Ion transport at the plasma-liquid interface

Abstract

Plasma-liquid interaction is a complex multidisciplinary topic that has drawn great attention from a variety of fields such as analytical chemistry, chemical engineering, nanoparticle synthesis, microfluidics, electrolysis, advanced oxidation engineering and medical applications. Interfacial transport of ions has been studied in a wide context ranging from chemical reaction, charge transfer and mass transfer. However, since plasma-liquid interface is a multi-faceted system involving electrohydrodynamic phenomena, bi-directional species transport and multiphase chemistry, there is much to be understood about the near-surface ion transport. In this thesis, I investigate cross-interface transport of ions in microwave-driven plasma in contact with DC-biased electrolyte. Visible line emissions of sodium are observed near the liquid surface when the solution is negatively biased, along with surface distortion and microdroplets spraying. To figure out the transport mechanism of sodium ions from liquid to plasma, we conducted high speed surface motion imaging and examined the correlation among the surface motion, optical emissions and potential drop in the plasma. From the experimental results, the ion transport can be described into three steps as a surface deformation induced by ionic wind, Taylor-cone formation and Electro-spraying ionization. Simplified sheath-liquid interface model is adapted to analyze the experimental observations on the surface. When the electric field at the sheath region reaches critical electric field overcoming capillary force, the surface deforms into Taylor cone shape and electro-spraying has been occurred at the tip of the cone. Finally, droplets ejected into the plasma bulk region are reduced to ions by evaporation and coulomb fission. We demonstrate that the emission of sodium ions is induced by the electrohydrodynamic instability of the liquid surface, commonly called electro-spraying ionization