Plasma Microjet Research Articles

Stagnant liquid layer as “Microreaction System” in submerged plasma Micro-Jet for formation of carbon quantum dots

This study aims to intensify reactivity via bespoke transient hydrodynamics in a plasma-activated three-phase catalyst system. Likewise, three-phase plasma systems in literature are not well designed, understood and commercially available. The synthesised N-doped carbon quantum dots, with application as fertilisers and wastewater treatment was evaluated as a model reaction. A plasma microjet was produced using a commercial plasma system, creating an almost flat, large interface covering a thin stagnant liquid layer with a catalyst bed underneath. In this hydrodynamic regime, plasma can penetrate the catalyst bed via the stagnant thin liquid film and polarise the plasma-liquid interface. The new process regime’s effectiveness is proven by the enhanced reaction rate achieved by raising liquid diffusivity toward the catalyst bed through solvent viscosity reduction. Results indicate that the reaction rate depends on the small surface area interacted with the plasma jet amid an excess of excited species, not the total gas–liquid interface area. The plasma process competes energetically with the best dielectric barrier discharge processes and uses less energy than microfluidic processing in chemical microreactors. High momentum transfer from the plasma jet to the liquid partly evaporates non-water additives, impacting process design and requiring reduction.

Study of uniformity of atmospheric pressure plasma microjet array for maskless parallel micropatterned etching

AbstractA novel atmospheric pressure plasma microjet (APPμJ) array is proposed to realize maskless parallel micropatterned etching of materials. Due to applying microfabrication technology, the dimension and distance of multiple jets in an array can be adjusted at a micro/nanoscale. However, interactions among multiple jets often lead to inconsistency of microjets and nonuniformity of sample etching. In this study, a 1 × 2 APPμJ array with 90‐μm nozzles and varying jet‐to‐jet distance is developed. The effects of different operating factors on the consistency of the APPμJ array are investigated. The experiment results show that the consistency of the microjet array and uniformity of sample etching can be greatly improved by optimizing operating parameters, such as applied voltage, gas flow rate, and jet‐to‐jet distance.

A Simple Gas-Kinetic Model for Dilute and Weakly Charged Plasma Micro-Jet Flows

This paper presents a simple model for slightly charged gas expanding into a vacuum from a planar exit. The number density, bulk velocity, temperature, and potential at the exit are given. The electric field force is assumed weaker than the convection term and is neglected in the analysis. As such, the quasi-neutral condition is naturally adopted and the potential field is computed with the Boltzmann relation. At far field, the exit degenerates as a point source, and simplified analytical formulas for flow and electric fields are obtained. The results are generic and offer insights on many existing models in the literature. They can be used to quickly approximate the flowfield and potential distributions without numerical simulations. They can also be used to initialize a simulation. Based on these results, more advanced models may be further developed.

The effects of grounded electrode geometry on RF-driven cold atmospheric pressure plasma micro-jet

With the argument that two-electrode DBD-like systems are much more operational than single-electrode systems in biomedical applications, targets sensitive to temperature and electric shock, the effects of parameters associated with the geometry of the grounded electrode such as its shape, size, and position it at the output of the atmospheric pressure RF plasma jet in two-electrode systems is investigated. By varying the position of the typical narrow ring grounded electrode on the dielectric tube toward the powered electrode, the ratio of the axial to radial electric field components depend on the externally applied potential to the plasma has been investigated and shown that the axial component of the electric field is maximized at certain position(s) of the grounded electrode. The analysis of the data indicates that there is an inverse relationship between the magnitude of the axial electric field in the plasma channel and the discharge ignition voltage, and a direct relationship with the plasma jet length. It is known that by increasing the width of the ground electrode until the full covering of dielectric, the jet length increases from the dielectric output to the neighborhood near the needle electrode, and reduces the discharge ignition threshold and consequently power consumption of the jet, but increasing its width to greater than the above values does not have a significant effect on jet output. It has also been shown that by tapering the dielectric end and fully covering it with its conical-shaped electrode, the output jet length increases and decreases its width.

Research on Process and Mechanism of Micro Punching driven by Laser-induced Cavitation

In this paper, a method of micro-punching driven by combining laser-induced cavitation was proposed. By analysing the surface roughness and the morphology of copper foil punched by the process, the influences of key process parameters including laser energy, cavitation position and punching times on the punching workpiece were systematically studied. With the laser focus position changing from -1 mm to +3 mm, the maximum depth of punching copper foil pits gradually decreased from 118.10 μm to 55.19 μm, and the ablative degree of the pit bottom reduced gradually. Meanwhile, it is found by high speed camera that the plasma shock, collapse shock and micro-jet produced by laser induced cavitation are the main driven power sources for deformation. Finally, the process is optimized with laser focus position of +2 mm, laser energy of 10.1 mJ and 5 laser pulses, the surface ablation could be completely avoided.

