| Plasma discharges in liquid has attracted much attention during recent years,due to their applications in areas such as medical treatment,environmental mediation and material synthesis.However,the mechanism of plasma discharge in liquid remains very controversial.For most cases it is generally believed that plasma discharges in liquid are produced and propagate inside bubbles,either preexisting in the liquid or generated with the application of high voltage through enthalpy change or cavitation.Due to the considerable complexity of liquid plasmas,previous studies paid particular emphasis on practical applications.Under such context,it is imperative to get a deeper insight into the fundamental streamer behavior and plasma development inside the bubbles.One of the main causes for currently limited fundamental insight in the physics of these plasmas is that only a few diagnostics are applicable.This is due to the complex discharge geometry and the surrounding liquid.In our experiments,discharges in a single bubble or a series of bubbles were studied using a pin-to-pin electrodes system.A useful diagnostic is provided in this paper,a high speed camera was used to capture the single breakdown event from plasma initiation to extinction to eliminate the stochastic behavior of the discharge.Time-resolved images of electrical discharge developments in different situations have been achieved.High speed camera is able to attain a frame rate of 90,000fps(frame per second).Different frame rates were used according to different situations.Influence of the electrodes'and the bubble's positions,the deionized water layer' thickness,the working gas species and the water solution's conductivity on the discharge have been investigated using different number of bubbles:(1)Influence of the electrodes' and the bubble's position on the discharge of single air bubble submerged in deionized water.Influence of electrodes' horizontal position:when both electrodes were in contact with the bubble,the discharge behaved similarly to the breakdown process in gas phase,where the streamer connected the high voltage electrode and the ground electrode through the shortest path,i.e.along the axis of the bubble,we call it the direct discharge mode;when one electrode was in contact with the bubble and the other one was not(it includes two different situations,one is H.V.electrode was in contact with the bubble and the ground electrode was not;the other one is ground electrode was in contact with the bubble and the H.V.electrode was not),the streamer would form along the inner surface of the bubble,we call it the dielectric barrier mode one;when both electrodes were not in contact with the bubble,streamer propagated along the axis of the bubble,we call it the dielectric barrier mode one;in the dielectric barrier mode one and two,the plasma acted like a power source after the discharge breakdown.Influence of electrodes' vertical position:when both the electrodes were perfectly aligned with the center of the bubble,no strong filamentation was observed inside the bubble,the discharge propagated along the upper and lower surface of the bubble wall and formed a complete circle around the inner surface;if the electrodes were shifted slightly above the centerline of the bubble,the streamer would only propagate along the upper surface of the bubble;similar effects were observed when the electrode was shifted below the centerline of the bubble.Here the bubble plasma was essentially a surface dielectric barrier discharge.(2)Influence of the deionized water layer' thickness on the discharge of single air bubble submerged in deionized water.Keep the H.V.electrode in contact with the air bubble and keep the ground electrode not in contact with the air bubble in the same time,simplify the deionized water layer' thickness into the distance d between the air bubble and the ground electrode.With the distance d increased from 50?m to 450?m,the time duration decreased from 24 ?s to less than 4 ?s,and the total energy injection into the bubble per breakdown decreased from 0.55 J to 0.15 J.The bubble-water-electrode system was capacitively coupled.When the ground electrode was retracted away from the bubble,it was essentially increasing the thickness of dielectric in the capacitor,resulting a decrease of the capacitance.As a result,as the value of d increased,the capacitance decreased,leading to a decrease in the injected energy.When the distance was larger than the threshold value(for our case,500 ?m),the injected energy was not sufficient to initiate the breakdown process,so no plasma could be observed.(3)Influence of the working gas species on the discharge of single bubble submerged in deionized water.Change the working gas from air to He,record the dynamic process of the discharges.Compare the results to the time-resolved images of the discharges using air as the working gas,we found that the two kind of dynamic processes were very similar.The voltages and currents of them were similar too.(4)Influence of the water solution's conductivity on the discharge of single bubble submerged in water solution.The results of the experiments showed:when the conductivity of the water solution was lower than 100?S/cm,we could observe discharge breakdown if the distance d between the ground electrode and the air bubble was suitable(lower than 350?m);when the conductivity of the water solution increased to 200?S/cm,we could observe discharge breakdown if the distance d between the ground electrode and the air bubble was lower than 50?m;when the conductivity of the water solution was up to 300?S/cm or higher,we could not observe any discharge breakdown even the two electrodes were both in contact with the air bubble.The current wave got closer to the voltage wave(a square wave)when the conductivity of the water increased,it means that the resistive component of the current occupied more place.(5)Influence of the bubbles' and the electrodes' positions on the discharge of multiple bubbles immersed in deionized water.Depending on the position of the bubbles and electrodes,the breakdown could be categorized into three modes:the axial mode,where the streamer propagates along the axis of the bubbles;the surface mode,where the streamer propagates along the inner curved surface of the bubbles;the hybrid mode,where the streamer propagates both along the axis and the inner surface simultaneously.For larger electrode gap,the discharge only initiates at the tip of the electrode,and doesn't propagate into the bubble. |
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