Generation of a direct-current, atmospheric-pressure microplasma at the surface of a liquid water microjet for continuous plasma-liquid processing

Plasmas at the surface of or inside liquids are of importance for emerging applications, and are often formed with stagnant liquids. Here, the authors present the generation of a direct-current, atmospheric-pressure microplasma at the surface of a liquid water microjet that enables solution species to be transported by forced convection. The water jet is formed by pumping conductive ionic solutions through a plastic capillary tube in a vertically falling geometry, and overcomes Plateau-Rayleigh instabilities by controlling the flow rate, resulting in a constant diameter jet of ∼0.45 mm over lengths of more than 30 mm. Analysis of the electrical characteristics of the complete microplasma-water jet system shows that the current-voltage (I-V) relationship is linear with a large positive slope when the solution conductivity is relatively low. The authors show that the primary contribution to this large resistance is the confined solution geometry. As proof-of-concept, the authors demonstrate that plasmonic Ag nanoparticles can be continuously produced at steady state from solutions of silver nitrate, opening up the possibility of scaled-up production of materials by plasma-liquid processes.