The experiments were performed in a unique coplanar dielectric barrier discharge (DBD)-like geometry, in which a thin layer of deionized water flows over flat metal electrodes separated by insulating plastic blades. The water itself becomes part of the dielectric barrier, and a short-lived plasma is generated by a transient discharge propagating along the water-air interface. A pulsed power supply delivers a positive high-voltage pulse followed by a negative pulse, separated by a delay of a few microseconds. This bipolar excitation scheme allowed the researchers to isolate and compare the discharges produced by each polarity within a single event, using deionized water with a conductivity of about 5 microsiemens per centimeter flowing at a constant rate of 20 milliliters per minute.

The positive-polarity discharge exhibits a distinct two-phase morphology. Fast imaging with sub-nanosecond resolution captured a bundle of double-headed streamers initiating at the insulating blade, propagating bidirectionally toward both the high-voltage and grounded electrode regions. These closely spaced, overlapping structures — a collective streamer front — subsequently transformed, after reaching a certain length, into discrete spark filaments. The double-headed streamer phenomenon is usually observed in sprite-halo events developing at altitudes below 75 kilometers in the upper atmosphere, where they arise from inhomogeneities in electron density initiated by lightning-induced electric fields. Observing them at a water surface in the laboratory provides a rare terrestrial analogue of these atmospheric events.

In contrast, the negative-polarity discharge reveals a fundamentally different morphology, characterized by thin, discrete filaments from the earliest stages that persist and prolong at later times. Crucially, the formative streamer pre-phase observed during the positive pulse is completely absent during the negative pulse. The negative discharge initiates directly as filamentary structures within the same channels formed during the preceding positive discharge. Photomultiplier tube waveforms confirmed this difference: the positive pulse produces a double-peak emission structure reflecting its two-phase development, while the negative pulse yields broader, single-peak waveforms consistent with direct spark evolution.

Analysis of optical emission spectra showed that the negative pulse yields significantly higher rotational temperatures, reaching up to 1300 Kelvin, compared to the preceding positive pulse, where temperatures approached 700 Kelvin in the blade region. The vibrational excitation of the N2(C3Πu) electronic state was also markedly elevated during the negative pulse. The vibrational temperature of the excited state increased from about 3500 ± 300 Kelvin during the positive pulse to 6000 ± 400 Kelvin during the negative pulse in the blade region, corresponding to ground-state vibrational temperatures rising from roughly 2000 Kelvin to above 5000 Kelvin. These measurements indicate that the negative discharge develops in a preheated, vibrationally excited medium left in the wake of the positive discharge.

The plasma electron density, estimated via the Stark-broadened Hα line, reached 2×10^17 per cubic centimeter for both polarities and exhibited a gradual decay over time with minimal spatial or polarity dependence. During the negative pulse, the Hα line dominates from the very beginning, allowing the electron density to be monitored over a relatively long interval of 50 to 250 nanoseconds. Densities around 10^17 per cubic centimeter persisted up to 200 nanoseconds, consistent with the extended atomic line emission captured in the spatiotemporal color maps derived from photomultiplier data.

The researchers attribute the polarity-dependent behavior to a combination of memory effects established by the preceding positive pulse. Thermal memory is most directly supported by the measurements: gas heating from roughly 300 Kelvin to 700 Kelvin during the positive pulse decreases the neutral gas density, enhancing the reduced electric field experienced by the subsequent negative pulse. Monte Carlo calculations using the LoKI-MC code showed that this thermally induced increase in the reduced electric field can shift discharge conditions from a regime where electron multiplication is suppressed into a regime enabling rapid electron multiplication in the preionized channel. The calculations also revealed that water vapor evaporating from the heated surface modifies the effective ionization coefficient, enhancing electron attachment at low reduced fields and shifting the balance toward net ionization at higher fields.

Preionization effects provide another memory mechanism. Although free electrons generated by the positive pulse decay rapidly through attachment to oxygen molecules within about 13 nanoseconds, residual negative ions can persist on microsecond time scales through slow ion-ion recombination. Once the electric field associated with the negative pulse is established, a substantial fraction of the residual negative-ion population can be rapidly converted into free electrons through field-induced detachment, with characteristic detachment times of approximately 10 to 40 nanoseconds at representative reduced fields. The resulting seed-electron density is comparable to values typically reported for streamer discharges, potentially substituting for the initial streamer phase and explaining its absence during the negative pulse.

Non-equilibrium vibrational kinetics add a further layer of memory. Vibrationally excited nitrogen molecules can influence electron kinetics through superelastic collisions that transfer internal molecular energy to electrons, enhancing the high-energy tail of the electron energy distribution function. Ionization from vibrationally excited states also requires less additional electron energy than ionization from the vibrational ground state. Since vibrational relaxation occurs on much longer time scales than electron density decay, a significant fraction of the vibrational excitation generated during the positive pulse persists until the arrival of the negative pulse. Long-lived excited states, atomic radicals, and water-related species may further modify the plasma chemistry during the subsequent pulse.

The 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. The synergy of positive and negative pulses allows the formation of transient spark filaments with a much higher concentration of atomic species directly at the water-air interface compared to what would be expected from a classical surface DBD. The negative pulse produces considerably higher concentrations of atomic hydrogen, oxygen, and nitrogen, as demonstrated by the spatiotemporal mapping of atomic line intensities. This has obvious implications for guiding future optimization of double-pulse discharge configurations to increase the production of atomic radicals, with potential applications ranging from plasma-activated water for agriculture to nitrogen fixation. The team is now working to complement their top-down diagnostics with lateral projections to reveal the characteristic thickness and vertical structure of the plasma layer on the water surface during various discharge phases.

Subject of Research: Nanosecond pulsed plasma discharges at the water-air interface and polarity-dependent memory effects

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

Article References: Arora, G., Fujera, J., Hoffer, P., Prukner, V., Bílek, P., Bonaventura, Z., & Šimek, M. (2026). Streamer-spark discharge at the water-air interface: spatio-temporal tracking of non-equilibrium kinetics and memory effects. Frontiers in Physics, 14, Article 1924513. https://doi.org/10.3389/fphy.2026.1924513

Image Credits: AI Generated

DOI: 10.3389/fphy.2026.1924513

Keywords: plasma physics, streamer discharge, spark discharge, water-air interface, dielectric barrier discharge, optical emission spectroscopy, memory effect, double-headed streamer, nanosecond pulses, electron density, vibrational excitation, plasma-activated water