The aim of this study was to clarify the initiation process and the propagation mechanism of positive underwater streamers under the application of pulsed voltage with a duration of 10 μs, focusing on two different theories of electrical discharges in liquids: the bubble theory and the direct ionization theory. The initiation process, which is the time lag from the beginning of voltage application to streamer inception, was found to be related to the bubble theory. In this process, Joule heating resulted in the formation of a bubble cluster at the tip of a needle electrode. Streamer inception was observed from the tip of a protrusion on the surface of this bubble cluster, which acted as a virtual sharp electrode to enhance the local electric field to a level greater than 10 MV/cm. Streak imaging of secondary streamer propagation showed that luminescence preceded gas channel generation, suggesting a mechanism of direct ionization in water. Streak imaging of primary streamer propagation revealed intermittent propagation, synchronized with repetitive pulsed currents. Shadowgraph imaging of streamers synchronized with the light emission signal indicated the possibility of direct ionization in water for primary streamer propagation as well as for secondary streamer propagation.

Recently, a plasma flow has been applied to medical treatment using effects of various kinds of stimuli such as chemical species, charged particles, heat, light, shock wave and electric fields. Among them, the chemical species are known to cause an inactivation of cell viability. However, the mechanisms and key factors of this event are not yet clear. In this study, we focused on the effect of H₂O₂ in plasma-treated culture medium because it is generated in the culture medium and it is also chemically stable compared with free radicals generated by the plasma flow. To elucidate the significance of H₂O₂, we assessed the differences in the effects of plasma-treated medium and H₂O₂-added medium against inactivation of HeLa cell viability. These two media showed comparable effects on HeLa cells in terms of the survival ratios, morphological features of damage processes, permeations of H₂O₂ into the cells, response to H₂O₂ decomposition by catalase and comprehensive gene expression. The results supported that among chemical species generated in a plasma-treated culture medium, H₂O₂ is one of the main factors responsible for inactivation of HeLa cell viability.

The spreading of water drops impinging on structured rough surfaces is studied experimentally. The rough surfaces are specially prepared with a regular pattern of surface asperities. The arrangement of the square-shaped surface asperities creates channel-like grooves on the surface. A video microscope along with a controlled light exposure system is used to construct the image sequences of the spreading process. The images are digitally analyzed to measure the temporal variation of the spreading drop diameter $2R$ . Results are obtained for three rough surfaces with varying asperity heights in the range of $100–500\mu \mathrm{m}$ and for different impact drop conditions with Weber number We in the range of 35–225. The results on the temporal variation of $2R$ show that, on the structured rough surfaces, the spreading occurs simultaneously both inside and above the texture pattern of the surfaces. For a given surface geometry, the volume of liquid flowing inside the grooves of the surface increases with increasing We. Consequently, the values of $2R$ measured inside the texture pattern are larger than those measured above the texture pattern, and their difference increases with increasing We. The arrangement of the surface asperities influences the spreading pattern of an impacting drop spreading axisymmetrically. For the texture geometry used in the present study, the spreading pattern resembles a regular rhombus shape for the impact of low We drops and becomes complex at high We. The spreading distances, measured both inside and above the texture pattern of the structured rough surfaces, are nearer to the measurements recorded on the smooth surface if the asperity height of the rough surface is smaller than the thickness of the spreading liquid lamella; however, the surface asperities influence the spreading pattern drastically and create a liquid splash.

A mechanism for sterilizing Escherichia coli by a flowing postdischarge and UV radiation of argon plasma at atmospheric pressure was investigated by analyzing the surviving cells and the potassium leakage of cytoplasmic material and by morphological observation. Inactivation of E. coli results from the destruction of the cytoplasmic membrane and the outer membrane under plasma exposure and the destruction of nucleic acids by exposure to ultraviolet radiation from the plasma source.

Plasma activated water (PAW) has emerged as a viable technique for sterilization process since it contains some long-lived reactive oxygen and nitrogen species along with some very short-lived but strong oxidation agents such as hydroxyl radical (OH ^* ) and superoxide anion (O ₂ ⁻ ). However, the concentrations of the reactive species such as nitrite, nitrate, and ozone are often insufficient in water for sterilization, or only a small amount of PAW can be generated. In this study, we developed a state-of-the-art approach by combining microbubbles (MBs) and plasma technology in a water recirculation system to enhance the dissolution of reactive species in a large amount of water. The investigations included plasma absorbing power measurement, visualization of plasma-activated microbubbles (PAMBs) generation, bubble size distribution, and the concentration measurements of various reactive species in water, after the air plasma jet and the PAMBs jet treatments, respectively. The results revealed that the plasma absorbing power efficiency could reach up to 71.7% for both plasma jet and PAMBs jet cases, respectively. For the PAMBs jet, the mean equivalent diameter of the bubble was estimated to be 290 μ m. The results of the concentrations of chemical species such as nitrite and ozone in PAMBs water could be increased up to twice as high as compared to PAW using the proposed device. Moreover, the concentration of nitrate in PAMBs water was found to be nearly three times greater than that in PAW. Thus, we found that employing the PAMBs jet can significantly increase the concentrations of chemical species in water, which could considerably widen the application.