Surface Micro-discharge Research Articles

The dynamics and transport of ozone in the gas and liquid phase generated by air surface microdischarge plasma at atmospheric pressure

AbstractThis contribution focuses on the spatial‐temporal behavior and reactive pathways of O3 produced by a surface air microdischarge in the gas and liquid phase using ultraviolet absorption spectroscopy. The findings demonstrate that mode transition from ozone to nitrogen oxide over time is observed at a constant input power higher than ~0.60 W/cm2. Due to the long‐lived characteristic and ionic wind, the perpendicular distribution of O3 is almost uniform. The maximum penetration depth is around 5 mm, and the gas–liquid mass transfer efficiency is approximately 0.4‱ at a depth of 1 mm, when the treatment time is 10 min. The mass transfer of O3 between gas and liquid phases is dominated by the liquid convention induced by ionic wind.

Comparative analysis of helium and air surface micro-discharge plasma treatment on the microbial reduction and quality attributes of beef slices

This work aimed to compare the effects of helium and air surface micro-discharge (SMD) plasma on the microbial safety and quality of beef tissues. For the beef tissue model, the concentration and diffusion depth of hydroxyl radical and ozone have different change patterns over plasma treatment time and distance in helium and air SMD plasma. The inactivation efficiency of helium plasma depended on the plasma treatment time and distance, while the inactivation efficiency of air plasma only depended on the treatment time. For the fresh beef slices, air SMD plasma treatment exhibited a higher antimicrobial activity against S. aureus and E. coli than helium SMD plasma treatment (1.5 versus 0.9; 0.9 versus 0.28 log CFU/g at 10 min). However, air SMD plasma treatment caused more adverse effects on beef quality, leading to a smooth surface, extensive lipid oxidation, protein structure damage, low pH and discoloration compared to helium SMD plasma treatment. This work provides valuable guidelines for the working gas choice in the practical application of plasma to meat decontamination.

Quantitative characterization of the temporal-spatial evolution of RONS in air surface micro-discharge using UV–VIS optical absorption spectroscopic diagnosis and modeling approach

The research of the chemically active species of cold atmospheric pressure plasmas is a essential step for a more in-depth comprehension of the effects of its interaction with the target. In this paper, the temporal and spatial evolution of key species O3, NO2 and NO3 produced by surface micro-discharge in air were investigated. UV–VIS optical absorption spectroscopy at 254 nm, 400 nm and 662 nm were used to measure the concentrations of O3, NO2 and NO3, respectively. The results show that the temporal evolution of O3, NO2 and NO3 are revealed a significant correlation with the surface power density (SPD). The phenomenon of O3 and NO3 quenching occur once the SPD overcomes a critical value of 0.15 W cm−2. An O3-enriched atmosphere (peak concentration around 3000 ppm) is formed when the SPD is below the critical value, and a NO2-enriched atmosphere (maximum NO2 density around 600 ppm) is formed at higher SPD. In addition, the concentration distribution of O3, NO2 and NO3 in the chamber ranging from 10–100 mm of the downstream of the mesh electrode tends to be uniform. Finally, a zero-dimensional model of the afterglow chemistry, validated using the experimental measurements, is developed to determined important reactions affecting O3, NO2 and NO3 respectively, and obtain insight into the evolutionary behavior of the considered reactive species.

Effect of powered electrode configuration on plasma structure of surface micro-discharge array

Surface micro-discharge (SMD) has received growing interest for its great prospect in various emerging applications spanning the fields of biomedicine, manufacturing, and agriculture. Discharge properties and the nature of the chemical species are heavily dependent on the electrode configuration. However, insight into the effect of electrode configuration on SMD remains scarce. This paper reports on the effect of the geometry of the powered planar electrode on the dynamic evolution of plasma in a helium SMD array at atmospheric pressure. The observed symmetric and asymmetric luminous patterns correspond to odd and even numbered current pulses in negative half-cycle, respectively. When the powered electrode is not covering the full area of the mesh unit of the grounded electrode, the luminous pattern dominated by the negative pattern consists of three parts: the distinct luminous channel, the flabelliform luminous area, and the discontinuous cambered channels under the incomplete powered electrode condition. The distinct luminous channel is ascribed to the propagation of the bullet-like plasma volume originated at the middle part of the mesh side before merging with other plasmas. The merging plasma forms a cambered front and continuous to propagate, leading to the formation of a flabelliform area. The discontinuous cambered channels result from the propagation of bullets ignited at the mesh corners, especially the slide of luminous area with maximum emission intensity along the streamer. Moreover, the mesh element without a powered electrode can also initiate breakdown due to the sufficiently high voltage and the propagation of surface charges, suggesting that the grounded mesh electrode should be extended to avoid redundant discharges.

