# Susceptibility of _Staphylococcus epidermidis_ to Argon Cold Plasma Jet by Oxygen Admixture
## 3 Results and Discussion  
### The Effect of Oxygen Gas Mixing on ACPPJ  
The generated reactive species (OH, NO, O) from the non-thermal APPJ can be enhanced by mixing the input gas with other gases. Therefore, oxygen gas is added with different ratios to reach optimum O-radical production [20]. The occurrence of oxygen causes all plasma processes rather complex, even in a simple binary gas mixture [22]. The effect of adding oxygen gas electrically and optically is presented in this section. Mixing oxygen gas with argon to generate non-thermal APPJ may seem quite obvious, as shown in Figure 2, which presents the current-voltage waveforms with pure argon in (a) and 0.4 % oxygen (0.018/4.5 slpm) mixed in (b) at fixed \(\approx\)14 kV and 29 kHz. Through this work, argon gas was mixed with oxygen at 4.5 argon flow rate. The current values presented in this work are the current peak values. Adding oxygen gas increases the total current from \(\approx\)11.3 to \(\approx\)18 mA while the ground current increased from \(\approx\)8.3 to \(\approx\)22.9 mA, which is slightly higher than the total current for the positive half cycle. For the negative half cycle, the total current rises from \(\approx\)11.6 to \(\approx\)17.2 mA and from \(\approx\)8.1 to \(\approx\)16.6 mA for the ground one. Moreover, the number of currents peaks for each half cycle normally increases with applied voltage [23]. However, the current-voltage waveform in case of mixing oxygen is presenting the same effect of increasing the current peaks per each half cycle, and this effect could be due to the increase in the residual charge carriers from the privies pulse owing to oxygen electron affinity [20]. The imaging of the non-thermal APPJ declared that adding oxygen to the plasma accelerates the mode transition from filamentary to diffused mode at lower flow rate values compared to the case of pure argon as illustrated in Figure 3. Normally, the filament is generated inside the diffused plasma at the operation condition 4.5 slpm, 14 kV, and 29 kHz [24]. However, the filament is disappearing by adding oxygen gas. Moreover, increasing the mixing of oxygen gas to the argon non-thermal APPJ causes the shrink of the plasma plume length. Figure 4 shows the generated non-thermal APPJ emission spectra in the ranges from 200 to 450 nm (Figure 4a) and from 450 to 850 nm (Figure 4b) at different O\({}_{2}\) mixture percentages from 0.2% to 1.2%. The emission spectra for pure argon flow show the domination of the nitrogen second positive system in the range from 300 to 450 nm, while Ar lines were the highest intensity in the range from 600 to 850 nm. As the oxygen percentage increases, the plasma plume intensity decreases, and that takes place on the emission spectra. The band and line intensities decrease by increasing the percentage of oxygen into the argon gas, as shown in Figure 4. Moreover, adding oxygen gas lets the three lines of oxygen spectrum be emitted clearly at 777.35, 777.6, and 777.7 nm as well as the fourth one at 844.80 nm, as shown in Figure 5. The measurements of the gas temperature and vibrational temperature result in an estimated 320 K gas temperature (rotational temperature) and 1200 K vibrational temperature. The estimated temperature results from the comparison between the measured and the simulated spectra for pure Ar and Ar + 0.2% O\({}_{2}\) at 14 kV peak-to-peak voltage and 29 kHz. These measurements prove that the generated plasma is non-thermal equilibrium plasma. Figure 3: Non-thermal APPJ images show its development as a faction of increasing the oxygen percentages, operating at fixed 14 kV and 29 kHz, and 4.5 slpm argon flow. Figure 2: Current-voltage waveforms with (**a**) pure argon and (**b**) mixed with 0.018 slpm (0.4%) oxygen to 4.5 slpm argon flow rate, \(\approx\)14 kV and 29 kHz. Figure 4: Emission spectra of the argon non-thermal APPJ in the range 200–850 nm at 14 kV, 29 kHz, and 4.5 slpm with different oxygen concentrations (**a**) and (**b**).