Maskless Surface Modification of Polyurethane Films by an Atmospheric Pressure He/O₂ Plasma Microjet for Gelatin Immobilization.

A localized maskless modification method of polyurethane (PU) films through an atmospheric pressure He/O2 plasma microjet (APPμJ) was proposed. The APPμJ system combines an atmospheric pressure plasma jet (APPJ) with a microfabricated silicon micronozzle with dimension of 30 μm, which has advantages of simple structure and low cost. The possibility of APPμJ in functionalizing PU films with hydroxyl (–OH) groups and covalent grafting of gelatin for improving its biocompatibility was demonstrated. The morphologies and chemical compositions of the modified surface were analyzed by scanning electronic microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The fluorescent images show the modified surface can be divided into four areas with different fluorescence intensity from the center to the outside domain. The distribution of the rings could be controlled by plasma process parameters, such as the treatment time and the flow rate of O2. When the treatment time is 4 to 5 min with the oxygen percentage of 0.6%, the PU film can be effectively local functionalized with the diameter of 170 μm. In addition, the modification mechanism of PU films by the APPμJ is investigated. The localized polymer modified by APPμJ has potential applications in the field of tissue engineering.

Separated Type Atmospheric Pressure Plasma Microjets Array for Maskless Microscale Etching

Maskless etching approaches such as microdischarges and atmospheric pressure plasma jets (APPJs) have been studied recently. Nonetheless, a simple, long lifetime, and efficient maskless etching method is still a challenge. In this work, a separated type maskless etching system based on atmospheric pressure He/O2 plasma jet and microfabricated Micro Electro Mechanical Systems (MEMS) nozzle have been developed with advantages of simple-structure, flexibility, and parallel processing capacity. The plasma was generated in the glass tube, forming the micron level plasma jet between the nozzle and the surface of polymer. The plasma microjet was capable of removing photoresist without masks since it contains oxygen reactive species verified by spectra measurement. The experimental results illustrated that different features of microholes etched by plasma microjet could be achieved by controlling the distance between the nozzle and the substrate, additive oxygen ratio, and etch time, the result of which is consistent with the analysis result of plasma spectra. In addition, a parallel etching process was also realized by plasma microjets array.

Expansion of a multicomponent current-carrying plasma jet into vacuum

An expression for the ion−ion coupling in a multicomponent plasma jet is derived for an arbitrary ratio between the thermal and relative velocities of the components. The obtained expression is used to solve the problem on the expansion of a current-carrying plasma microjet emitted from the cathode surface into vacuum. Two types of plasmas with two ion components are analyzed: (i) plasma in which the ion components of equal masses are in the charge states Z1= +1 and Z2= +2 and (ii) plasma with ions in equal charge states but with the mass ratio m1/m2 = 2. It is shown that, for such plasmas, the difference between the velocities of the plasma components remains substantial (about 10% of the average jet velocity in case (i) and 15% in case (ii)) at distances of several centimeters from the emission center, where it can be measured experimentally, provided that its initial value at the emitting cathode surface exceeds a certain threshold. This effect is investigated as a function of the mass ratio and charge states of the ion components.

Ion acceleration in multi-species cathodic plasma jet

A general expression for ion-ion coupling in a multi-species plasma jet was obtained. The expression is valid for any value of the inter-species velocity. This expression has enabled us to review a hydrodynamic problem of expanding the cathodic plasma microjet with two ion species within the respective charge states Z1 = +1 and Z2 = +2 into a vacuum. We were able to illustrate that in scenario when the initial (i.e., acquired during a process of emission from cathode's surface) difference for ion's species velocity exceeds a threshold value, the difference remains noticeable (roughly about 10% of the average jet's velocity) at a distance of a few centimeters from the emission center. At this point, it can be measured experimentally.

Enhancing wettability of high density polyethylenes (HDPE) through microplasma jets

This paper aims at the characterization of plasma microjets generated by open microhollow cathodes to ionize gases such as air, N2, He and O2. The electrical and optical parameters were raised. Through the optical emission spectroscopy, gas temperature (Tg) and the excitation temperature were estimated. Tg ranging from 2100K to 600K for a gas flow rate ranging from 0 to 400ml/min. From 150 to 400ml/min Tg remained close to 700K. As an application, the treatment of smooth surfaces of high density polyethylenes (HDPE) was performed with a view to improve the hydrophilic property of the surface.

Production of gold nanoparticles using atmospheric pressure glow microdischarge generated in contact with a flowing liquid cathode – a design of experiments study

dc-μAPGD with a miniature flow Ar plasma microjet and a small-sized flowing liquid cathode was characterized with respect to the multivariate effects of selected factors on the particle size of synthesized AuNPs by using the DOE and RSM approach.