Process optimization of nanosecond pulsed packed‐bed discharge plasma for SF6 degradation based on response surface methodology

AbstractIn this work, a nanosecond pulsed packed‐bed discharge (PBD) plasma was developed for SF6 degradation under different dilution gases. The discharge form transfers from filamentary discharge to the combination of surface discharge and microdischarge after introducing the packing material regardless of the dilution gas used, and the local electric field strength and SF6 degradation performance are promoted. The mean electron energy and rate coefficient for SF6 excitation in SF6/Ar system are significantly higher than those in SF6/N2 or SF6/air system, which results in higher SF6 degradation efficiency in SF6/Ar system. Moreover, the response surface methodology based on the Box–Behnken design model was constructed to optimize several key process parameters including pulsed voltage, gas flow rate, oxygen content, and relative humidity, as well as assess the contribution of each parameter to SF6 degradation. The proposed optimization model shows a satisfactory correlation between the predicted and the experimental results. The energy yield concerning SF6 degradation in nanosecond pulsed PBD plasma achieves approximately 75.3 g/kWh with nearly complete degradation. This work is expected to provide an efficient and energy‐saving plasma technique for SF6 degradation.

Operation-mode recognition of surface microdischarge based on visible image and deep learning

Discharging images contain useful information regarding the operation mode of surface microdischarge (SMD). To solve the shortcomings of low efficiency, high cost, and long operation time of existing SMD operation-mode recognition methods, a convolutional neural network (CNN) based on deep learning is introduced herein. The visible image library of SMD at different applied voltages, dielectric sheets with different dielectric constants, and dielectric sheets with different thicknesses and exposure times are constructed using a digital camera. The typical structure of a CNN is discussed, and the hyperparameters, including the number of network layers, convolution kernel size, number of neurons in the fully connected layer, and activation function type that affect the recognition accuracy of the CNN are investigated. The optimal structure of the CNN for the SMD operation-mode recognition is obtained via training. The recognition accuracy of the CNN is compared with those of three traditional machine learning methods: support vector machine (SVM), decision tree (DT), and random forest (RF). Test results show that the recognition accuracy based on the CNN is 99.745%, which is better than those of the SVM, DT, and RF. Finally, an SMD operation-mode online recognition method based on the CNN is proposed.

Nitrox surface discharge used for water activation: the reactive species and their correlation to the bactericidal effect

Plasma activated water (PAW) is a promising green antibacterial agent and the bactericidal effect is complicatedly affected by electron bombardment, ultraviolet radiation, interface reaction, and cascade chemical reaction. In this paper, a case of preparing PAW by treating aqueous solutions with afterglow gas is constructed based on surface micro-discharge (SMD), which focuses on the effect of afterglow gas–liquid mass transfer and liquid phase chemistry on PAW sterilization. The correlation of the bactericidal effect of PAW to the reactive species was studied based on the model of methicillin-resistant Staphylococcus aureus. The production of reactive oxygen and nitrogen species (RONS) in PAW with the regulation of N2/O2 ratios in the working gas for SMD. The RONS in both gas and liquid phases and the physicochemical properties of PAW were measured through optical and chemical methods. In addition, the effects of liquid types, liquid conductivity, and storage time on the bactericidal effects of PAW were explored. The key species for bacteria inactivation were identified by equivalent mixed solutions and specific scavengers. The results demonstrated that control of the N2/O2 ratios in the working gas can effectively improve the RONS in plasma and PAW. The bactericidal effect of PAW is correlated with peroxynitrite, superoxide anion, and their synergistic effects in an acidic liquid environment. This study provides a new strategy for insight into the bactericidal mechanism of PAW in biomedical applications.

