Gas Flow Formation by Plasma Discharge on Water Surface

T. Shimizu1,2, M. Hara3,†, N. Kishimoto3,‡, D. Yoshino4, G. E. Morfill1,2, and T. Sato4

1Max-Planck Institute for Extraterrestrial Physics, Germany2Terraplasma GmbH, Germany

3Graduate School of Engineering, Tohoku University, Japan4Institute of Fluid Science, Tohoku University, Japan

Abstract—Atmospheric plasmas have large potential in many industrial applications including biomedicine because they canproduce relevant reactive species for reactions. When atmospheric plasmas are applied to biological samples, these samples are oftencovered by a liquid layer. It is important to understand the transport of reactive species since the plasma discharge itself also drivesa gas flow. In this study, a plasma discharge was produced between a wire electrode and the surface of water by high voltage.This plasma discharge consists of several streamer discharges. The development of a thermal field between the wire electrode andthe surface water as well as in the water was observed using the Schlieren visualization technique and a high speed camera. Inaddition, discharge photos with short exposure time were taken with an image intensifier. A thermal field was found to develop inthe gas phase and a circulating flow was formed in the water in the case of low conductivity of 0.8 µS/cm due to the formation ofgas flow. Our experiments shows that generation of higher gas velocity could occur with lower conductivity of water.

Keywords—Atmospheric plasma, plasma medicine, gas and water flow formation, water conductivity

I. INTRODUCTION

Atmospheric plasmas have large potential in industrial ap-plications. The plasma systems can be simple and cheap sincean expensive vacuum system is not required and productionin open air is possible. They are already applied in surfacetreatment such as etching, modification and cleaning [1].

An important aspect in the usage of the atmospheric plasmasis that the plasma discharge can drive a gas flow throughthe collisions between neutral gas molecules/atoms and ionsaccelerated by an existing electric field. Atmospheric plasmahas been applied for the active manipulation of gas flow tocontrol laminar-turbulent transition in flows at a boundary area[2]. Such gas flow formation is also very important when theplasma discharges are used as a source of chemically activeproducts (reactive species). The transport of the products isstrongly influenced by the driven gas flow [3].

Recently, cold atmospheric plasmas have been investigatedfor biomedical applications because they can produce relevantspecies for biological reactions such as those of reactiveoxygen species and reactive nitrogen species [4]–[8]. It hasalready been demonstrated that cold atmospheric plasmashave an inactivation effect on bacteria, including antibiotic-resistant bacteria, e.g., methicillin-resistant Staphylococcusaureus, fungi, spores, and viruses [9]–[11]. Moreover, theplasma discharge can provide a healing effect [12]. Thesecharacteristics have opened a new application window inbiomedicine.

In order to optimize these effects by the plasma discharge,it is very important to control the production and transport

Corresponding author: Takehiko Satoe-mail address: sato@ifs.tohoku.ac.jp†Currently, Mitsubishi Materials Co.,‡Currently, ShinMaywa Industries Ltd.

Presented at the 2014 International Symposium on Electrohydrodynamics(ISEHD 2014), in June 2014

of reactive species. Plasma-treated objects are often coveredby a liquid layer when the plasma treatment is applied toliving tissues/ microorganisms. For example, when chronicwounds are treated by cold atmospheric plasma [13], [14],these wounds are covered by a body liquid (e.g., blood).Therefore, the aim of this study was to observe gas flowformation driven by atmospheric plasma on the surface ofwater. The plasma discharge was produced between a wireelectrode over 1 cm in length and the water surface. The gasflow formation was observed using the Schlieren visualizationtechnique and a high speed camera. The temporal evolutionof generated gas flows and flow formation mechanisms wereexamined.

II. EXPERIMENTAL SETUP

In this study, in principle, atmospheric plasma of thestreamer discharge mode was produced, as shown in Fig.1. Water of 0.8 µS/cm or 10 mS/cm in conductivity waspoured into a quartz glass cell (10×10×8 mm3) until thelevel of the water surface was as high as the glass cell. Belowthe glass cell, there was a metal plate which was connectedto an electrical ground. A wire electrode made of coppercoated with polymide film was placed 1 mm above the watersurface. The diameter of the wire electrode was 0.2 mm andits length for the plasma discharge was 10 mm, i.e. the wireelectrode was longer than 10 mm, however only a part ofthe wire electrode of 10 mm long contributed to the plasmadischarge. The whole setup was in still atmospheric air andno gas flow was applied externally. A plasma discharge wasproduced between the tip of the wire electrode and the watersurface by applying a high voltage of 7.5 kV in amplitude withrespect to the electrical ground with a square waveform to thepin electrode using a high-voltage amplifier, model PD05034,TREC. The repletion frequency of the applied voltage was5 kHz, and the time duration of the high voltage was 100 µs.

