Effects of initial surfactant concentration on plasma-induced liquid flows

Clarifying the mechanisms of liquid flows induced by plasma irradiation is necessary to understand the plasma–liquid interaction. In the present work, we studied the effects of the initial surfactant (lauryl betaine) concentration on the plasma-induced liquid flows using particle image velocimetry. The results indicated that the initial surfactant concentration affected the generation characteristics of a liquid flow that linearly moved in the depth direction of the liquid. The intensity of the driving force strongly depended on the initial surfactant concentration and varied with the plasma-irradiation time. At a concentration of 20 mg/l, the liquid flow changed in the opposite direction during plasma irradiation. In the concentration range of 50–150 mg/l, the liquid flows were mostly enhanced in the same direction. The liquid flows became extremely weak in the surfactant solution above a critical micelle concentration (approximately 400 mg/l). We focused on the Marangoni flow generated by a surface-tension gradient as the driving force. The surface tension was measured before and after plasma irradiation. The difference in the surface tension due to plasma irradiation was also maximum in the concentration range of 50–150 mg/l. The experimental results demonstrated that the change in the surface-tension difference due to plasma irradiation was strongly related to the direction and intensity of the liquid flows. The mechanisms of the plasma-induced liquid flows were qualitatively explained in relation to the initial surfactant concentration.