The electron heating mode transition induced by ultra-high frequency in atmospheric-pressure microplasmas was investigated using particle-in-cell simulation with a Monte Carlo collision. Interestingly, this discharge mode transition is accompanied by non-monotonic evolution of electron kinetics such as effective electron temperature, plasma density, and electron energy on the electrode. In this study, the highest flux of energetic electrons (ɛ > 4 eV) usable for tailoring the surface chemistry in atmospheric microplasmas is obtained at the specific frequency (400 MHz), where an optimal trade-off is established between the amplitude of sheath oscillations and the power coupled to electrons for sub-millimeter dimensions (200 µm).

A microwave-excited atmospheric-pressure plasma jet (uAPPJ) exhibited a synergistic sterilization effect when combined with hydrogen peroxide (H 2 O 2 ), distilled water (DW) and titanium dioxide (TiO 2 ) photocatalysis. The sterilization efficacy of H 2 O 2 -uAPPJ increased as the H 2 O 2 concentration increased. The addition of TiO 2 also remarkably increased the sterilization efficacy. To find the main factor for the sterilization effect, optical emission spectra and the degradation rate of a methylene blue solution were measured. Numerical analysis, a newly developed global modeling, was also conducted to discover the mechanisms. Both experimental measurements and global modeling results suggested that combinations of H 2 O 2 , DW and TiO 2 increased the generation of hydroxyl radicals (•OH), which are known to be strong bactericidal agents. It was revealed that charged species, especially electrons, have a dominant role in the increase of •OH.

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The formation of secondary energetic electrons induced by an abnormal electron-heating mode in pulsed microwave-frequency atmospheric microplasmas was investigated using particle-in-cell simulation. We found that additional high electron heating only occurs during the first period of the ignition phase after the start of a second pulse at sub-millimeter dimensions. During this period, the electrons are unable to follow the abruptly retreating sheath through diffusion alone. Thus, a selfconsistent electric field is induced to drive the electrons toward the electrode. These behaviors result in an abnormal electron-heating mode that produces high-energy electrons at the electrode with energies greater than 50 eV. V C 2014 AIP Publishing LLC. [http://dx.

Particle-in-cell/Monte Carlo simulations and numerical analysis of a single particle motion are performed for atmospheric He microplasmas at microwave frequencies to determine the characteristics of non-Maxwellian to Maxwellian transition. The left and the right regimes of Paschen curve, divided by this transition, reveal that the transition frequencies depend on the gap of electrodes and the neutral gas pressure to follow scaling laws for a new extended Paschen law. The fluid models are reasonable at the right-side regime of Paschen breakdown areas, but not on the left side, which is highly kinetic for electrons. The plasmas driven by weaker electric fields of high enough frequencies at the right-side Paschen regime breed more energetic electrons.

A comparative study of electron kinetics between single-frequency (SF) microplasmas and their equivalent dual-frequency (DF) microplasmas with matching effective frequencies in atmosphericpressure helium discharges was performed using particle-in-cell simulation with a Monte Carlo collision. The effective-frequency concept helps in analyzing DF microplasmas in a fashion similar to SF microplasmas with effective parameters. In this study, the plasma characteristics such as the plasma potential, density, and electron energy probability functions of the SF microplasma and its DF counterpart were almost the same. However, the oscillating sheath edge was pushed further into the electrode for a substantial fraction of the time and the sheath width decreased in DF microplasmas. As a result, the transportation of the energetic electrons (e > 4 eV) usable for tailoring the surface chemistry in atmospheric microplasmas is enhanced in DF microplasmas as compared to SF microplasmas. V C 2013 American Institute of Physics. [http://dx.

The characteristics of low-frequency (LF) and microwave-powered plasmas were investigated. The optical emission of these two plasmas indicated that more chemicals were generated by microwave plasma than by LF plasma with the intensities being higher by factors of about 9, 3, 5, and 1.6 for OH (309 nm), O (777 nm), NO (247 nm), and Ca2+ (290 nm), respectively. Application experiments were also conducted. A steel plate became hydrophilic after 45 s of microwave plasma treatment. This is more than ten times faster than in the case of LF plasma treatment, an action related to the generation of reactive species (e.g., OH, O, and NO) as measured by optical emission spectroscopy (OES). Ca2+ generation was verified by blood coagulation experiment. Microwave-plasma-induced coagulation was twice faster than LF-plasma-induced coagulation. Simulation results that explain the chemical generation in microwave plasma were also included. High-energy electrons were considered a major factor for microwave plasma characteristics.

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