Experimental Validation on the Ionic Strength and Charge Effect in Plasma-Induced Liquid Mobility

This study utilized a direct current-needle system for plasma generation and liquid flow inducement. The liquid flow was visualized and analyzed by particle image velocimetry. Electrolyte solutions of potassium chloride, potassium bromide, potassium iodide, calcium chloride and chromium(III) nitrate with concentrations ranging from 0.1 to 1.0 mM were studied. The results indicate that the plasma induces an upward liquid flow with an area mean velocity of up to 3.0 mm/s. The flow speed decreases with increasing electrolyte concentration and shows a strong dependence on the solution's conductivity. This study proposed a physical model based on these findings. The plasma generates short-lived ions and electrons, which shift the hydrogen bonds among the water molecules through their electrical effect. This process creates an intermolecular force gradient and induces liquid flow on the water surface. The distance that electrostatic effect of a charged particle can persist in an electrolyte solution is defined as Debye length. This physical quantity decreases with increasing ionic strength or electrical conductivity. Thus, the plasma induces slower liquid flow in solutions with higher electrolyte concentration. Based on the regression analysis, the characteristic flow velocity is significantly proportional to the square of the solution's Debye length, with a coefficient of determination of 0.9365.