22P010-O
High Speed Camera Diagnostics of Dielectric Barrier Discharges with Bubble
Generating Porous Graphite Electrode in Solution
 !" #$Sonia Muradia1, Jun Watanabe1, Yohei Mochizuki2, Akihisa Ogino1, Masaaki Nagatsu1,21
2 1 1,2
 1, 1Graduate School of Science and Technology, Shizuoka University,
3-5-1, Johoku, Nakaku, Hamamatsu 432-8561, Japan
432-8561
3-5-1
2Faculty of Engneering, Shizuoka University,
3-5-1, Johoku, Nakaku, Hamamatsu 432-8561, Japan432-8561
3-5-1

Dielectric barrier discharge(DBD) plasma is produced inside water using bubble discharge method.
Bubbles are produced inside water using a porous graphite electrode. A high voltage pulsed device is used
for power supply. The breakdown voltage is observed at 3kV with frequency of 5 kHz. The characteristicsof plasma are studied using optical emission spectra, ozone analysis and oscillographs. To understand therole of bubbles in producing discharge inside solution and in order to improve it, high speed camera is used
to study their behavior in our experiment. We also used methylene blue as organic pollutant to determine
the efficacy of plasma in the degradation of wastewater.

1. Introduction
So far, there have been many researches on theproduction of plasma discharges inside the liquid
to apply them for environmental fields orsynthesis of novel materials. To produce plasma
inside the liquid medium, it is a commontechnique to introduce the micro-bubble inside
the liquid between the electrodes driven by highvoltages. Recently, boiling point heating of thewater by high power RF and microwave is usedwhich consumes a lot of energy [1][2].
In the present study, we propose to use theporous graphite plate as the one of DBD
electrodes and as well as the bubble generator. It
has an advantage that the discharge in the gas
bubble can be initiated at significantly lower
input power, compared to the RF and microwave
plasmas.
2. Experimental setup
Figure 1 shows a schematic drawing of theexperimental set-up. The parallel-plate DBD
configuration consists of a punched metal plate andporous graphite plate which is mounted metal
holder. A very thin mica sheet is used as dielectricmaterial. The space between two electrodes is kept
variable but the most stable discharge is observed at
5mm of gap spacing. Figure 2 shows the DBD
electrode. From the graphite electrode holder, when
the gas is introduced through the tube, gas opens
into the solution directly forming a large number ofPorous Carbon
Electrode

Punched Metal
Electrode
Bubbles

Gas

WaterPower
SupplyFig. 1 Schematic drawing of the experimental setup.
Gas inlet

Side view

Punched metal
Dielectricsheet
sheet Porous graphite
electrodeFig. 2 DBD electrode setup

bubbles. On applying high pulsed voltage across theelectrodes, we observed discharges in gas bubbles.
Typical square waves voltages applied between
electrodes were 2.0 ~5.0 kV at a frequency of 1~5
kHz (Figure 3). Optical emission spectroscopy isused to study the atomic or molecular composition
generated in the plasma discharges in bubbles.

3. Results and discussion
Typical plasma with He and Ar gas inside water
are shown in Fig. 4 (a) and (b) respectively. The
photos are taken using camera from the top of theelectrode set-up. The typical voltage is 3kV in bothcases. Because of a high gas flow rate of ~ 200sccm,
the plasma looks moving inside the water andplasma does not look uniform from the top.
We determine the efficiency of our plasma forenvironmental applications by using it for thedegradation of 10mg/l methylene blue solution. In
order to check its applications for commercial
purposes, all experiments are with 2 liters ofsolution. Figure 5 shows the degradation of MBwith time. It takes 6 hours for the 96% reduction ofthe MB.

(a)occurs.
Different gases with various gas flow rate is usedin the experiment in order to understand the bubbles
discharge. The number and size of the bubbles areof prime concern in our experiment as discharge
inside the gas bubble produces a large amount ofoxidizing species which is expected to be the reason
behind wastewater treatment. So, It is obvious that
more the number of bubbles discharge, more is theefficiency of plasma for applications. The numberand size of the bubbles are partially dependent on
the pore size of graphite bed of the graphite
electrode but the physical nature of the bubbles are
remarkably affected by the neighbor bubbles as thebubbles are continuously moving inside the water athigh rate in the small space between two electrodes.
In order to improve the discharge occurring in thebubbles, we need to understand the critical size andexact location of the bubbles between the electrodes.
To understand behavior of the bubbles, we use highspeed camera and we will show our results at theconference.

(b)

Fig. 5 Degradation of MB using Ar discharge
Fig. 3 Typical waveforms of (a) voltage and (b) current.

(a)(b)
Fig. 4 Typical pictures of plasma discharges produced
between two electrodes in the water; (a) Gas: Heand (b) Ar.

Since, time is an important parameter; our motive
is to increase the efficiency of our plasma without
input of more energy. So, one of the ways is toincrease the number of bubbles in which discharge

4. Conclusion
In this study, we presented the preliminary
results on DBD plasma produced inside water using
bubble discharge method. The DBD plasma wassuccessfully produced inside the water. In order todetermine the efficacy of plasma in the degradationof wastewater, we demonstrated the decompositionof methylene blue as organic pollutant. To
understand the role of bubbles in producing
discharge inside solution, we carried out
measurement using a high speed camera. These
results will be presented and discussed at theconference.
References[1] T. Maehara et. al. J. Hazard Mater. 174 (2010) 4736.
[2] H. Aoki et.al. Plasma Sources Sci. Tech. 17 (2008)
5006.