Cold Atmospheric Plasma Decontamination of FFP3 Face Masks and Long-Term Material Effects

The use of cold atmospheric plasmas (CAP) to sterilize sensitive surfaces is an interesting new field of applied plasma physics. Motivated by the shortages of face masks and safety clothing at the beginning of the corona pandemic, we conducted studies on the sterilization of FF3 face masks with CAP and the resulting material effects. Therefore, the bactericidal and sporicidal efficacy of CAP afterglow sterilization of FFP3 mask material was investigated by inoculating fabric samples with test germs Escherichia coli (E. coli) and Bacillus atrophaeus (B. atrophaeus) and subsequent CAP afterglow treatment in a surface-micro-discharge (SMD) plasma device. In addition, a detailed analysis of the changes in long-term plasma treated (15h) mask material and its individual components - ethylene vinyl acetate (EVA) and polypropylene (PP) - was carried out using surface analysis methods such as laser microscopy, contact angle measurements, X-ray photoelectron spectroscopy (XPS) as well as fabric permeability and resistance measurements. The experiments showed that E. coli and B. atrophaeus could both be effectively inactivated by plasma treatment in nitrogen mode (12 kVpp, 5 kHz). For B. atrophaeus inactivation of more than 4-log was achieved after 30 minutes. E. coli population could be reduced by 5-log within one minute of CAP treatment and after five minutes a complete inactivation (> 6-log) was achieved. Material analysis showed that long-term (> 5 h) plasma treatment affects the electrostatic properties of the fabric. From this it can be deduced that the plasma treatment of FFP3 face masks with the CAP afterglow of an SMD device effectively inactivates microorganisms on the fabric. FFP3 masks can be plasma decontaminated and reused multiple times but only to a limited extent, as otherwise the permeability levels no longer meet the DIN EN 149 specifications.

Operation mode recognition of surface microdischarge based on the gray level histogram

AbstractA visible image of gas discharge contains rich information. This study investigates the visible light information of surface microdischarge (SMD) under different operation modes and develops a method to realize online monitoring of the operation modes based on a gray level histogram (GLH). Two SMD sources were used to verify the effectiveness of the method. The distribution curve in the GLH gradually shifted to a large gray level as the operation mode transferred from ozone mode to NOx mode. The mean, variance, energy, entropy, and uniformity threshold values used to distinguish the operation modes were found to be approximately the same in both sources. Finally, a simplified distribution map based only on entropy and uniformity is proposed for the fast operation mode recognition of SMD.

Inside Back Cover: Plasma Process. Polym. 2/2022

Inside Back Cover: The dielectric parameters of a surface microdischarge (SMD) device were found to have significant effect on both mode transformation and energy cost for nitrogen fixation. The thickness of the dielectric sheet dd has a negative correlation with the power consumption, while a larger permittivity εd could effectively lower the applied voltage for mode transformation and decrease the energy cost. Increasing εd and reducing dd could further decrease the energy cost in SMD. Further details can be found in the article by Chen Lu, Xingyu Chen, Yuqi Wang, Yu Zhu, Zhenping Zou, and Zilan Xiong (e2100107).

In vivo Assessment of Cold Atmospheric Pressure Plasma Technology on the Bioactivity of Spirulina.

The present study challenges the in vivo assessment of cold atmospheric pressure plasma (CAPP) technology on the bioactive activity (antioxidant/antiaging and antimicrobial potential) of Spirulina powder, using Caenorhabditis elegans as an animal model. Surface microdischarge cold atmospheric pressure plasma (SMD-CAPP) treatment was 3.3 W discharge power for 7 min. C. elegans lifespan and egg laying were used as indicators of antioxidant/antiaging potential of Spirulina (1 mg/mL), when grown with Spirulina CP-treated [E_SCP] and untreated [E_S], compared with a control [E_0] (non-supplemented with Spirulina). According to our results, under both Spirulina supplemented media [E_SCP and E_S] and for the first 17 days, nematodes experienced an increase in lifespan but without significant differences (p > 0.05) between control and Spirulina CP-treated. Regarding the in vivo assay of the antimicrobial potential of Spirulina against Salmonella enterica serovar Typhimurium (infected worms), no significant differences (p > 0.05) were found between the three exposure scenarios (control [S_0]; Spirulina supplemented media [S_S]; CP-treated Spirulina supplemented media [S_SCP]). According to present results, CAPP-treatment do not influence negatively the lifespan of C. elegans but a reduction in the Spirulina antiaging potential was found. No in vivo modifications in antimicrobial activity seem to be linked to CAPP-processed Spirulina.

