Electric wind and water surface stabilization under impingement of an atmospheric pressure plasma jet

An electric wind or ionic wind is a neutral gas flow that occurs in weakly ionized gases due to the electrohydrodynamic force generated by the charged particle drag as a result of the momentum transfer from charged particles to neutrals (Nature Comm. 9 (2018) 371). In atmospheric pressure plasmas, although the convective flow of neutrals by electric winds can significantly contribute to the transport of radicals, it has not been considered so far and must be considered. For gas-liquid two phase systems, such as water under impingement of a plasma jet, these electric winds also give rise to many interesting physical phenomena. Gas jets can create dimple-like depressions in the liquid surface. As the gas jet speed increases, the cavity becomes unstable and starts bubbling and splashing. Our study, for the first time, revealed that an ionized gas jet blowing onto water produces a more stable interaction with the water surface compared to a neutral gas jet (Nature 592 (2021), 49). It has been found that when a plasma jet is impinged toward the water surface, deeper digging of the water surface occurs by the electric wind generated by the plasma. At the same time, the cavity undergoes a damped oscillation of about 100 Hz, which eventually becomes stable, i.e., the stability of the surface is improved despite the deeper digging of the water surface. We found that the Kelvin-Helmholtz instability becomes stabilized due to the strong electric field parallel to the water surface produced by high-speed ionization waves. The experimental observation was confirmed by computational modelling for the plasma jet and water surface. In the modelling, the plasma characteristics were analyzed by calculating the spatiotemporal change of the plasma, and the electric field near the water surface was quantitatively obtained. From this computational modelling, the experimentally identified improvement in surface stability and deepening of the water surface cavity was cross-validated.