Assessment of the Physicochemical Properties and Biological Effects of Water Activated by Non‐thermal Plasma Above and Beneath the Water Surface

A direct current atmospheric pressure cold plasma microjet (PMJ) was used to produce plasma activated water (PAW) in two generation modes above (PAW‐A) and beneath (PAW‐B) the water surface. The physicochemical characteristics and biological effects of them were measured respectively. Results showed that PAW‐B exhibited stronger disinfection efficiency than PAW‐A, which was associated with ORP and conductivity, but not pH value, temperature, H2O2, NO3− or NO2−. Furthermore, the cell membrane integrity and membrane potential of S. aureus were destroyed more severely by PAW‐B. More importantly, a significant increase of intracellular ROS was induced by PAW‐B, which was supposed to contribute most to the sterilization. This work provides valuable guidance for the production and wide applications of PAW.

Wound healing with nonthermal microplasma jets generated in arrays of hourglass microcavity devices

Clinical studies are reported in which artificial wounds in rat epidermal and dermal tissue have been treated by arrays of sub-500 µm diameter, low temperature plasma microjets. Fabricated in Al/nanoporous alumina (Al2O3) by wet chemical and microablation processes, each plasma jet device has a double parabolic (hourglass) structure, and arrays as large as 6 × 6 devices with 500 µm diameter apertures have been tested to date. Treatment of 1 cm2 acute epidermal wounds for 20–40 s daily with an array of microplasma jets generated in He feedstock gas promoted wound recovery significantly, as evidenced by tissue histology and measured wound area. Seven days after wound formation, the wound area of the untreated control was 40 ± 2% of its initial value, whereas that for an identical wound treated twice daily for 20 s was 9 ± 2% of its original surface area. No histological distinctions were observed between wounds treated twice each day for 10 or 20 s – only the full recovery time differed. Spectra produced in the visible and ultraviolet by He jets in room air are dominated by atomic oxygen (3p 5P → 3s 5S) at 777 nm and violet fluorescence (391.4 nm) from N2+, a species produced when the He (2s 3S1) metastable is deactivated by Penning ionization of N2. Although the combined cross-sectional area of the jets in the array is only 7% of the wound area, the microplasma treatment results in spatially uniform, and accelerated, wound healing. Both effects are attributed to the increased surface area of the jet array (relative to a single jet having an equivalent diameter) and the concomitant enhancement in the generation of molecular radicals, and metastable atoms and molecules (such as ).

The Efficacy, Safety, Stability, and Mechanism of Tooth Whitening by a Cold Atmospheric Pressure Air Plasma Microjet Assisted With or Without Hydrogen Peroxide

The aim of this paper was to assess the effectiveness, safety, stability, and mechanism of tooth whitening using a direct current cold atmospheric pressure air plasma microjet (PMJ) with the assistance of whitening gels (blank or containing 3% H2O2). The results were compared with those obtained from teeth treated with blank gel only and with gel containing 35% H2O2 in a cold light bleaching system. Tooth whitening effectiveness right after various treatments as well as 18 months after treatment was evaluated via CIELAB colorimetry. Type K thermocouple, scanning electron microscope and energy dispersive spectroscopy were employed to study the temperature of pulp chamber, tooth surface morphology, and elemental composition of enamel surface, respectively. Teeth in the plasma groups were effectively whitened in 5 min, though no significant difference was observed between the two plasma groups. Pulp chamber temperature was shown to remain below 35 °C. Morphological and surface elemental analysis showed similar changes to tooth enamel in the plasma groups and in the cold light group. These observations suggested that PMJ with blank gel was more effective for tooth whitening with acceptable safety and good stability. Optical emission spectroscopy results revealed that reactive oxygen species, such as O, •OH, and NO, in plasma may play a dominant role in the process of plasma tooth whitening.

A study of oxidative stress induced by non-thermal plasma-activated water for bacterial damage

Ar/O2 (2%) cold plasma microjet was used to create plasma-activated water (PAW). The disinfection efficacy of PAW against Staphylococcus aureus showed that PAW can effectively disinfect bacteria. Optical emission spectra and oxidation reduction potential results demonstrated the inactivation is attributed to oxidative stress induced by reactive oxygen species in PAW. Moreover, the results of X-ray photoelectron spectroscopy, atomic absorption spectrometry, and transmission electron microscopy suggested that the chemical state of cell surface, the integrity of cell membrane, as well as the cell internal components and structure were damaged by the oxidative stress.

Atmospheric-pressure cold plasma treatment of contaminated fresh fruit and vegetable slices: inactivation and physiochemical properties evaluation

A direct-current, atmospheric-pressure air cold plasma microjet (PMJ) was applied to disinfect Salmonella directly deposited on fresh fruit and vegetable slices. Effective inactivation was achieved on sliced fruit and vegetables after 1 s plasma treatment. The physiochemical properties of the slices, such as water content, color parameters, and nutritional content were monitored before and after plasma treatment. It was found that the physiochemical properties changes caused by the plasma were within an acceptable range. Reactive oxygen species, which are believed to be the major bactericidal agents in the plasma, were detected by electron spin resonance spectroscopy and optical emission spectroscopy.