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Fig. 1. Electrode configuration in this study. Between the wire electrode andthe water surface, a plasma discharge was produced as shown in the inset.

The total power consumption was estimated to be around1.5 W by the Lissajous figure mehod with 10 nF insertedbetween the metal electrode and the electrical ground [15].The applied voltage and the electrical current were monitoredby an oscilloscope (LeCroy, WaveSurfer 62Xi) and a currenttransformer (Pearson, model 6585).

In order to observe the density gradient in gas and water,the Schlieren flow visualization technique was applied with ahigh-speed camera (Photoron, SA5), collimators, a knife edgeand a light source. The high speed-camera was synchronizedwith the measurements of the applied voltage and the electricalcurrent. Details of when gas flow was driven are discussedbelow.

III. RESULTS AND DISCUSSION

Typical waveforms of the applied voltage and current areshown in Fig. 2. These waveforms were obtained using 0.8µS/cm of water. In this study, voltage with positive polaritywas applied. Around 0 s, the applied voltage started to increasefrom 0 kV. During the increase phase of the voltage, therewere displacement current of approximately 0.1 A and apeak current of about 1 A corresponding to the discharge.The displacement current was from the capacitance of ourexperimental system. After the applied voltage reached 7.5 kV,there was no current until the voltage started to decrease. Thespike current which appeared at the increase of the appliedvoltage contained several components of spike currents, i.e.,several discharges were ignited in a short time during thevoltage increase. When the voltage decreased to 0 kV, therewas a displacement current again and a very small spikecurrent was observed. This observation of a high spike currentat the increase of the applied voltage and a smaller spikecurrent at the decrease was confirmed in many cases. Typicallyin our experiments, the full-width half-maximum of the spikecurrents was about 20-30 ns.

A series of photos between the wire electrode and the watersurface using the Schlieren visualization technique is shown inFig. 3. Here the photos are shown every 20 µs. The exposuretime for each image was 10 µs. The signal in the photoswas very faint, so that the difference between each imageand the same image before the start of plasma discharge was

Fig. 2. Waveforms of the applied voltage and current using 0.8 µS/cm ofwater conductivity. Around 0 ms, the voltage started to increase and a spikecurrent corrsonding to the discharges was observed.

Fig. 3. Schlieren images taken between the wire electrode and the watersurface. The image were taken from the side of the wire electrode.

calculated. Note that by the Schlieren visualization, a densityvariation was detected. Since pressure in our experiments wasconstant at atmospheric pressure, the temperature field is givenin the photos. The high-speed camera was synchronized withthe measured waveforms in Fig. 2, i.e., the shown time from0 to 140 µs corresponds to that in Fig. 2.

At 20 µs, after a peak current was observed as shown inFig. 2, a hot region developed between the wire electrode andwater surface. Since the gas was heated, there was, at least, onedischarge before 20 µs. The color change in the vicinity of thewire electrode was higher than that near the water surface; thetemperature increase near the wire electrode was higher. Thewidth of the hot region increased until 60 µs due to thermaldiffusion. At 100-200 µs, the color in the hot region, especiallynear the wire electrode, became darker, suggesting that therewas another discharge at the decrease of the applied voltageat 100 µs.

In order to see how the discharges were produced betweenthe wire electrode and water surface, photos of the dischargewere taken using an image intensifier. The exposure time forthis photo was only during the increase of the applied voltage,i.e. the photo shows discharges produced only at the increaseof the applied voltage. From the side, there was almosthomogeneous light emission between the wire electrode andthe water surface. As seen in Fig. 4 (right), there were severaldischarges produced between the wire electrode and watersurface. The discharge was not homogeneous along the wireelectrode. However, when each discharge was observed, the

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Fig. 4. Discharge photo between the wire electrode and the water surfacein the case of 0.8 µS/cm. For the image, the shutter was opened only at theincrease phase of the applied voltage.

Fig. 5. Development of the thermal field in the water in the case of 0.8 µS/cmusing Schlieren visualization technique. These images were taken from theside of the wire electrode. The observed flow pattern is illustrated at 1.2 s.

emission from the discharge was almost homogenous betweenthe wire electrode and the water surface.

When biological samples are treated by atmospheric plasma,an important point is to understand how the reactive speciesare transported in water since the samples are often covered bya liquid layer. In order to understand the transport in water, weconducted Schlieren visualization in the water treated by theatmospheric plasma, as shown in Fig. 5. These images were

Fig. 6. Discharge photo between the wire electrode and the water surfacein the case of 10 mS/cm. For the image, the shutter was opened only at theincrease phase of the applied voltage.

