Plasma-Induced Oxidation in Micro-Droplets: Quantifying H₂O₂ and OH Fluxes and Transport Limitations

The plasma treatment of micro-droplets significantly enhances the reactivity transfer of gas phase species into the liquid phase and enables more efficient conversion of chemical compounds. While OH fluxes to the droplet have been obtained using gas phase density measurements, the determination of these fluxes involved assumptions. In this work, the H₂O₂ production and OH flux to the droplet have been quantified using a combined approach of liquid phase measurement and 1D reaction-diffusion modeling. It was found that H₂O₂ is majorly produced in the gas phase. To quantify the OH flux, four compounds (formate, ascorbic acid, ferrocyanide, caffeine) that readily react with OH were treated at varying initial concentrations. Two transport limited trends were observed: (1) solute diffusion limited conversion for lower initial concentrations, and (2) gas phase species flux limited conversion for higher initial concentrations. The latter limit allows for the OH flux determination. Furthermore, it was found that competing reactive chemistry in the liquid phase, as in the cases of ferrocyanide and caffeine, can result in reaction limited conversion and skew the OH flux quantification. The OH flux derived from the formate and ascorbic acid measurements showed excellent agreement with previous OH gas phase measurements and are recommended to be used for OH flux measurements in plasma-liquid setups for which the liquid phase chemistry is not dominated by other oxidizing species such as ozone.