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 (N 2 ) and oxygen (O 2 ) 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 O 2 in the surrounding gas plays a key role in enhancing the downward flow. Increasing the O 2 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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