Plasma-induced flow of de-mineralised water is governed by the aqueous grounding configuration

Plasma-induced liquid flow is a topic that is receiving accelerated interest in recent years as a possible method to control the distribution of reactive oxygen and nitrogen species through a liquid, thereby improving the efficiency of plasma-liquid interactions. Here we investigate the significance of electro-hydrodynamic forces for the interaction of a monopolar pulsed non-thermal helium atmospheric pressure plasma jet and de-mineralised water through a combination of experimental methods and scaling laws. Our study finds that the flow in electrically floating liquids exhibits behaviour characteristic of gas phase electro-hydrodynamic (EHD) forces being the dominant driving force. We come to this conclusion primarily through linking the flow velocity to gas phase ionisation. In line with this mechanism, we consistently observe that positive jets, which are known to cause stronger gas-phase ionisation, induce stronger liquid flows than negative jets. In electrically grounded liquids, this strong influence of the jet polarity was not observed, which suggests that introducing a grounded electrode into the liquid fundamentally modifies the main driving force of the flow, potentially via interfacial or bulk liquid EHD forces. The results of this study therefore clearly show the important role that the electrical grounding configuration plays in the mechanisms behind plasma-induced liquid flow.