Hydrodynamic tuning of nitrogen plasma in a fast pulsed channel for liquid activation stability

This study investigates the hydrodynamic and physicochemical behavior of a nitrogen Fast Pulsed Discharge plasma system for treating deionized water (DIW) and 0.75% hyaluronic acid gel (HAG). The discharge channel consists of hemispherical electrodes (2 mm gap, 26 cm length) powered by a locally assembled high-voltage source (≤15 kV), with nitrogen injected centrally and distributed along the plasma channel. Flow rates from 1 to 15 L/min were examined, and key physical parameters—Mach number, Reynolds number, dynamic pressure, and residence time—were calculated. Plasma irradiation was applied to the liquids for 5, 10, 15, and 20 min, with both theoretical and experimental results confirming that 5 L/min represents the optimal condition, yielding subsonic flow (Mach ≈ 0.025), a transitional regime (Re ≈ 1900), and a stable plasma glow. Post-treatment monitoring over four days revealed that HAG exhibited superior physicochemical stability compared to DIW. Measurements of pH, ORP, and EC showed that DIW underwent pronounced fluctuations with only partial recovery, whereas HAG maintained nearly stable values, reflecting its enhanced buffering and activation capacity. In contrast, zeta potential was measured only at the 20-min irradiation point, where both HAG and DIW displayed significant surface charge modification, providing further evidence of plasma-induced interfacial changes. These findings demonstrate that a 5 L/min nitrogen feed ensures optimal plasma uniformity and liquid activation, supporting the development of plasma-activated media with improved storage capability for biomedical applications.