Impact of Water pH on Streamer Dynamics in Liquid Water*

In this study, multi-physics simulations are conducted to understand the formation of plasma and streamer dynamics in liquid-phase water. A special focus is placed on understanding the impact of water pH on streamer propagation. The system features a vertical needle-plate configuration, with the needle electrode positioned at the top and subjected to a nanosecond lightning impulse voltage characterized by a fast rise time and an extended decay time. The process initiates with electron detachment from negative hydroxyl ions (OH⁻), which requires lower threshold energy compared to electron impact ionization and field-dependent ionization of water molecules. During the initial discharge stage, the detached electrons from OH⁻ ions form an electron stream. The concentration of OH⁻ ions, which correlates with the pH level of the water, significantly influences this stage. In alkaline water, characterized by a higher pH, a greater concentration of OH⁻ ions leads to an increased initial electron population, thereby extending the discharge propagation from the powered electrode. Elevated pH levels induce stronger electrical forces that create larger density variations in the liquid, significantly enhancing bulk fluid velocity and inducing shock-like conditions. For acidic water, which has lower OH^- ions, the streamer development is suppressed and takes a longer time to form, as the ionization channel is primarily dependent on electron impact or field-dependent ionization of water molecules. Parametric studies are performed to understand the influence of pH on the development speed of streamers and the density of the charged particles. These findings underscore the critical interplay between electrical forces and chemical composition in plasma-liquid interactions, providing valuable insights for optimizing plasma systems across a wide range of applications.