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When cold atmospheric plasma comes into contact with water and biological media, antimicrobial or antitumor effects are induced, representing great potential for applications in biomedicine and agriculture. The need to control and tune the chemical composition and biomedical effects of plasma activated water/media (PAW/PAM) is emerging. By comparing two nonthermal air plasma sources, streamer corona and transient spark, interacting with water in open and closed reactors, and by enhancing the plasma-liquid interaction by water electrospray through these discharges, we demonstrate that the plasma gaseous products strongly depend on the discharge regime, its deposited power and gas flow conditions. The streamer corona strongly leads to the formation of ozone and hydrogen peroxide, while the more energetic transient spark leads to nitrogen oxides and hydrogen peroxide. The gaseous products then determine the chemical properties of the PAW and the dominant aqueous reactive oxygen and nitrogen species (RONS). The production of hydrogen peroxide depends on water evaporation and hydroxyl radical formation, which is determined by the discharge power. A transient spark produces higher concentrations of gaseous and aqueous RONS and induces stronger antibacterial effects than a streamer corona; however, the RONS production rates per joule of deposited energy are comparable for both studied discharge regimes. The net production rate per joule of gaseous nitrogen oxides strongly correlates with that of aqueous nitrites and nitrates. The antibacterial effects of the PAW tested on Escherichia coli bacteria are determined by the aqueous RONS: in the lower power streamer corona, this is ascertained mainly by the dissolved ozone and hydrogen peroxide, and in the higher power transient spark, by the combination of hydrogen peroxide, nitrite and acidic pH, while in the transient spark in the closed reactor it is determined by the acidified nitrites present.

We present a study of the streamer-to-spark transition in a self-pulsing dc-driven discharge called a transient spark (TS). The TS is a streamer-to-spark transition discharge with short spark duration (∼10-100 ns), based on charging and discharging of the internal capacity of the electric circuit with repetition frequency 1-10 kHz. The TS can be maintained under relatively low energy conditions (0.1-1 mJ pulse −1). It generates a very reactive non-equilibrium air plasma applicable for flue gas cleaning or bio-decontamination. Thanks to the short spark current pulse duration, the steady-state gas temperature, measured at the beginning of the streamers initiating the TS, increases from an initial value of ∼300 K only up to ∼550 K at 10 kHz. The streamer-to-spark transition is governed by the subsequent increase in the gas temperature in the plasma channel up to ∼1000 K. This breakdown temperature does not change with increasing repetition frequency f. The heating after the streamer accelerates with increasing f , leading to a decrease in the average streamer-to-spark transition time from a few µs to less than 100 ns.

The paper presents an analysis of electrical characteristics of streamer-to-spark transition discharge in air at atmospheric pressure named transient spark (TS). The TS is applicable for flue gas cleaning or bio-decontamination and has potential in plasma shielding, combustion and flow control applications. Despite the dc applied voltage, TS has a pulsed character with short (∼10-100 ns) high current (>1 A) pulses, with repetitive frequencies 1-20 kHz. Estimation of the temporal evolution of electron density shows that n e ≈ 10 16 cm −3 at maximum and ∼10 11 cm −3 on average are reached using relatively low power delivered to the plasma (0.2-3 W). Thanks to the high repetition frequency, n e between two current pulses does not fall below a critical value and therefore plasma exists during the whole time. A detailed analysis of the TS control by electrical circuit parameters is presented. With appropriate circuit components, the current pulse tail (>1 mA) can be extended and the electron density can be held above ∼10 13 cm −3 for several tens of µs.

Generation of microdischarges inside porous ceramics by AC high voltage was explored. The physical properties of the microdischarges were investigated by electrical measurements, photographic visualization, and optical emission spectroscopy. The effects of pore size, discharge power, and gas mixture composition on the discharge properties and its development are described. The optimum generation and distribution of the microdischarges was observed with pore sizes of 50 and 80 µm. The emission spectra of the microdischarges indicated their strongly non‐equilibrium character. A theoretical analysis of the microdischarge mechanism is provided, accounting for the pore size and the elementary processes such as ionization, recombination, and ambipolar diffusion. magnified image

Air transient spark (TS) discharge generates cold plasma, which is a rich source of reactive oxygen and nitrogen species (RONS). The gaseous products both in ambient air and air humidified by water electrospray (ES) are nitrogen oxides (NO and NO2). The rotational and vibrational temperatures determined by optical emission spectroscopy (OES) are lower with water ES through the discharge than in ambient air, which reduces the formation of NOx. The investigation of the specificity of Griess colorimetric assay for the detection of long‐lived nitrites in plasma activated solutions confirms its accuracy by comparison with ion chromatography (IC) and excludes possible interferences with hydrogen peroxide by using the enzyme catalase. Examination of the specificity of the Indigo blue assay for ozone detection shows strong interferences with the peroxynitrite chemistry. Phenol as the chemical probe confirms that the TS air discharge produces no aqueous ozone.

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The gliding arc discharge plasma reactors are known as a source of non-equilibrium plasma at atmospheric pressure. In the present study, generation of dominant reactive oxygen and nitrogen species in gaseous and liquid phase in water by the compact gliding arc device (mini-GAD) and corresponding bactericidal effects were investigated. Water and phosphate buffer solutions were used as model liquids. Mini-GAD is a strong source of nitrogen oxides (up to 800 ppm NO and 200 ppm NO 2) that result in high concentrations of nitrites and nitrates in water solutions. The highest bactericidal efficacy towards Escherichia coli was achieved for non-buffered water solution.