Fig. 7. Development of the thermal field in the water in the case of 10 mS/cmusing Schlieren visualization technique. These images were taken from theside of the wire electrode.

taken from the side of the wire electrode. At 0 s, productionof the plasma discharge was started. After 0.2 s, a thermalfield developed just below the wire electrode. This flow fieldexpanded toward the wall of the glass cell and downward. At0.8 s, the thermal field reached the sidewall of the glass celland along the side wall there was movement of the thermalfield downward along the sidewall. Around 1.0 s, circulatingflow patterns were observed on both sides in the water. Thecirculating flows started right under the wire electrode. Theypropagate to the walls and downward. Around the middle in

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Fig. 8. Experiment for which the polarities change.

height, their directions changed to the center of the cell andupward at the end. The circulating flow pattern is illustratedat 1.2 s in Fig. 5. Even after the circulating flow patternsdeveloped, the movement of the thermal field at the centerof the wire electrode was present and from 1.2 s, a complexthermal flow was observed in the entire glass cell.

This flow formation and the development of the thermalfield in water can be explained as resulting from the gas flowformation by the atmospheric plasma. As shown in ref. [2], [3],gas flow is formed through collisions between neutral particlesand accelerated ions. Similar liquid flow patterns were alsoreported by several research groups [16], [17]. The gas flowwhich is formed can give momentum to the water surface, andwater flow as well as a thermal field is developed in water.

Photos of the discharge between the wire electrode and thewater surface, and development of the thermal field in the wa-ter are shown in Figs. 6 and 7, respectively, when 10 mS/cm ofwater conductivity was used. In this case, the emission widthfrom the discharge increased as shown in Fig. 8. For evaluationof the emission width, three individual measurements wereconducted. However, circulating flow formation in the waterwas not observed. The shade formed in the water expandedsemi-spherically from the water surface. This result impliesthat the heat transfer observed as light and shade was mainlycaused by heat conduction from the discharge.

Since the development of the flow field using 0.8 µS/cmwater conductivity was different from that using 10 mS/cm,the flow formation in water was strongly dependent on theelectrical property of water. Generation of higher gas velocitycould occur in water of lower conductivity.

In the water, there were several reactive species dissolved,which are relevant to biological reactions. They were mainlyproduced in the gas by the plasma discharge and dissolved inthe water. In both cases, ∼30 mg/l of ozone and 15 mg/l ofH2O2 were detected after 5 min of irradiation by the plasmadischarge. However, the dissolved HNO2 was ∼7 mg/l for0.8 µS/cm and ∼12 mg/l for 10 mS/l, and the dissolved HNO3was ∼90 mg/l for 0.8 µS/cm and ∼140 mg/l for 10 mS/cm.

IV. SUMMARY

In this study, development of a thermal field in gas andformation of flow in water by atmospheric plasma were

observed. Plasma discharge was produced between the wireelectrode and the water surface by applying a high voltage of7.5 kV. The thermal field as well as the gas flow formationwas monitored by the Schlieren visualization technique usinga high speed camera.

The thermal field was found to develop between the wireelectrode and the water surface and the atmospheric plasmadischarge was confirmed to be responsible for the thermalfield. Photos of the plasma discharges show that there wereseveral streamer discharges between the wire electrode andwater surface over 1 cm of the wire electrode using both waterconductivities of 0.8 µS/cm and 10 mS/cm. The dischargeswere not distributed homogeneously. In the water, water flowwas formed in the case of low conductivity of 0.8 µS/cm dueto gas flow formation. On the other hand, no flow formationwas observed using 10 mS/cm of water conductivity. Only theheat transfer was observed from the discharge point on thewater surface toward the bottom of the glass cell. This impliesthat generation of higher gas velocity could occur with lowerconductivity of water.

In the water, several reactive species were found to dissolvefor water conductivities of both 0.8 µS/cm and 10 mS/cm.They are relevant to biological reactions. The quality of aprocess is determined by the production of reactive speciesas well as transport. To optimize a process using atmosphericplasma, it is necessary to control the gas flow driven by theplasma discharge. Especially when the area treated by theplasma discharge is large, it is important to consider formationof a complex flow and the development of a thermal field inwater.

ACKNOWLEDGMENT

This study was partly supported by a Grant-in-Aid forScientific Research from JSPS and the Collaborative ResearchProject of the Institute of Fluid Science, Tohoku University.The authors would like to thank Mr. T. Nakajima (TohokuUniversity) for technical support.

REFERENCES

[1] A. Schutze, J. Jeong, S. Babayan, J. Park, G. Selwyn, and R. Hicks,“The atmospheric-pressure plasma jet: A review and comparison toother plasma sources,” IEEE Transactions on Plasma Science, vol. 26,pp. 1685–1694, 1998.

[2] E. Moreau, “Airflow control by non-thermal plasma actuators,” Journalof Physics D: Applied Physics, vol. 40, pp. 605–636, 2007.

