Amanda M. Lietz

Air plasma treatment of liquid covered tissue: long timescale chemistry

Atmospheric pressure plasmas have shown great promise for the treatment of wounds and cancerous tumors. In these applications, the sample is usually covered by a thin layer of a biological liquid. The reactive oxygen and nitrogen species (RONS) generated by the plasma activate and are processed by the liquid before the plasma produced activation reaches the tissue. The synergy between the plasma and the liquid, including evaporation and the solvation of ions and neutrals, is critical to understanding the outcome of plasma treatment. The atmospheric pressure plasma sources used in these procedures are typically repetitively pulsed. The processes activated by the plasma sources have multiple timescales—from a few ns during the discharge pulse to many minutes for reactions in the liquid. In this paper we discuss results from a computational investigation of plasma–liquid interactions and liquid phase chemistry using a global model with the goal of addressing this large dynamic range in timescales. In modeling air plasmas produced by a dielectric barrier discharge over liquid covered tissue, 5000 voltage pulses were simulated, followed by 5 min of afterglow. Due to the accumulation of long-lived species such as ozone and N x O y , the gas phase dynamics of the 5000th discharge pulse are different from those of the first pulse, particularly with regards to the negative ions. The consequences of applied voltage, gas flow, pulse repetition frequency, and the presence of organic molecules in the liquid on the gas and liquid reactive species are discussed.

Plasma-induced flow instabilities in atmospheric pressure plasma jets

Pulsed plasma excitation of rare gases flowing into air has been shown to impact the stability of the flow in non-equilibrium atmospheric pressure plasma jets (APPJs). In this paper, the results from a numerical modeling investigation of the stability of a round He APPJ with a powered electrode exposed to the gas flow are discussed. Localized gas heating at the powered electrode occurs on the time scale of the voltage pulse, tens to 100 ns, which is short compared to the fluid timescales. An acoustic wave propagates from this heated, expanding gas and exits the jet. The wave disturbs the shear layer between the He and surrounding humid air, exciting a shear instability which grows downstream with the flow and increases the mixing of the humid air into the He. The effects of the eddy-dominated flow on ionization wave (IW) propagation in an APPJ were investigated. The IW followed the regions of the highest helium concentration, resulting in an increased production of NO, HO2, and NO2.

The consequences of air flow on the distribution of aqueous species during dielectric barrier discharge treatment of thin water layers

The desired outcomes of wet tissue treatment by dielectric barrier discharges (DBDs) strongly depend on the integrated fluences of reactive species incident onto the tissue, which are determined by power, frequency and treatment time. The reactivity produced by such plasmas is often expected to be proportional to treatment time due to the accumulation of radicals in the liquid over the tissue. However, one of the typically uncontrolled parameters in DBD treatment of liquids and tissue is gas flow, which could affect the delivery of plasma produced radicals to the tissue. Gas flow can redistribute long-lived, plasma produced gas phase species prior to solvating in the liquid, while not greatly affecting the solvation of short-lived species. Gas flow can therefore potentially be a control mechanism for tailoring the fluences of reactive species to the tissue. In this paper, we report on a computational investigation of the consequences of gas flow on treatment of liquid layers covering tissue by atmospheric DBDs by up to 100 pulses. We found that gas flow (through residence time of the gas) can control the production of gas phase species requiring many collisions to form, such as reactive nitrogen species (RNS). The resulting solvation of the RNS in turn controls the production of aqueous species such as and (aq denotes an aqueous species). With the exception of O3 and O3aq, reactive oxygen species (ROS) are less sensitive to gas flow, and so OHaq and H2O2aq, are determined primarily by discharge properties.

Plasma kinetics in a nanosecond pulsed filamentary discharge sustained in Ar-H 2 O and H 2 O

The plasma kinetics of Ar–H2O and H2O at atmospheric pressure are of interest for applications in biotechnology where rare-gas plasma jets treat liquid surfaces and in water treatment where discharges are generated in bubbles or directly in liquid water. Due to evaporation resulting from heat transfer to the liquid, for many conditions the mole fraction of water in the plasma can be large—approaching nearly pure water. In this paper, results are discussed from a combined experimental and computational investigation of the chemical kinetics in a high electron density plasma filament sustained in Ar–H2O at atmospheric pressure. The chemical kinetics were simulated using a 0D global model, validated by measurements of the absolute OH and H densities by laser induced fluorescence (LIF) and two-photon absorption LIF. The primary sources of H and OH during the discharge pulse are dissociative excitation transfer from metastable Ar atoms and Ar dimer excimers at low water concentration and electron impact dissociation of H2O at high water concentration. In spite of their similar sources, the density of OH was measured to be two orders of magnitude smaller than that of H at power densities on the order of 105 Jm−3. This disparity is due to electron impact dissociation of OH during the discharge pulse and rapid reactions of OH in the presence of high H and O densities in the afterglow. It is often assumed that OH is the dominant non-selective reactive species in water-containing plasmas. These results reinforce the importance of atomic species such as H and O in water containing high energy density plasmas. A numerical parametric study revealed that the lowest energy cost for H2O2 production is achieved at low energy densities in pure water. The high concentration of atomic radicals, which rapidly recombine, results in an overall lower energy efficiency of reactive species production. In particular, the selectivity of H2O2 production decreases with increasing power density which instead favors H2 and O2 production.

