Zdenko Machala

Plasma–liquid interactions: a review and roadmap

Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.

Plasma agents in bio-decontamination by dc discharges in atmospheric air

Bio-decontamination of water and surfaces contaminated by bacteria (Salmonella typhimurium) was investigated in two types of positive dc discharges in atmospheric pressure air, in needle-to-plane geometry: the streamer corona and its transition to a novel regime called transient spark with short high current pulses of limited energy. Both generate a cold non-equilibrium plasma. Electro-spraying of treated water through a needle electrode was applied for the first time and resulted in fast bio-decontamination. Experiments providing separation of various biocidal plasma agents, along with the emission spectra and coupled with oxidation stress measurements in the cell membranes helped to better understand the mechanisms of microbial inactivation. The indirect exposure of contaminated surfaces to neutral active species was almost as efficient as the direct exposure to the plasma, whereas applying only UV radiation from the plasma had no biocidal effects. Radicals and reactive oxygen species were identified as dominant biocidal agents. (Some figures in this article are in colour only in the electronic version)

Transient spark: a dc-driven repetitively pulsed discharge and its control by electric circuit parameters

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 ne ≈ 1016 cm−3 at maximum and ~1011 cm−3 on average are reached using relatively low power delivered to the plasma (0.2–3 W). Thanks to the high repetition frequency, ne 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 ~1013 cm−3 for several tens of μs.

Removal of cyclohexanone in transition electric discharges at atmospheric pressure

Two new types of streamer-induced electric discharges operating in a non-uniform electric field in air at atmospheric pressure were applied to the removal of volatile organic compounds (VOCs). The first type is a pulseless dc discharge with physical properties corresponding to the glow discharge. The second, also supplied by a dc high voltage of both polarities, is a spontaneously pulsing discharge operating in the regime of the streamer-to-spark transition, the spark phase being too short to reach local thermodynamic equilibrium conditions. Both discharges are able to generate a non-thermal plasma, as resolved from their rotational and vibrational temperatures. The influences of these discharges on the removal of cyclohexanone at various gas flow rates and concentrations (600-6000 ppm) were compared. The removal efficiencies achieved were about 50-60%, and the energy costs were 16-100 eV/molecule at various energy densities. Special conditions where CO2 and other gaseous products are minor and dominant products appear in the condensed phase can be obtained, especially in the spontaneously pulsing transition discharge. We explain some plasmochemical processes induced by the discharges by considering heterogeneous effects of the copper electrode surface. The role of active nitrogen and the formation of the NCO radical are probably key factors leading to the formation of the condensation product based on amino acids, here produced for the first time from VOCs, as well as in the overall energy cycle resulting in low energy costs of the process. The small pilot-scale reactor based on the spontaneously pulsing transition discharge has been successfully applied to the removal of cyclohexanone in the mixture with other VOCs with no noxious gas output. This validates the possibility of the application of such a type of reactor for larger scales.

DC Discharges in Atmospheric Air and Their Transitions

We present images and basic characteristics and describe transition mechanisms between three dc discharges in atmospheric air point-to-plane or point-to-water gap. With increasing applied voltage, a streamer corona transits to a transient spark: a repetitive spark with very short (~100-ns) current pulses (~1 A) of very limited energy. With an appropriate ballast resistor, this transient regime evolves into a pulseless glow discharge. These three discharges generate nonequilibrium plasmas of high chemical activity interesting for environmental and biomedical applications.

