Streamer-spark discharge at the water-air interface: spatio-temporal tracking of non-equilibrium kinetics and memory effects

This study investigates the space- and time-resolved dynamics of nanosecond pulsed discharges propagating along a water-air interface within a unique coplanar dielectric barrier discharge (DBD)-like geometry. Using a pulsed power supply that delivers a positive high-voltage pulse followed by a negative pulse, the plasma characteristics were evaluated using four-channel fast imaging and time-resolved optical emission spectroscopy. The positive-polarity discharge exhibits a distinct two-phase morphology, initiating as bundle of double-headed streamers at the insulating blade and subsequently, after reaching certain length, developing into discrete spark filaments. In contrast, the negative-polarity discharge reveals a different morphology, characterized by thin, discrete filaments from the early stages that persist and prolong at later times. Analysis of optical emission spectra shows that the negative pulse yields significantly higher rotational temperatures (reaching up to 1300 K) and elevated vibrational excitation of N 2 ( C 3 Π u ) electronic state compared to the preceding positive pulse. The plasma electron density, estimated via the H α line, reached 2 × 1 0 17 cm − 3 for both polarities and exhibited a gradual decay over time with minimal spatial or polarity dependence. These findings provide important insights into the role of pre-existing plasma environments in the dynamics of surface-discharge evolution at gas-liquid interfaces under double-pulse excitation. Particularly showing that the negative discharge is initiated in the environment with localized memory of the preceding positive discharges, including synergetic effects of increase in the gas temperature, significant vibrational excitation, as well as increased reactive species and free charge carriers densities. All these effects working together to facilitate the negative discharge initiation and development.