The transport of reactive nitrogen species from a non-equilibrium gas discharge is dominated by electrohydrodynamic forces created within the plasma.

Cold atmospheric plasma offers great potential for decontamination of heat-sensitive foods. CAP research is often case specific, due to its focus on specific target microorganisms and food products. In this work, the impact of different factors influencing the CAP efficacy is assessed: CAP setup (DBD electrode, operating on He/O 2), type of pathogen, model system properties and experimental protocol. Controlled tests on model systems indicate that intrinsic and extrinsic stresses impact the CAP efficacy. Taking into account all influencing factors, this work provides guidelines (i.e., with respect to setup , microorganisms, food properties and treatment protocol) that need to be included to ensure successful CAP treatment.

In an atmospheric pressure surface barrier discharge the inherent physical separation between the plasma generation region and downstream point of application reduces the flux of reactive chemical species reaching the sample, potentially limiting application efficacy. This contribution explores the impact of manipulating the phase angle of the applied voltage to exert a level of control over the electrohydrodynamic forces generated by the plasma. As these forces produce a convective flow which is the primary mechanism of species transport, the technique facilitates the targeted delivery of reactive species to a downstream point without compromising the underpinning species generation mechanisms. Particle Imaging Velocimetry measurements are used to demonstrate that a phase shift between sinusoidal voltages applied to adjacent electrodes in a surface barrier discharge results in a significant deviation in the direction of the plasma induced gas flow. Using a two-dimensional numerical air plasma model, it is shown that the phase shift impacts the spatial distribution of the deposited charge on the dielectric surface between the adjacent electrodes. The modified surface charge distribution reduces the propagation length of the discharge ignited on the lagging electrode, causing an imbalance in the generated forces and consequently a variation in the direction of the resulting gas flow.

Control of the plasma chemistry is essential for the effectiveness of atmospheric pressure plasmas in many applications. For this, the effects of the humidity of the feed gas on the discharge chemistry need to be considered. Detailed studies are scarce and many of them are dominated by surface interactions, obscuring any volume effects. Here, a negative nanosecond pulsed discharge is generated in a pin-pin 3 mm gap geometry in He+H2O that enables the study of volume kinetics due to minimal surface area. The effect of humidity on the discharge development, electric field and electron density is investigated through experiments and modelling. It is found that the presence of water vapour affects both the electron density at the start of the pulse (remaining from the previous pulse) and the ionisation rates during the ignition phase, leading to a complex dependence of the discharge development speed depending on the water concentration. The electron decay is studied using the 0D global kinetics model GlobalKin. The dominant reactions responsible for the electron decay depending on the concentration of water vapour are determined by comparing experimental and simulated results and these reactions are grouped in simplified kinetic models. It is found that with water concentrations increasing from 0 to 2500 ppm, the complexity of the dominant reactions increases with in particular O2 +, H2O3 + and water clusters becoming important for high water concentrations. This work also provides experimental data for validation of kinetic models of plasmas in controlled environments.

It has been shown that plasma generated in contact with liquid can be tailored to tune the composition of plasma functionalized liquids. For biomedical applications, it is necessary to understand the generation of the plasma treated liquids to modulate the composition and thus the biological response. In this work, two distinct discharge compositions were realized by modifying the location of the ground electrode in a pin-to-liquid plasma system. Through this simple modification to the configurations, the spatiotemporal characteristics of the discharge were significantly affected which, in turn, affected the composition of the generated plasma activated water (PAW). Colorimetric testing of the PAW generated from each system revealed that only one configuration was able to generate PAW with a high concentration of H2O2. Using time-, space-, and wavelength-resolved imaging of excited plasma species [OH, N2 (SPS), N2+ (FNS), and atomic O], the differences in PAW composition were linked to the differences observed in the discharge dynamics between the two configurations.