Discharge mechanism of Aqua-plasma inside an oscillating bubble in electrolyte

Summary form only given. Recently, plasmas in liquids are spotlighted in biomedical and environmental fields. The “Aqua-plasma”, which is discharged inside a bubble on the electrode immersed in electrolyte, is understood that the complete coverage of bubble on the electrode surface is important condition for generation of plasma. However, the detailed mechanism of aqua-plasma generation is still unclear. In this work, the discharge mechanism of aqua-plasma inside the oscillating bubble is studied experimentally and numerically. The aqua-plasma is discharged in saline with the electrical conductivity of 1.6 S/m by applied the 380 kHz AC driven voltage on immersed electrode. The range of applied voltage is 234-280 V_rms. In the experimental results, it is found that the aquaplasma is not continuously but periodically discharged with the period of 1.5-2 msec due to oscillation of bubble. The period of photo-multiplier tube measured optical emission signal from aqua-plasma is consisted with the shrunken period of bubble from High Speed Camera (HSC) observation. The aqua-plasma is discharged only when the bubble shrunken period during oscillation. From V-I signal, different discharge current and breakdown voltage are observed with the polarity of electrode voltage. In positive voltage, discharge current and breakdown voltage are 0.7 A and 620 V, respectively. On the contrary, 1.5 A of discharge current and -580 V of breakdown voltage is observed in negative voltage. It seems that the positive sodium ion is accumulated on bubble surface because of higher mobility than negative chloride ion and bubble surface is charged in positive. In order to understand the effect of surface charge on bubble oscillation, a 1-D thermo-hydrodynamic model is established considering surface charge and temperature effect. The model estimated bubble size is well agreed to HSC results in range of 170-230 μm. Based on this work, the discharge dynamics of aqua-p- asma can be explained by three steps; 1) aqua-plasma heating on bubble surface increase evaporation rate than condensation rate and bubble expands until electric field lower than threshold value for breakdown. 2) Absence of aqua-plasma causing higher condensation than evaporation and bubble shrinks. 3) When the electric field is recovered to breakdown value, aqua-plasma re-discharged.