Atmospheric pressure plasma jets interacting with liquid covered tissue: touching and not-touching the liquid

In the use of atmospheric pressure plasma jets in biological applications, the plasma-produced charged and neutral species in the plume of the jet often interact with a thin layer of liquid covering the tissue being treated. The plasma-produced reactivity must then penetrate through the liquid layer to reach the tissue. In this computational investigation, a plasma jet created by a single discharge pulse at three different voltages was directed onto a 200 µm water layer covering tissue followed by a 10 s afterglow. The magnitude of the voltage and its pulse length determined if the ionization wave producing the plasma plume reached the surface of the liquid. When the ionization wave touches the surface, significantly more charged species were created in the water layer with H₃O⁺_aq, O₃⁻_aq, and O₂⁻_aq being the dominant terminal species. More aqueous OH_aq, H₂O_2aq, and O_3aq were also formed when the plasma plume touches the surface. The single pulse examined here corresponds to a low repetition rate plasma jet where reactive species would be blown out of the volume between pulses and there is not recirculation of flow or turbulence. For these conditions, N_xO_y species do not accumulate in the volume. As a result, aqueous nitrites, nitrates, and peroxynitrite, and the HNO_3aq and HOONO_aq, which trace their origin to solvated N_xO_y, have low densities. (paper)