Molecular admixtures and impurities in atmospheric pressure plasma jets

A more complete understanding of reactive chemistry generated by atmospheric pressure plasma jets (APPJs) is critical to many emerging medical, agricultural, and water treatment applications. Adding molecular gases to the noble working gas which flows through the jet is a common method to tailor the resulting production of reactive oxygen and nitrogen species (RONS). In this paper, results are discussed from a computational investigation of the consequences of H2O and O2 admixtures on the reactive chemistry of He APPJs flowing into humid air. This investigation, performed with a 2-dimensional plasma hydrodynamics model, addresses the RONS that are initially produced and the evolution of that chemistry on longer time scales. Without an admixture, the impurities in 99.999% pure helium are a major source of RONS. The addition of H2O decreases the production of reactive nitrogen species (RNS) and increases the production of reactive oxygen species (ROS). The addition of O2 significantly decreases the production of RNS, as well as hydrogen-containing ROS, but increases the production of ROS without hydrogen. This selectivity comes from the lower ionization energy of O2 compared to N2 and H2O, which then allows for charge exchange reactions. These charge exchange reactions change the RONS which are produced in the afterglow by dissociative recombination. The consequences of impurities were also examined. Humid air impurities as low as 10 ppm in the helium can account for 79%-98% of the production of most RONS in the absence of an intentional admixture. The degree to which the impurities affect the RONS production depends on the electrode configuration and can be reduced by molecular admixtures.A more complete understanding of reactive chemistry generated by atmospheric pressure plasma jets (APPJs) is critical to many emerging medical, agricultural, and water treatment applications. Adding molecular gases to the noble working gas which flows through the jet is a common method to tailor the resulting production of reactive oxygen and nitrogen species (RONS). In this paper, results are discussed from a computational investigation of the consequences of H2O and O2 admixtures on the reactive chemistry of He APPJs flowing into humid air. This investigation, performed with a 2-dimensional plasma hydrodynamics model, addresses the RONS that are initially produced and the evolution of that chemistry on longer time scales. Without an admixture, the impurities in 99.999% pure helium are a major source of RONS. The addition of H2O decreases the production of reactive nitrogen species (RNS) and increases the production of reactive oxygen species (ROS). The addition of O2 significantly decreases the producti...

An array of atmospheric pressure plasma jets from a single ionization wave

Summary form only given. Atmospheric pressure plasmas are being investigated for wound healing, agricultural enhancements, sterilization, and functionalization of materials. An often used configuration is the atmospheric pressure plasma jet (APPJ). Although highly effective, the plasma emanating from the APPJ covers a small area. In order to increase the area treated arrays of APPJs have been developed. The individual jets in such arrays often have inter-jet interactions and may require multiple power supplies. In this paper, we discuss results from a computational investigation of an APPJ geometry which enables multiple jets to be generated from a single ionization wave and has a minimum of jet-to-jet interactions. The configuration consists of a dielectric tube having a row of holes along its length. Helium flows through the dielectric tube and out the holes as a He plume into ambient humid air. The discharge is initiated by a powered annular electrode inside the tube and a grounded ring electrode outside of the tube which overlaps the inner electrode. An ionization wave (IW) propagates along the inside of the tube and over the holes. Depending on the size of the holes, secondary IWs are launched from the holes perpendicularly to the tube into the He plumes, thereby producing a comb of plasma jets. This configuration is based on the work of Robert et al.[1] The multi-APPJ and IW behavior was investigated using 2-D plasma hydrodynamics model, nonPDPSIM. We found that the ability to produce the secondary IWs from the holes depends on the size of the hole and voltage magnitude and polarity, parameters which are investigated in this study. In general the holes must be commensurate to or larger than the Debye length in order for the axial IW to penetrate into the hole to launch the secondary IWs. Electric field enhancement is significant within the holes and assists in the propagation of the IWs out of the main tube. As the IW approaches and enters a hole, the IW initially propagates along one side of the hole, eventually developing into a full APPJ. If the hole is too large, the secondary IW may propagate at an angle. The production of reactive oxygen and nitrogen species (RONS) is critically important for biological applications. The effect of different pulse polarities on RONS production will also be discussed.