Inactivation of Candida Biofilms by Non-Thermal Plasma and Its Enhancement for Fungistatic Effect of Antifungal Drugs

We investigated the antifungal effect of non-thermal plasma, as well as its combination with common antifungal drugs, against Candida biofilms. A direct current atmospheric pressure He/O2 (2%) plasma microjet (PMJ) was used to treat Candida biofilms in a 96-well plate. Inactivation efficacies of the biofilms were evaluated by XTT assay and counting colony forming units (CFUs). Morphological properties of the biofilms were evaluated by Scanning Electron Microscope (SEM). The sessile minimal inhibitory concentrations (SMICs) of fluconazole, amphotericin B, and caspofungin for the biofilms were also tested. Electron Spin Resonance (ESR) spectroscopy was used to detect the reactive oxygen species (ROS) generated directly and indirectly by PMJ. The Candida biofilms were completely inactivated after 1 min PMJ treatment, where severely deformed fungal elements were observed in SEM images. The SMICs of the tested antifungal drugs for the plasma-treated biofilms were decreased by 2–6 folds of dilution, compared to those of the untreated controls. ROS such as hydroxyl radical (•OH), superoxide anion radical (•O2 -) and singlet molecular oxygen (1O2) were detected by ESR. We hence conclude that He/O2 (2%) plasma alone, as well as in combination with common antifungal drugs, is able to inactivate Candida biofilms rapidly. The generation of ROS is believed to be one of the underlying mechanisms for the fungicidal activity of plasma.

A dc non-thermal atmospheric-pressure plasma microjet

A direct current (dc), non-thermal, atmospheric-pressure plasma microjet is generated with helium/oxygen gas mixture as working gas. The electrical property is characterized as a function of the oxygen concentration and show distinctive regions of operation. Side-on images of the jet were taken to analyze the mode of operation as well as the jet length. A self-pulsed mode is observed before the transition of the discharge to normal glow mode. Optical emission spectroscopy is employed from both end-on and side-on along the jet to analyze the reactive species generated in the plasma. Line emissions from atomic oxygen (at 777.4 nm) and helium (at 706.5 nm) were studied with respect to the oxygen volume percentage in the working gas, flow rate and discharge current. Optical emission intensities of Cu and OH are found to depend heavily on the oxygen concentration in the working gas. Ozone concentration measured in a semi-confined zone in front of the plasma jet is found to be from tens to ∼120 ppm. The results presented here demonstrate potential pathways for the adjustment and tuning of various plasma parameters such as reactive species selectivity and quantities or even ultraviolet emission intensities manipulation in an atmospheric-pressure non-thermal plasma source. The possibilities of fine tuning these plasma species allows for enhanced applications in health and medical related areas.

Reactive Oxygen Species in a Non‐thermal Plasma Microjet and Water System: Generation, Conversion, and Contributions to Bacteria Inactivation—An Analysis by Electron Spin Resonance Spectroscopy

AbstractHydroxyl radical (•OH) and singlet oxygen (1O2) were detected by electron spin resonance (ESR) spectroscopy in a direct current He/O2 (2%) non‐thermal plasma microjet‐water system. ${}^{ \bullet }{\rm O}_{{\rm 2}}^{- } $ is shown to be the precursor of •OH. The concentrations of 1O2 and •OH are evaluated to be around 6 × 10−4 and 1.2 × 10−5 M, respectively. The survival rates of S. aureus exposed to plasma for 20 s in 1 ml H2O, SOD (100 U, for scavenging ${}^{ \bullet }{\rm O}_{{\rm 2}}^{- } $), D‐Man (0.15 M, for scavenging •OH), and L‐His (0.15 M, for scavenging •OH and 1O2) solutions were 0.7, 1.6, 13.4, and 40.9%, respectively, indicating that 1O2 contributes the most to the inactivation.magnified image

Raman spectroscopy of nonstacked graphene flakes produced by plasma microjet deposition

An extensive Raman investigation of few‐layer graphene structures, obtained using a plasma microjet technique, is presented. Raman spectroscopy represents a unique method to characterize specific features of these systems. Excitation energies both in the visible and in the deep ultraviolet range are exploited, allowing to extract the main structural properties of the in‐house deposited samples. Particular attention is given to the determination of the stacking order properties of these few‐layer graphene structures. The results presented here also validate the plasma microjet as an efficient deposition technique to obtain graphene‐based systems with a low number of layers and reduced coupling on well defined and spatially localized areas. Copyright © 2011 John Wiley & Sons, Ltd.