Effect of dielectric parameters on the transformation of operation mode and the energy cost of nitrogen fixation of surface microdischarge in air

AbstractIn this study, we examine the effect of dielectric parameters (permittivity of the dielectric sheet and thickness of the dielectric sheet ) on the operational mode transformation and energy cost of nitrogen fixation in surface microdischarge (SMD). Three dielectric materials with different thicknesses were used. It was found that has a negative correlation with the power consumption, while a larger could effectively lower the applied voltage for mode transformation and decrease the energy cost. The trend of energy cost has the shape of a checkmark symbol (√), and the lowest energy cost in this investigation was 489.07 GJ/tN. Increasing and reducing could further decrease the energy cost in SMD. Finally, the mechanism was briefly discussed.

On the mechanisms of surface microdischarge plasma treatment of onychomycosis: Penetration, uptake, and chemical reactions

AbstractDespite becoming increasingly established as a treatment technique, the mechanism underlying the plasma treatment of onychomycosis has yet to be elucidated. Here, we focus on the interactions between the nail plate and a surface microdischarge plasma, including penetration, uptake, and chemical reactions. Results show that long‐lived gaseous species from plasma can penetrate the nail plate effectively and are primarily responsible for the instantaneous antionychomycosis effect. The amount of uptake far exceeded that of penetration. Attenuated total reflection‐Fourier‐transform infrared spectroscopy and X‐ray photoelectron spectroscopy results show that antimicrobial compounds form on the nail surface. Combined with the uptake of active species in the nail plate, these antimicrobial compounds may inhibit microorganism growth, thereby promoting long‐term protection against onychomycosis.

Dynamics of plasma streamers in a helium surface micro-discharge array at atmospheric pressure

The dynamic evolution of plasma optical emission from an array of surface micro-discharges has been investigated by optical emission imaging. The array was operated in helium at atmospheric pressure and driven at 2.0 W at a frequency of 30 kHz. The findings indicate that surface charges and external voltage have a significant contribution to the splitting of the plasma streamer, with luminous fronts moving at velocities of 8.3–22.4 km s−1. The split plasmas induce new discharge events within a single hexagonal cell. Furthemore, we present the case of two co- and counter-propagating streamers generated within one hexagon mesh element. Experimental evidence reveals that the co-propagating streamers merge and produce a new streamer front with enhanced intensity under the combined effects of electrostatic repulsion, gas dynamic interaction and a photolytic process. As the spacing between the counter-propagating streamers decreases, the streamers interact electrically, resulting in a modification of the shape of these streamers as well as a decrease in their velocities and emission intensities. The emergence of secondary streamers is also observed. This behavior is related to surface charges accumulated during a previous half cycle and their redistribution due to the turbulence fluctuations dominated by electrohydrodynamic force. From the propagation of an individual streamer, it is shown that surface charges accumulated in a previous negative half cycle can determine the plasma path to some extent. The ionization wave propagates over the rim electrode with a velocity of about 20 km s−1, resulting in a distinct discharge channel and a strong interaction between neighboring hexagonal units in an array. The ionization wave leads to the propagation of plasma across the dielectric surface of the array.

Outside Front Cover Picture: Plasma Process. Polym. 12/2020

Outside Front Cover: Surface microdischarge has shown great potential in practical applications. In our work, the effects of macroscopic parameters on the temporal evolution of OH radicals are investigatedin both plasma layer and afterglow region. The results reveal that OH lifetime dependence on the power is more pronounced compared to the modulation time, and the density decay is mainly ascribed to the transport process and recombination reactions of OH with itself. Further details can be found in the article by Zhiwei Wang, Chunlei Feng, Liang Gao, and Hongbin Ding (e2000122).

Detection of long‐lived species in plasma‐activated water, based on digital colorimetry

AbstractTo fit a specific application, a cheap and effective manner of measurement of chemistries in plasma‐activated water (PAW) is in demand. We propose a digital colorimetry‐based method of quantifying the long‐lived species in PAW using only smartphones and specialized software. Pin−plate discharge and surface microdischarge were used to generate the PAWs. H2O2 and NO2− were selected as representatives for the test. Various color parameters were extracted from images of a solution consisting of testing species and colorimetric reagent. Specialized curve‐fitting strategies were developed during data processing. The results were consistent with those measured by UV–Vis spectrometry, but they provided better performance and simplified operation. Additionally, the proposed PAW generator design can be integrated with the detecting module based on this method.

