Understanding the mechanisms behind plasma-induced liquid flow is important for the transport of reactive species in liquid. In this study, we studied the effects of the surrounding gas compositions of a plasma-jet on the plasma-induced downward liquid flow using particle image velocimetry. Nitrogen (N2) and oxygen (O2) mixtures in different mixing ratios were supplied as surrounding gas around a helium (He) plasma jet at a constant flow rate. The results clearly indicated that O2 in the surrounding gas plays a key role in enhancing the downward flow. Increasing the O2 concentration increased the downward flow in the depth direction. An emission spectroscopy analysis suggested that reactive species related to excited atomic O were considered to be important for inducing downward flows. The relationship between the downward flows and oxidation reactions on a liquid target were discussed to determine the reasons responsible for the driving forces.
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