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Abstract Plasma can remediate nitrogen oxides (NO_x) emission from combustion processes. Nitric oxide (NO) oxidation reactions have been studied extensively. However, NO decomposition by plasma in a non-oxidizing environment, the timescales for NO dissociation reactions and their coupling with transport processes, the focus of this work, remains relatively unexplored. We report on axially resolved laser-induced fluorescence measurements of NO densities in a plasma in helium (He) with small admixtures of NO generated in a capillary tube by two outer ring electrodes, where one ring is powered by radio frequency (rf) high voltage. A limited number of chemical reactions describe the He/NO model system, and this description of the plasma chemistry allows for the quantification of the kinetic and transport mechanisms associated with the plasma-mediated NO decomposition. A 1D plug flow model shows the dominant role of electrons in addition to helium metastable species, and atomic nitrogen for NO decomposition, while the effect of metastables is largely counteracted by the ${\mathrm{NO}}^{+}$ recombination at the capillary wall, yielding a significant source of NO. A 2D reaction transport model assuming a given distribution of short-lived species in the plasma zone is also able to describe the NO recovery experimentally found in the plasma effluent. The observed NO recovery as a result of inhomogeneous NO decomposition by short-lived species and the resulting radial transport underlines the limitations of the widely used plug flow approximation for such systems. 

Herpes simplex virus type 1 (HSV-1) is a lifelong pathogen characterized by asymptomatic latent infection in the trigeminal ganglia (TG), with periodic outbreaks of cold sores caused by virus reactivation in the TG and subsequent replication in the oral mucosa. While antiviral therapies can provide relief from cold sores, they are unable to eliminate HSV-1. We provide experimental results that highlight non-thermal plasma (NTP) as a new alternative therapy for HSV-1 infection that would resolve cold sores faster and reduce the establishment of latent infection in the TG. Additionally, this study is the first to explore the use of NTP as a therapy that can both treat and prevent human viral infections. The antiviral effect of NTP was investigated using an in vitro model of HSV-1 epithelial infection that involved the application of NTP from two separate devices to cell-free HSV-1, HSV-1-infected cells, and uninfected cells. It was found that NTP reduced the infectivity of cell-free HSV-1, reduced viral replication in HSV-1-infected cells, and diminished the susceptibility of uninfected cells to HSV-1 infection. This triad of antiviral mechanisms of action suggests the potential of NTP as a therapeutic agent effective against HSV-1 infection. 

Abstract Plasmas in contact with liquids can degrade organic molecules in a solution, as reactive oxygen and nitrogen species produced in the plasma solvate into the liquid. Immersing small droplets (tens of microns in diameter) in the plasma can more rapidly activate the liquid compared to treating a large volume of liquid with a smaller surface-to-volume ratio. The interactions between a radio frequency glow discharge sustained in He/H 2 O and a water droplet containing formate (HCOO − aq ) immersed in and flowing through the plasma were modeled using a zero-dimensional global plasma chemistry model to investigate these activation processes. HCOO − aq interacts with OH aq , which is produced from the solvation of OH from the gas phase. The resulting HCOO − aq concentrations were benchmarked with previously reported experimental measurements. The diameter of the droplet, initial HCOO − aq concentration, and gas flow rate affect only the HCOO − aq concentration and OH aq density, leaving the OH density in the gas phase unaffected. Power deposition and gas mixture (e.g. percentage of H 2 O) change both the gas and liquid phase chemistry. A general trend was observed: during the first portion of droplet exposure to the plasma, OH aq primarily consumes HCOO − aq . However, O 2 − aq , a byproduct of HCOO − aq consumption, consumes OH aq once O 2 − aq reaches a critically large density. Using HCOO − aq as a surrogate for OH aq -sensitive contaminants, combinations of residence time, droplet diameter, water vapor density, and power will determine the optimum remediation strategy. 

Interactions at the interface between atmospheric pressure plasmas and liquids are being investigated to address applications ranging from nanoparticle synthesis to decontamination and fertilizer production. Many of these applications involve activation of droplets wherein the droplet is fully immersed in the plasma and synergistically interacts with the plasma. To better understand these interactions, two-dimensional modeling of radio frequency (RF) glow discharges at atmospheric pressure operated in He with an embedded lossy dielectric droplet (tens of microns in size) was performed. The properties of the sheath that forms around the droplet were investigated over the RF cycle. The electric field in the bulk plasma polarizes the dielectric droplet while the electron drift in the external electric field is shadowed by the droplet. The interaction between the bulk and sheath electric fields produces a maximum in E/N (electric field/gas number density) at the equator on one side of the droplet where the bulk and sheath fields are aligned in the same direction and a minimum along the opposite equator. Due to resistive heating, the electron temperature T e is maximum 45° above and below the equator of the droplet where power deposition per electron is the highest. Although the droplet is, on the average, negatively charged, the charge density on the droplet is positive on the poles and negative on the equator, as the electron motion is primarily due to diffusion at the poles but due to drift at the equator. 

Abstract Microbial biofilms are of critical concern because of their recalcitrance to antimicrobials. Cold atmospheric plasmas (CAP) represent a promising biofilm remediation strategy as they generate reactive oxygen and nitrogen species (RONS), but mechanisms underpinning CAP‐biofilm interactions remain unknown. We assess the impact of treatment modality on biofilm inactivation and show that CAP killing of Staphylococcus aureus biofilms is dependent on treatment conditions, including solution chemistry. In dry treatments, biofilms are locally ablated due to plasma‐produced O flux. For saline‐submerged biofilms, while we show that ClO − is generated at high concentrations in larger treatment volumes, CAP inactivation at low ClO − concentrations implicates other reaction pathways. Finally, we demonstrate CAP efficacy over conventional antimicrobials, underscoring its promise as a biofilm treatment approach.