[3] T. Shimizu, Y. Iwafuchi, G. Morfill, and T. Sato, “Formation of thermalflow fields and chemical transport in air and water by atmosphericplasma,” New Journal of Physics, vol. 13, p. 053025, 2011.

[4] M. Kong, G. Kroesen, G. Morfill, T. Nosenko, T. Shimizu, J. Van Dijk,and J. Zimmermann, “Plasma medicine: An introductory review,” NewJournal of Physics, vol. 11, p. 115012, 2009.

[5] G. Fridman, G. Friedman, A. Gutsol, A. Shekhter, V. Vasilets,and A. Fridman, “Applied plasma medicine,” Plasma Processes andPolymers, vol. 5, pp. 503–533, 2008.

[6] M. Laroussi, “Nonthermal decontamination of biological media byatmospheric-pressure plasmas: review, analysis, and prospects,” IEEETransactions on Plasma Science, vol. 30, pp. 1409–1415, 2002.

[7] K. Weltmann, E. Kinde, T. Von Woedtke, M. Hahnel, M. Stieber, andR. Brandenburg, “Atmospheric-pressure plasma sources: Prospectivetools for plasma medicine,” Pure and Applied Chemistry, vol. 82, pp.1223–1237, 2010.

[8] E. Stoffels, Y. Sakiyama, and D. Graves, “Cold atmospheric plasma:Charged species and their interactions with cells and tissues,” IEEETransactions on Plasma Science, vol. 36, pp. 1441–1457, 2008.

86 International Journal of Plasma Environmental Science & Technology, Vol.10, No.1, MARCH 2016

[9] T. Sato, T. Miyahara, A. Doi, S. Ochiai, T. Urayama, and T. Nakatani,“Sterilization mechanism for Escherichia coli by plasma flow atatmospheric pressure,” Applied Physics Letters, vol. 89, p. 073902,2006.

[10] T. Klampfl, G. Isbary, T. Shimizu, Y.-F. Li, J. Zimmermann, W. Stolz,J. Schlegel, G. Morfill, and H.-U. Schmidt, “Cold atmosphericair plasma sterilization against spores and other microorganisms ofclinical interest,” Applied and Environmental Microbiology, vol. 78, pp.5077–5082, 2012.

[11] J. Zimmermann, K. Dumler, T. Shimizu, G. Morfill, A. Wolf,V. Boxhammer, J. Schlegel, B. Gansbacher, and M. Anton, “Effectsof cold atmospheric plasmas on adenoviruses in solution,” Journal ofPhysics D: Applied Physics, vol. 44, p. 505201, 2011.

[12] J. Heinlin, J. Zimmermann, F. Zeman, W. Bunk, G. Isbary,M. Landthaler, T. Maisch, R. Monetti, G. Morfill, T. Shimizu,J. Steinbauer, W. Stolz, and S. Karrer, “Randomized placebo-controlledhuman pilot study of cold atmospheric argon plasma on skin graftdonor sites,” Wound Repair and Regeneration, vol. 21, pp. 800–807,2013.

[13] T. Shimizu, B. Steffes, R. Pompl, F. Jamitzky, W. Bunk, K. Ramrath,M. Georgi, W. Stolz, H.-U. Schmidt, T. Urayama, S. Fujii,and G. Morfill, “Characterization of microwave plasma torch fordecontamination,” Plasma Processes and Polymers, vol. 5, pp.577–582, 2008.

[14] G. Isbary, G. Morfill, H. Schmidt, M. Georgi, K. Ramrath, J. Heinlin,S. Karrer, M. Landthaler, T. Shimizu, B. Steffes, W. Bunk, R. Monetti,J. Zimmermann, R. Pompl, and W. Stolz, “A first prospectiverandomized controlled trial to decrease bacterial load using coldatmospheric argon plasma on chronic wounds in patients,” BritishJournal of Dermatology, vol. 163, pp. 78–82, 2010.

[15] U. Kogelschatz, “Advanced ozone generation,” in Process Technologiesfor Water Treatment, S. Stucki, Ed. Boston, MA: Springer US, 1988,pp. 87–118.

[16] R. Ohyama, K. Inoue, and J. Chang, “Schlieren optical visualizationfor transient EHD induced flow in a stratified dielectric liquid undergas-phase ac corona discharges,” Journal of Physics D: AppliedPhysics, vol. 40, pp. 573–578, 2007.

[17] S. Kanazawa, H. Kawano, S. Watanabe, T. Furuki, S. Akamine,R. Ichiki, T. Ohkubo, M. Kocik, and J. Mizeraczyk, “Observation ofOH radicals produced by pulsed discharges on the surface of a liquid,”Plasma Sources Science and Technology, vol. 20, 2011.

Shimizu et al. 87