Influence of air diffusion on the OH radicals and atomic O distribution in an atmospheric Ar (bio)plasma jet

Treatment of samples with plasmas in biomedical applications often occurs in ambient air.
Admixing air into the discharge region may severely affect the formation and destruction of
the generated oxidative species. Little is known about the effects of air diffusion on the spatial
distribution of OH radicals and O atoms in the afterglow of atmospheric-pressure plasma jets.
In our work, these effects are investigated by performing and comparing measurements in
ambient air with measurements in a controlled argon atmosphere without the admixture of air,
for an argon plasma jet. The spatial distribution of OH is detected by means of laser-induced
fluorescence diagnostics (LIF), whereas two-photon laser-induced fluorescence (TALIF) is
used for the detection of atomic O. The spatially resolved OH LIF and O TALIF show that, due
to the air admixture effects, the reactive species are only concentrated in the vicinity of the
central streamline of the afterglow of the jet, with a characteristic discharge diameter of
∼1.5 mm. It is shown that air diffusion has a key role in the recombination loss mechanisms of
OH radicals and atomic O especially in the far afterglow region, starting up to ∼4mm from the
nozzle outlet at a low water/oxygen concentration. Furthermore, air diffusion enhances OH
and O production in the core of the plasma. The higher density of active species in the
discharge in ambient air is likely due to a higher electron density and a more effective electron
impact dissociation of H2O and O2 caused by the increasing electrical field, when the
discharge is operated in ambient air.