Jet in NO-rich gas admixtures by means of FTIR and Mass Spectrometry on the simple biological model cysteine

Cold atmospheric plasmas (CAP) can be used as an effective treatment for chronic wounds, cancer, and even antibiotic-resistant bacterial infections1. Among the myriad reactive species that are produced in CAPs, nitric oxide (NO) is theorized to play a crucial role in these applications. Research has shown that NO restricted bioavailability is one of the main causes of complications in wound healing, infections, and decreased tissue microcirculation2. Since chemical modifications can have complex reaction pathways, the amino acid cysteine was used to represent a simple biological model, as previously introduced, due to its amino, carboxyl, and thiol groups that are prevalent in well studied biological macromolecules3. In this research, we use the Capacitively Coupled Atmospheric Pressure Microplasma Jet (COST-jet)4, with a synthetic air admixture (1 slm He, 2 sccm O2, and 8 sccm N2) that has been shown to produce the highest NO densities in the effluent5, in order to understand the chemical modifications that this NO-rich plasma produces in a simple biological model: cysteine. These modifications were analyzed through mass spectrometry and Fourier Transform Infrared (FTIR) spectroscopy for different treatment times in order to understand their possible biological relevance.    

Insights into hydrogen peroxide generation and reaction chemistry at a dc plasma-liquid interface by multiphysics modeling and chemical probe characterization

The plasma-liquid interface is a complex reaction system that presents both challenges and opportunities for emerging applications in water treatment, materials synthesis, and chemical conversion. Most of the studies to date have been experimental, including spectroscopic characterization of the plasma and chemical characterization of liquid-phase species. Here, we present a multi-physics model of the plasma-liquid interface that encompasses both the plasma and plasma-liquid interface using the MOOSE-based drift-diffusion-reaction software, Zapdos-Crane. The model was used to study a humid argon dc plasma over a water electrode, with results supported by experimental measurements. In this system, one of the reactions that occurs is the formation and dissolution of hydroxide (OH) radicals, which subsequently produce hydrogen peroxide. We studied potential mechanisms for H2O2 production with the plasma operated as both the cathode and anode. From both modeling and experiments, our results reveal that H2O2 production is increased during anodic plasma treatment due to elevated water vapor dissociation reactions near the interface. Solvated electrons generated during cathodic plasma treatment are shown to directly degrade aqueous H2O2, substantially inhibiting its accumulation.   

Electron generation during plasma formation in liquid water: The role of negative hydroxyl ions

Multiphysics simulations are performed to identify the role of negative hydroxyl ions on liquid phase plasma discharge formation. The simulations are conducted for a needle-like powered electrode with two different nano-second rise time voltage profiles – a linear and an exponential rise both having a peak voltage of 15 kV.  The predictions show that the electron detachment from negative hydroxyl ions which has a much lower threshold energy requirement provides stream of electrons at low applied voltage during the initial rise time. The electrical forces from the electron detachment process generates stronger compression but a weaker expansion regime in the liquid resulting in ~40% increase in the density and only ~1% decrease. The electron detachment tunneling process is found to be not limited by the electric field, rather the availability of negative hydroxyl ions in the system and ceases when these ions are depleted. The role of tunneling ionization of water molecule was also assessed and was found to form an electron wave at a higher applied voltage, but the resulting peak electron number density is typically six orders of magnitude larger than the detachment tunneling. The higher electron number density allows the recycling of depleted negative hydroxyl ions in the system and can reestablish tunneling detachment. Path flux analysis are conducted to determine the kinetics responsible for the recycling of the negative hydroxyl ions.     

Modelling of electron multiplication mechanism in nanovoids initiating the breakdown in liquid water

The explanation of discharge initiation in liquid water by a nanosecond electric pulse poses a challenge, both theoretically and experimentally. The short duration of the pulse does not allow for a conventional gas breakdown initiated by electron avalanche – the liquid water does not have enough time to heat to form water vapor. Therefore, alternative mechanisms that operate on sub-nanosecond and sub-micrometer scales are assumed to be present. One of the prevailing hypotheses explains the initiation of the breakdown by the generation and expansion of nano-cavities due to strong negative pressure caused by the inhomogeneous electric field around the electrode. In this work, we present a combination of modeling approaches directed at analyzing the processes that would govern the discharge initiation. In particular, we use fluid dynamics to model the propagation of negative pressure in the liquid. The resulting negative pressure values are then used for the simulation of the cavity growth. Finally, the electron multiplication occurring in the cavities is modeled using the particle-tracking GEANT4-DNA software. The combination of methods allows us to peek into the time-resolved dynamics of discharge initiation in cavitating water.  The results reveal secondary electron gain per primary electron per volume for various voltage pulses and electrode shapes and are compared with published experimental observations.   

Memory propagation in barrier discharge at water interface

Barrier discharges are known for strong memory effects caused by residual charges deposited on the dielectric surface or metastable states and free charges present in the surrounding volume. These memory effects create the bond between the present and previous states and determine the future, they influence the statistical probabilities for the discharge pulses generation in both time and space, and are essential for proper understanding of the discharge physics and stochasticity of their behaviour. We present an experimental study of the memory propagation in a surface barrier discharge operated at the water interface, employing electrical measurements and synchronised 2D intensified CCD imaging. Analysis of hundreds of subsequent periods recorded with high temporal resolution and large dynamic range and their evaluation using conditional and unconditional discharge pulse occurrence probabilities confirmed known memory pathways and revealed new memory patterns. A memoryless state within the strongly memory-driven system was found as well.   

Modeling of plasma-activated aqueous chemistry

Plasma-activated water is gaining increasing attention in biomedical applications due to its potential to produce abundant reactive species. However, plasma-induced aqueous chemistry is complicated and its mechanism is not fully understood. A newly developed time-dependent one-dimensional numerical model reveals the effects of the influx of various plasma-induced reactive oxygen/nitrogen species on aqueous chemistry. In particular, the emphasis is on elucidating the chemistry of interactions with nitrogen, oxygen, carbon, chlorine and water driven by plasma species.