Generating enhanced chemical reactions inside highly charged microscale droplets for remote delivery of reactive radicals and high purity nanomaterials*

The OH^· radical plays an important role in areas including atmospheric chemistry, water and pollution remediation, disinfection, tumour therapy and protein folding studies. Atmospheric pressure plasmas (APP) are very efficient generators of radicals including OH^·. Most configurations involve plasmas that are in contact or directly coupled to the substrate leading to complexity in species diagnostics and control. The RF plasma configuration can limit plasma and photon fluxes as well as electric field and current effects to the near electrode afterglow region, allowing remote delivery of mainly radical chemical species only and further restricted to predominantly OH^· and H₂O₂, which can be useful for validation of chemical models. However radical recombination in flight over extended plasma-free distances has not been characterised and may limit the technological benefits of remote delivery. We report measured OH^· and H₂O₂ fluxes over distances up to 200 mm from the RF plasma. We then present details of enhanced delivery using low-density aerosol streams. The fundamental mechanisms derive from a complex interplay between plasma and liquid microdroplets, involving charged and neutral species bombardment, interfacial electric field generation, solvated electron and chemical species surface reactions, evaporation and high-density vapour layer. While complex, the ultra-small volume, high surface to volume ratio and the well-defined spherical geometry offer opportunities for modelling within and beyond the plasma region. We present initial outcomes from one-dimensional spherical simulation of liquid reaction – diffusion chemistry with reference to direct measurements of downstream OH^· and H₂O₂ fluxes as well as measurements of droplet charge, from which estimates of electron and ion fluxes to droplet within the plasma can be obtained.