Effects of air transient spark discharge and helium plasma jet on water, bacteria, cells, and biomolecules

Atmospheric pressure DC-driven self-pulsing transient spark (TS) discharge operated in air and pulse-driven dielectric barrier discharge plasma jet (PJ) operated in helium in contact with water solutions were used for inducing chemical effects in water solutions, and the treatment of bacteria (Escherichia coli), mammalian cells (Vero line normal cells, HeLa line cancerous cells), deoxyribonucleic acid (dsDNA), and protein (bovine serum albumin). Two different methods of water solution supply were used in the TS: water electrode system and water spray system. The effects of both TS systems and the PJ were compared, as well as a direct exposure of the solution to the discharge with an indirect exposure to the discharge activated gas flow. The chemical analysis of water solutions was performed by using colorimetric methods of UV-VIS absorption spectrophotometry. The bactericidal effects of the discharges on bacteria were evaluated by standard microbiological plate count method. Viability, apoptosis and cell cycle were assessed in normal and cancerous cells. Viability of cells was evaluated by trypan blue exclusion test, apoptosis by Annexin V-FITC/propidium iodide assay, and cell cycle progression by propidium iodide/RNase test. The effect of the discharges on deoxyribonucleic acid and protein were evaluated by fluorescence and UV absorption spectroscopy. The results of bacterial and mammalian cell viability, apoptosis, and cell cycle clearly show that cold plasma can inactivate bacteria and selectively target cancerous cells, which is very important for possible future development of new plasma therapeutic strategies in biomedicine. The authors found that all investigated bio-effects were stronger with the air TS discharge than with the He PJ, even in indirect exposure.

The streamer-to-spark transition in a transient spark: a dc-driven nanosecond-pulsed discharge in atmospheric air

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 ?s to less than 100?ns.

DC Glow Discharges in Atmospheric Pressure Air

Abstract We present experimental investigations of DC glow discharges in atmospheric pressure air with the aim of producing nonequilibrium air plasmas with high electron density (~1012 cm-3) and relatively low gas temperature (less than 2000 K). Such plasmas are potentially interesting for many applications, including air pollution control. The discharge of our study is ignited by a streamer-to-spark transition, but thanks to an appropriate ballast resistor, it operates in a pulseless regime with currents from 2 to 500 mA, current densities of 1-10 A/cm2 , and electric fields of 3000-300 V/cm. Spectroscopic and electrical measurements show that the discharge is of the glow type and generates a nonequilibrium air plasma. We also describe an innovative approach where thermionic cathodes and tubes with swirl gas flow are employed. With this approach, electron densities of up to 1013-1014 cm-3 can be obtained and the production of relatively large plasma volumes is possible.

Air spark-like plasma source for antimicrobial NOx generation

We demonstrate and analyse the generation of nitrogen oxides and their antimicrobial efficacy using atmospheric air spark-like plasmas. Spark-like discharges in air in a 1 L confined volume are shown to generate NOx at an initial rate of about 1.5 × 1016 NOx molecules/J dissipated in the plasma. Such a discharge operating in this confined volume generates on the order of 6000 ppm NOx in 10 min. Around 90% of the NOx is in the form of NO2 after several minutes of operation in the confined volume, suggesting that NO2 is the dominant antimicrobial component. The strong antimicrobial action of the NOx mixture after several minutes of plasma operation is demonstrated by measuring rates of E. coli disinfection on surfaces and in water exposed to the NOx mixture. Some possible applications of plasma generation of NOx (perhaps followed by dissolution in water) include disinfection of surfaces, skin or wound antisepsis, and sterilization of medical instruments at or near room temperature.

Streptococci biofilm decontamination on teeth by low-temperature air plasma of dc corona discharges

Non-thermal plasmas of atmospheric pressure air direct current corona discharges were investigated for potential applications in dental medicine. The objective of this ex vivo study was to apply cold plasmas for the decontamination of Streptococci biofilm grown on extracted human teeth, and to estimate their antimicrobial efficiency and the plasma’s impact on the enamel and dentine of the treated tooth surfaces. The results show that both positive streamer and negative Trichel pulse coronas can reduce bacterial population in the biofilm by up to 3 logs in a 10 min exposure time. This bactericidal effect can be reached faster (within 5 min) by electrostatic spraying of water through the discharge onto the treated tooth surface. Examination of the tooth surface after plasma exposure by infrared spectroscopy and scanning electron microscopy did not show any significant alteration in the tooth material composition or the tooth surface structures.