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The susceptibility of Candida albicans biofilms to a non-thermal plasma treatment has been investigated in terms of growth, survival and cell viability by a series of in vitro experiments. For different time periods, the C. albicans strain SC5314 was treated with a microwave-induced plasma torch (MiniMIP). The MiniMIP treatment had a strong effect (reduction factor (RF) = 2.97 after 50 s treatment) at a distance of 3 cm between the nozzle and the superior regions of the biofilms. In addition, a viability reduction of 77% after a 20 s plasma treatment and a metabolism reduction of 90% after a 40 s plasma treatment time were observed for C. albicans. After such a treatment, the biofilms revealed an altered morphology of their cells by atomic force microscopy (AFM). Additionally, fluorescence microscopy and confocal laser scanning microscopy (CLSM) analyses of plasma-treated biofilms showed that an inactivation of cells mainly appeared on the bottom side of the biofilms. Thus, the plasma inactivation of the overgrown surface reveals a new possibility to combat biofilms., (© 2019 The Authors. Microbial Biotechnology published by John Wiley & Sons Ltd and Society for Applied Microbiology.)

Experiments and modeling of the plasma-microwave interaction have been performed in a coaxial microwave plasma source at a field frequency of 2.45 GHz generating argon plasmas at pressures of 20 and 40 millibars and a ratio of flow rate to pressure of 0.125 sccm/Pa. The incident microwave power between 100W and 300W is supplied in a regime of a pulse-width modulation with cycle duration of 110 ms and a power-on time of 23ms. The experiments are based on heterodyne reflectometry and microwave interferometry at 45.75 GHz. They provide the temporal behaviour of the complex reflection coefficient, the microwave power in the plasma, as well as the electron density in the afterglow zone of the discharge. The self-consistent spatially two-dimensional and time-dependent modeling complements the analysis of the plasma-microwave interaction delivering the plasma and electromagnetic field parameters. The consolidating experimental observations and model predictions allow further characterizing the plasma source. The generated plasma has a core occupying the region close to the end of the inner electrode, where maximum electron densities above 1020 m-3 and electron temperatures of about 1 eV are observed. Due to a longer outer electrode of the coaxial structure, the plasma region is extended and fills the volume comprised by the outer electrode. The electron density reaches values of the order of 1019 m-3. The heating of the gas occurs in its great part due to elastic collisions with the plasma electrons. However, the contribution of the convective heating is important especially in the extended plasma region, where the gas temperature reaches its maximum values up to approximately 1400 K. The temporally and spatially resolved modeling enables a thorough investigation of the plasma-microwave interaction which clearly shows that the power in-coupling occurs in the region of the highest electron density during the early stage of the discharge. In the steady state phase, however, the power in-coupling occurs close to the source walls where the electron density is significantly lower than on the discharge axis. [ABSTRACT FROM AUTHOR]

A two-dimensional model of microwave-induced plasma (field frequency 2.45 GHz) in argon at atmospheric pressure is presented. The model describes in a self-consistent manner the gas flow and heat transfer, the in-coupling of the microwave energy into the plasma, and the reaction kinetics relevant to high-pressure argon plasma including the contribution of molecular ion species. The model provides the gas and electron temperature distributions, the electron, ion, and excited state number densities, and the power deposited into the plasma for given gas flow rate and temperature at the inlet, and input power of the incoming TEM microwave. For flow rate and absorbed microwave power typical for analytical applications (200-400 ml/min and 20 W), the plasma is far from thermodynamic equilibrium. The gas temperature reaches values above 2000 K in the plasma region, while the electron temperature is about 1 eV. The electron density reaches a maximum value of about 4 ×lO21. The balance of the charged particles is essentially controlled by the kinetics of the molecular ions. For temperatures above 1200 K, quasineutrality of the plasma is provided by the atomic ions, and below 1200 K the molecular ion density exceeds the atomic ion density and a contraction of the discharge is observed. Comparison with experimental data is presented which demonstrates good quantitative and qualitative agreement. [ABSTRACT FROM AUTHOR]