Liquid movement driven by surface charge accumulation and Debye length dependence via experimental validation

This study focuses on the driving mechanism of the plasma-induced liquid flow (PILF) in electrolyte solutions. The examination was conducted by means of systematic analysis for the liquid flow induced in KCl solutions with a concentration ranging from 10−5 to 100 mol/l by a pin-to-liquid DBD reactor. Through a series of experimental examinations, we systematically verified and excluded the contributions of thermal effects, ionic wind, and the applied electric field. We found that a maximum temperature rise of only 8 °C corresponded to a negligible change in liquid properties, and the specific flow pattern cannot be repeated by the impingement of helium. These findings confirm that the occurrence of plasma discharge is the key factor for initiating the liquid flow. These findings lead to the proposal of a driving mechanism, which is a surface tension gradient caused by charge accumulation and the disruption of hydrogen bond network. As the range of electrical effect for charged particles depends on the Debye length of the electrolyte solution and decreases with electrolyte concentration, a negative dependency between the PILF velocity and the electrolyte concentration was anticipated and confirmed (e.g., 7.36 mm/s for the case of [KCl] = 10−5 mol/l vs 0.62 mm/s for [KCl] = 100 mol/l). Additionally, as this mechanism relies on the existence of hydrogen bonds, a negligible flow was predicted in non-polar liquids. This prediction was experimentally validated in the experiment using non-polar and low-viscosity silicone oil.