Laser-induced photodetachment of negative oxygen ions in the spatial afterglow of an atmospheric pressure plasma jet

Negative ions are an important constituent of the spatial afterglow of atmospheric pressure plasmas, where the fundamental plasma-substrate interactions take place that are vital for applications such as biomedicine, material synthesis, and ambient air treatment. In this work, we use laser-induced photodetachment to liberate electrons from negative ions in the afterglow region of an atmospheric pressure plasma jet interacting with an argon-oxygen mixture, and microwave cavity resonance spectroscopy (MCRS) to detect the photodetached electrons. This diagnostic technique allows for the determination of the electron density and the effective collision frequency before, during and after the laser pulse was shot through the measurement volume with nanosecond time resolution. From a laser saturation study, it is concluded that O− is the dominant negative ion in the afterglow. Moreover, the decay of the photodetached electron density is found to be dominantly driven by the (re)formation of O− by dissociative attachment of electrons with O2. As a consequence, we identified the species and process responsible for the formation of negative ions in the spatial afterglow in our experiment.

Plasma dynamics, instabilities and OH generation in a pulsed atmospheric pressure plasma with liquid cathode: a diagnostic study

While plasma-liquid interactions have been an important focus in the plasma research community, the impact of the strong coupling between plasma and liquid on plasma properties and processes remains not fully understood. In this work, we report on the impact of the applied voltage, pulse width and liquid conductivity on the plasma morphology and the OH generation for a positive pulsed DC atmospheric pressure plasma jet with He-0.1% H2O mixture interacting with a liquid cathode. We adopted diagnostic techniques of fast imaging, 2D laser induced fluorescence (LIF) of OH and Thomson scattering spectroscopy. We show that plasma instabilities and enhanced evaporation occur and have a significant impact on the OH generation. At elevated plasma energies, it is found that the plasma contracts due to a thermal instability through Ohmic heating and the contraction coincides with a depletion in the OH density in the core due to electron impact dissociation. For lower plasma energies, the instability is suppressed/delayed by the equivalent series resistor of the liquid electrode. An estimation of the energy flux from the plasma to the liquid shows that the energy flux of the ions released into the liquid by positive ion hydration is dominant, and significantly larger than the energy needed to evaporate sufficient amount of water to account for the measured H2O concentration increase near the plasma-liquid interface.

Establishing an Equivalent Circuit of a Quasihomogeneous Discharge Atmospheric-Pressure Plasma Jet with a Breakdown-Voltage-Controlled Breaker and Power-Supply Circuit

To simulate the I-V diagram of plasma homogeneous and filamentary discharge with equivalent circuit model more accurately, this study employed a breaker and passive circuit components and calculated the discharge parameters, such as equivalent discharge resistances and potential distribution etc., in atmospheric-pressure plasma jet (APPJ). In addition, this study calculated the gas-gap and dielectric capacitances of the APPJ and added a power supply equivalent circuit. Compared with other circuit models that adopted switches or a time-controlled current source to simulate the discharges, our present circuit model used a breakdown-voltage-controlled breaker for the homogeneous discharge and resistors with high-frequency switches for the filamentary discharge. We employed potential simulation to obtain the equivalent dielectric capacitance in the APPJ and then derived the gas-gap capacitance. We also replaced the ideal sine wave power supply with the equivalent circuit of the common double-peak-waveform power supply. The MATLAB Simulink was used to construct an equivalent circuit model and the discharge area ratio, breakdown voltage and filamentary equivalent resistance were obtained via I-V waveform fitting. We measured the plasma I-V waveform with a 20-kHz frequency, various voltages (6, 12, and 15 kV), a gas flow rate of 30 SLM, and two types of gas (Ar and He). The simulated and experimental I-V waveforms were very close under different conditions. In summary, the proposed equivalent circuit model more meaningfully describes the plasma physics to simulate homogenous and filamentary discharge, achieving results that were compatible with our experimental observations. The findings can help with investigating plasma discharge mechanisms and full-model simulations of plasma.

