Atmospheric discharge plasma in aqueous solution: Importance of the generation of water vapor bubbles for plasma onset and physicochemical evolution

Abstract

Discharge plasma formed in aqueous solutions has attracted much attention for its applications in environmental purification and material syntheses. The onset and evolution of the discharge plasma in an aqueous solution and transient reactive species formed in it are successfully monitored with micrometer spatial resolution and nanosecond temporal resolution. The combination of a custom-made microscopic discharge system and a high-speed camera provides direct evidence that water vapor bubbles form before the discharge with the thermal phase transition of aqueous solution at the electrode tip. The water vapor bubbles, i.e., locally formed space in the gas phase, connect the gap between the tips of the opposed electrodes. The local gas area formed in aqueous solution plays a crucial role in the ignition and continuance of the discharge plasma. It is also found that the initially formed plasma lasts for under 100 ns and quenches rapidly. However, plasma regenerates in the water vapor bubble and successively bridges the opposing electrodes during the pulsed-voltage application (ca. 1 μs). These two temporally distinct generations of plasma, i.e., the initial plasma (IP) and the following successive plasma (SP), can be seen to correspond to the dielectric breakdown and glow-like plasma, respectively. These results provide an important picture for the proposed mechanism for plasma evolution in water and also important information for the efficient control of the discharge plasma with its applications in waste-water treatments, nanomaterial syntheses with plasma oxidation–reduction reactions, and the chemical modification of the material surfaces in aqueous solutions as a form of “green chemistry.”

Structure and Physicochemical Properties of Water Treated under Nitrogen with Low-Temperature Glow Plasma

“…They involve plasma generated with a variety of methods and various modes of their application. Thus, an abundant number of papers [4][5][6][7][8][9][10][11][12][13][14][15][16][17] describe procedures in which plasma was generated by electric static or pulsed either dc low-voltage or RF discharges inside gas bubbles formed in water. Such treatment produced ozone [8] and hydrogen peroxide [11,13,18,19].…”

Initial stages of pulsed discharge in saline solutions in a vicinity of threshold voltages

“…The container is equipped with quartz windows. One of the windows is used to take photographs of the interelectrode gap by means of the CCD camera SensiCam, with a typical exposure time of (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) μs. Another window is used for obtain the waveforms of the temporal behavior of the discharge luminosity by means of the photomultiplier tube PMT.…”

“…The electrode system is placed in a saline solution and consists of the active pin electrode and the opposite grounded plane electrode [16-18, 22, 23]. In general, the scenario of discharge development seems to be understandable [18,19,[24][25][26][27][28][29]. Due to the current flowing via the bulk of the solution and the corresponding power dissipation, the microbubbles are generated in a vicinity of the active electrode and a part of them is attached to this electrode.…”