Controlled plasma–droplet interactions: a quantitative study of OH transfer in plasma–liquid interaction

Abstract

Plasmas in contact with liquids are a rich source of OH radicals and have been extensively studied in the last decade to leverage the ability to generate chemically reactive species in gas phase plasmas to decompose organics. Multiphase transfer of OH radicals is highly transport limited and to overcome transport limits, the plasma activation of aerosols, small liquid droplets, interspersed in the plasma has been proposed. In this work, we report a combined experimental and modeling study of a controlled plasma–droplet interaction experiment using a diffuse RF glow discharge in He + 0.2% H2O with detailed plasma diagnostics, ex situ analysis of the plasma-induced chemistry in the droplet containing formate, droplet trajectory and size measurements. This enables a quantitative study of the reactivity transfer of OH from the gas phase plasma to the liquid phase and how its diffusion limitations impact formate decomposition in the water droplet. For a droplet with a diameter of 36 μm, we observed 50% reduction in formate concentration in the droplets after plasma treatment for droplet residence times in the plasma of ∼10 ms. These short droplet residence times in the plasma allow in some cases for droplet size reductions of ∼5% in spite gas temperatures of 360 K. A one-dimensional reaction–diffusion model was used to calculate the OH transport and formate oxidation inside the droplet and was able to predict the conversion of formate by plasma in a droplet without any fitting parameters. The model further shows that formate conversion is dominated by near-interfacial reactions with OH radicals and is limited by diffusion of formate in the droplet. The results show that a controlled plasma–micro-droplet reactor as reported in this study might be an excellent tool for detailed quantitative plasma–liquid interaction studies.

Non-Thermal Plasma as a Novel Strategy for Treating or Preventing Viral Infection and Associated Disease

“…Short-lived species usually have a limited penetration depth (determined by their lifetimes) and will react near the plasma-liquid interface, making the reactivity transfer extremely transport limited. For example, OH radicals have penetration depths up to a few μm in water [116][117][118]. The solvation of neutral species into liquid water depends on its Henry's law solubility constant.…”

“…While the effect of OH against NDV in plasma-activated solution [163] is suggested, other studies imply that OH had no role in virus inactivation [99,121]. The demonstration that OH radicals can decompose 50% of the hydrocarbons in a droplet of 60 µm [117] suggests that a similar effect could be anticipated for virus in small droplets or virus particles present in the plasma. Although the role of plasma generated O radicals on virus inactivation has not been reported to date, several studies report plasma conditions that enable the inactivation of Penicillium digitatum spores [164], cancer (THP-1 leukemia) cells [165], bacteria (E. coli) [166], and yeast cells (Saccharomyces cerevisiae) [167] by O radicals.…”

Characterization of an RF-driven argon plasma at atmospheric pressure using broadband absorption and optical emission spectroscopy

“…The plasma reactor used in this study generates an RF-driven capacitively coupled glow discharge in a water-cooled parallel plate electrode type configuration. A detailed description of the reactor is provided in 31,32 . The reactor is designed to facilitate access for optical diagnostics along the length of the plasma (19.1 mm) and within the inter-electrode spacing of 2 mm.…”

Transport of Gaseous Hydrogen Peroxide and Ozone into Bulk Water vs. Electrosprayed Aerosol

“…Transformation of bulk water into fine droplets or aerosol results in an increasing surface-to-volume ratio and thus accelerates the transport of RONS into the water, which is of vital importance for slowly soluble species, such as ozone. This concept has been adopted by several research groups [38][39][40][41], as well as our previous work [31,32,42]. For instance, in two different cold air plasma sources (streamer corona and transient spark discharge), PAW was prepared by the electrospray of fine aerosol droplets directly through the active plasma zone, which resulted in a very efficient transfer of gaseous RONS into water.…”