Observation of OH radicals produced by pulsed discharges on the surface of a liquid

Abstract

The hydroxyl radical (OH) plays an important role in plasma chemistry at atmospheric pressure. OH radicals have a higher oxidation potential compared with other oxidative species such as free radical O, atomic oxygen, hydroperoxyl radical (HO 2 ), hydrogen peroxide(H 2 O 2 ) and ozone. In this study, surface discharges on liquids (water and its solutions) were investigated experimentally. A pulsed streamer discharge was generated on the liquid surface using a point-to-plane electrode geometry. The primary generation process of OH radicals is closely related to the streamer propagation, and the subsequent secondary process after the discharge has an influence on the chemical reaction. Taking into account the timescale of these processes, we investigated the behavior of OH radicals using two different diagnostic methods. Time evolution of the ground-state OH radicals above the liquid surface after the discharge was observed by a laser-induced fluorescence (LIF) technique. In order to observe the ground-state OH, an OH [A 2 + (v = 1) ← X 2 (v = 0)] system at 282 nm was used. As the secondary process, a portion of OH radicals diffused from gas phase to the liquid surface and dissolved in the liquid. These dissolved OH radicals were measured by a chemical probe method. Terephthalic acid was used as an OH radical trap and fluorescence of the resulting 2-hydroxyterephthalic acid was measured. This paper directly presents visualization of OH radicals over the liquid surface by means of LIF, and indirectly describes OH radicals dissolved in water by means of a chemical method.

Initiation process and propagation mechanism of positive streamer discharge in water

“…12,17,19 Underwater streamer discharges at positive polarity also have two propagation modes: a primary streamer and a secondary streamer. [20][21][22][23][24][25][26][27] The primary streamer appears at lower applied voltage with a propagation velocity of 2-3 km/s, having a semi-spherical brush-like structure ($500 lm) composed of many filaments. 22,26 All but a few filaments disappear within 400 ns, resulting in a tree-like structure.…”

“…23,24,26 Above a certain threshold voltage, the secondary streamer propagates with a velocity of greater than 10 km/s, for example, 25 reports, forming a filamentary structure of a few millimeters, dependent on gap distance. [20][21][22][23][24][25]27 In the case of low conductive water, secondary streamer propagation is followed by re-illuminations, which are restarts of the discharge in the formed gas channel, and show a dielectric-barrier discharge (DBD) like behavior with a stepwise propagation. 21,23,24 The secondary streamer and the re-illuminations are characterized by a continuous current with a duration of around 100 ns and subsequent spike currents.…”

Application of a Non-thermal Atmospheric Pressure Plasma Jet to the Decomposition of Salicylic Acid to Inorganic Carbon

“…This mass transport may have occurred due to an electrohydrodynamic flow in liquid and a flow caused by an ionic wind generated above the liquid, which induces convection and diffusion in liquid. 25 In conclusion, we demonstrated the decomposition of salicylic acid to inorganic carbon using non-thermal atmospheric pressure plasma jet. Large amounts of OH radical were generated by plasma irradiation, and the radicals directly decomposed salicylic acid within 10 min.…”

Roles of individual radicals generated by a submerged dielectric barrier discharge plasma reactor during Escherichia coli O157:H7 inactivation

“…Other researchers also show that the rate of OH radical formation varies from 10 −8 to 10 −10 M/s depending on plasma generation conditions. [44][45][46] It was observed that after 6-8 min of DBD operation, the intensity or concentration of 2-hydroxyterephthalic acid is reduced Fig. 4A, whereas the concentration of HTA is supposed to either increase or remain constant; it is to be noted that according to the reaction stoichiometry, the concentration of HTA is equivalent to the concentration of OH radicals.…”