Two different modes of plasmas in microbubbles formed in saline solution

Summary form only given. Various types of atmospheric pressure plasma sources in liquid medium have been developed for biological and biomedical treatments. For example, non-thermal plasmas generated in saline solution have been investigated for in-vivo medical treatments. Excited species, shock waves, and water jets produced there are believed to be responsible for tissue removal in surgery. However, the detailed mechanisms of the plasma formation and interaction with the biomaterial surface are not fully understood. In the present study, we examined influences of bubble formation in plasma generation in normal saline solution (0.85 wt% NaCl in H₂O). A titanium wire (126 mum in diameter), covered by glass except at its tip, was inserted into saline solution. High frequency (-100 kHz) and high voltage (-500 V) was applied to the titanium wire. Another exposed titanium wire was placed in the same solution to act as ground return electrode. Plasmas were generated in microbubbles near the electrode without gas injection. By controlling the magnitude and polarity of the applied voltage, we found that the bubble behavior can be switched between a bubble cloud mode and stable single bubble mode with repetitive plasma generation. In the cases of repetitive plasma generation at negative voltage phase, bubble cloud was formed. The discharge current was up to several hundred mA. We observed emission from OH. When plasmas were generated at positive voltage phase, a stable single bubble was formed at the high-voltage electrode surface. In that case, the discharge current was less than 10 mA. The emission from OH was less intense than the bubble cloud mode. The gas composition in the microbubbles and the chemical products in the liquid medium will be also discussed.