Temporal evolution of OH density in a pulse‐modulated surface microdischarge

AbstractLaser‐induced fluorescence was used to determine the absolute OH density in a pulse‐modulated atmospheric‐pressure surface microdischarge in helium. Measurements were carried out during the discharge and post‐discharge in both plasma layer and downstream region. Experimental findings show that dissipated power, modulation on‐time duration, and the distance from the dielectric surface have more significant effects on the OH(X) lifetime than OH(X) density. The lifetime dependence on power is more pronounced as compared with the modulation duration. Moreover, a dielectric barrier discharge model was presented to estimate the density evolution and loss mechanisms of OH radicals. The results indicate that the decay of OH density is mainly due to the transport process and recombination reactions.

Mode transition of air surface micro-discharge and its effect on the water activation and antibacterial activity

The chemical characteristics of atmospheric pressure dielectric barrier discharge plasma generated in air are very sensitive to some factors, such as environment temperature and discharge power. There are two typical modes, namely the ozone mode and nitrogen oxides mode. In this paper, the air surface micro-discharge plasma in the ozone mode and nitrogen oxides mode are respectively generated at environment temperatures of 5 °C and 50 °C when discharge power is almost kept at 6 W, and the plasma-activated water (PAW) is prepared by means of passing the plasma exhaust gases into deionized water. By comparing the reactive species in PAW under these two modes and their sterilization effects, it is found that the concentrations of H+, H2O2, NO2 −, NO3 − and ONOO− in PAW for the nitrogen oxides mode are about 2–3 fold over those for the ozone mode, while the concentration of O3 in PAW for the ozone mode is more than 28 times that in the nitrogen oxides mode. The sterilization effect under the nitrogen oxides mode is two orders of magnitude higher than that in the ozone mode. For the biological effects, it is found that treatment of a methicillin-resistant Staphylococcus aureus suspension by PAW leads to approximately a four-log reduction in the nitrogen oxides mode, which is higher than that in the ozone mode, leading to speculations that the reactive nitrogen species represented by ONOO− in PAW may be the critical species in sterilization. Furthermore, the transition between ozone mode and nitrogen oxides mode can be achieved by adjusting the discharge power. Interestingly, both of the compositions and concentrations of the reactive species in gas and liquid phases under 5 °C & 9.5 W and 50 °C & 6 W conditions are roughly identical, indicating that the effect of environment temperature on the mode transition is consistent with that of discharge power.

Investigation of air plasma generated by surface microdischarge for decellularized porcine aortic valve leaflets modification

AbstractIn this study, the surface microdischarge (SMD) device was used to modify the mechanical properties of decellularized porcine aortic valve leaflets (DPAVL). It was found that SMD treatment under NOx mode considerably improved the ultimate tensile strength and the Young's modulus of DPAVL, whereas treatment under O3 mode had no effect. The absorbance band peak at ~1,375 cm−1, measured by Fourier‐transform infrared spectrometer, increased with treatment time in the freeze‐dried DPAVL group under NOx mode treatment. The microstructure was not destroyed and blood compatibility test showed no toxicity. The reaction between plasma‐activated species and the DPAVL and the pH change, other than temperature/UV irradiation, plays a major role. All of the results indicate that cold atmospheric plasma treatment seems to be a potential alternative for DPAVL modification.

Surface modifications caused by cold atmospheric plasma sterilization treatment

Inactivation of microorganisms on sensitive surfaces by cold atmospheric plasma is one major application in the field of plasma medicine because it provides a simple and effective way to sterilize heat-sensitive materials. Therefore, one has to know whether plasma treatment affects the treated surfaces, and thus causes long-term surface modifications. In this contribution, the effect of cold atmospheric Surface micro-discharge (SMD) plasma on different materials and its sporicidal behavior was investigated.Hence, different material samples (stainless steel, different polymers and glass) were plasma-treated for 16 hours, simulating multiple plasma treatments using an SMD plasma device. Afterwards, the material samples were analyzed using surface analysis methods such as laser microscopy, contact angle measurements and x-ray photo-electron spectroscopy. Furthermore, the device was used to investigate the behavior of Bacillus atrophaeus endospores inoculated on material samples at different treatment times.The interaction results for plasma-treated endospores show, that a log reduction of the spore count between 4.3 and 6.2 can be achieved within 15 min of plasma treatment. Besides, the surface analysis revealed, that there were three different types of reactions the probed materials showed to plasma treatment, ranging from no changes to shifts of the materials’ free surface energies and oxidation.As a consequence, it should be taken into account that even though cold atmospheric plasma treatment is a non-thermal method to inactivate microorganisms on heat-sensitive materials, it still affects surface properties of the treated materials. Therefore, the focus of future work must be a further classification of plasma-caused material modifications.