Experimental investigation on the interaction of a nanopulsed plasma jet with a liquid target

Although the majority of atmospheric pressure plasma jet (APPJ) applications involve the interaction between the plasma and a surface, up to now the number of published papers focusing on this subject is limited, even though the nature of the target may strongly influence the plasma characteristics, the discharge structure, the generated reactive species, and consequently, the overall process. Under this framework, we investigated an APPJ impinging on a liquid surface and the effects of changing the stand-off distance, the applied peak voltage, and the pulse repetition frequency, looking at them as variable parameters often used to optimize plasma surface processes. Intensified charge-coupled device (iCCD) and Schlieren acquisitions suggest a key effect of gap width and peak voltage on the discharge morphology, velocity of the ionization front, and effluent fluid-dynamic behavior. The presence of a grounded liquid substrate enhances the electric field downstream of the source outlet: the smaller the gap the faster the ionization wave and the shorter the time for it to reach the surface. Consequently, a small gap favors the charging of the surface capacitance and the formation of surface ionization waves over the liquid target. Schlieren acquisitions highlight the formation of a transient turbulent structure propagating downstream of the gas flow, starting hundreds of microseconds after the initiation of the plasma discharge. The achieved results support the hypothesis that the formation of the turbulence is caused by a heating effect of the high-voltage electrode on the He gas flow. Another observed effect is the variation of the dimple caused by the He flow on the liquid surface as a consequence of the turbulence generated by the plasma discharge. The results presented here confirm how the gas dynamics and the discharge behavior are strongly affected by the presence of the liquid substrate and by its position with respect to the APPJ.