Cold Atmospheric Pressure Plasma Jet Operated in Ar and He: From Basic Plasma Properties to Vacuum Ultraviolet, Electric Field and Safety Thresholds Measurements in Plasma Medicine

Application desired functionality as well as operation expenses of cold atmospheric pressure plasma (CAP) devices scale with properties like gas selection. The present contribution provides a comparative investigation for a CAP system operated in argon or helium at different operation voltages and distance to the surface. Comparison of power dissipation, electrical field strength and optical emission spectroscopy from vacuum ultraviolet over visible up to near infrared ((V)UV-VIS-NIR) spectral range is carried out. This study is extended to safety relevant investigation of patient leakage current, induced surface temperature and species density for ozone (O3) and nitrogen oxides (NOx). It is found that in identical operation conditions (applied voltage, distance to surface and gas flow rate) the dissipated plasma power is about equal (up to 10 W), but the electrical field strength differs, having peak values of 320 kV/m for Ar and up to 300 kV/m for He. However, only for Ar CAP could we measure O3 up to 2 ppm and NOx up to 7 ppm. The surface temperature and leakage values of both systems showed different slopes, with the biggest surprise being a constant leakage current over distance for argon. These findings may open a new direction in the plasma source development for Plasma Medicine.

Effect of atmospheric-pressure plasma irradiation on the surface tension of water

We have shown that measuring the surface tension is a useful scheme to examine the plasma-liquid interface in real-time. The surface tension was measured using a method based on the dispersion relation of an acoustic capillary wave excited on the water surface. The surface tension gradually increased with time, when the water surface was irradiated with the outside region of the spatial afterglow of an atmospheric-pressure plasma. The Marangoni effect associated with the localized increase in the surface tension was observed during the plasma irradiation. The surface tension decreased after the termination of the discharge. A correlation was found between the transient decrease in the surface tension and the variation of the OH radical density in the gas phase. No increase in the surface tension was observed in the solution containing a trapping agent for liquid-phase OH radicals. These experimental results suggest that OH radicals act to increase the surface tension. However, the behavior of the surface tension cannot be explained perfectly by considering only the action of OH radicals.

Efficiency of cold atmospheric plasma, cleaning powders and their combination for biofilm removal on two different titanium implant surfaces

Objectives Biofilm removal is the decisive factor for the control of peri-implantitis. Cold atmospheric pressure plasma (CAP) can become an effective aid due to its ability to destroy and to inactivate bacterial biofilm residues. This study evaluated the cleaning efficiency of CAP, and air-polishing with glycine (APG) or erythritol (APE) containing powders alone or in combination with CAP (APG + CAP, APE + CAP) on sandblasted/acid etched, and anodised titanium implant surface. Materials and methods On respective titanium discs, a 7-day ex vivo human biofilm was grown. Afterwards, the samples were treated with CAP, APG, APE, APG + CAP, and APE + CAP. Sterile and untreated biofilm discs were used for verification. Directly after treatment and after 5 days of incubation in medium at 37 °C, samples were prepared for examination by fluorescence microscopy. The relative biofilm fluorescence was measured for quantitative analyses. Results Air-polishing with or without CAP removed biofilms effectively. The combination of air-polishing with CAP showed the best cleaning results compared to single treatments, even on day 5. Immediately after treatment, APE + CAP showed insignificant higher cleansing efficiency than APG + CAP. Conclusions CAP supports mechanical cleansing and disinfection to remove and inactivate microbial biofilm on implant surfaces significantly. Here, the type of the powder was not important. The highest cleansing results were obtained on sandblasted/etched surfaces. Clinical relevance. Microbial residuals impede wound healing and re-osseointegration after peri-implantitis treatment. Air-polishing treatment removes biofilms very effectively, but not completely. In combination with CAP, microbial free surfaces can be achieved. The tested treatment regime offers an advantage during treatment of peri-implantitis.

Plasma Co-Polymerization of HMDSO and Limonene with an Atmospheric Pressure Plasma Jet

Plasma co-polymers (co-p) were deposited with an atmospheric pressure plasma jet (APPJ) using a precursor mixture containing hexamethyldisiloxane (HMDSO) and limonene. A coating with fragments from both precursors and with siloxane, carbonyl and nitrogen functional groups was deposited. The flow rate of limonene was found to be an important parameter for plasma co-polymerization to tune the formation and structure of the functional groups. The FTIR and XPS analysis indicates that with increasing flow rate of limonene a higher proportion of carbon is bound to silicon. This is related to a stronger incorporation of fragments from limonene into the siloxane network and a weaker fragmentation of HMDSO. The formation mechanism of the nitroxide and carboxyl groups can be mainly differentiated into in-plasma and post-plasma reactions, respectively.