1. GENERATION OF PIN-HOLE DISCHARGES IN LIQUIDS František Krčma, Zdenka Kozáková, Michal Vašíček, Lucie Hlavatá, Lenka Hlochová Faculty of Chemistry, Brno University of Technology Czech Republic Patrick Vanraes Department of Applied Physics, Ghent University Belgium

2. E Electrical discharges in water  Highly polar liquid with large relative permittivity (ε r =81) and high dielectric strength E > 1 MV/cm compared to ~ 30 kV/cm of air  Relatively dense environment with high concentration of ions (H +, OH - etc.) that determine electrolytic conductivity of water (i.e. resistance)  Low mobility of ions compared to electrons => ions alter propagation of discharge channel in water by compensation of space charge electrical field on the streamer head  Solution conductivity strongly affects electrical breakdown of water => high requirements on power supply and reactor design  To ensure electrical breakdown of water under moderate (reasonable) conditions is necessary to use non-uniform electrode configurations point-point, point-plane, wire-cylinder, pin-hole Physical properties of water

3. Experimental devices and parameters Experimental parameters: High voltage: DC 1-3 kV Discharge current: 90-250 mA Input power: 90-300 W Electrodes: planar, stainless steel, Pt Dielectric diaphragm: PET, 0.25 mm, shapal ceramics 0.3 - 2 mm Pin-hole: centred, initial diameter 0.2 - 1.0 mm supporting electrolyte: NaCl, NaBr, NaNO 3, Na 2 HPO 4 ∙12H 2 O, Na 2 SO 4, etc. Optimal solution conductivity: 150-1300 μS∙cm -1 Devices: Batch discharge reactor HV source: DC, constant voltage

4. Pin-hole configurations d l d is typically 0.1 – 2 mm l ≈ d diaphragm discharge l » d capillary discharge

5. Pin-hole configurations d l d is typically 0.1 – 2 mm l ≈ d diaphragm discharge l » d capillary discharge symmetricasymmetric

6. Diagnostics electrical measurements of voltage and current sound generation light generation – PMT, high speed camera, iCCD optical emission spectroscopy – non-time resolved

7. Principle of pin-hole discharge formation Theories of electrical discharge creation in liquids:  thermal (bubble) theory – bubble formation due to Joule heating  electron theory – analogy to Townsend´s theory in gases

8. Principle of pin-hole discharge formation Theories of electrical discharge creation in liquids:  thermal (bubble) theory – bubble formation due to Joule heating  electron theory – analogy to Townsend´s theory in gases + ̶ positive streamersnegative streamers

9. Principle of diaphragm discharge formation in liquids Cathode space – positive plasma streamers P = 75 WP = 90 WP = 120 WP = 160 W Anode space – negative plasma streamers P = 200 W positive streamersnegative streamers + ̶

10. Mean current-voltage characteristics V-A characteristic for gas (red line) and NaCl solution (blue crosses)

11. Mean current-voltage characteristics V-A characteristic for gas (red line) and NaCl solution (blue crosses) U RdRd U R s R d R s R d « R s liquid gas

12. Time resolved current-voltage characteristics - electrolysis sound [a.u.] light [a.u.] current [mA] voltage [V]

13. Time resolved current-voltage characteristics - bubbles sound [a.u.] light [a.u.] current [mA] voltage [V]

14. Time resolved current-voltage characteristics - bubbles sound [a.u.] light [a.u.] current [mA] voltage [V]

15. Time resolved current-voltage characteristics - breakdown sound [a.u.] light [a.u.] current [mA] voltage [V]

16. Time resolved current-voltage characteristics - breakdown sound [a.u.] light [a.u.] current [mA] voltage [V]

17. Time resolved current-voltage characteristics - breakdown sound [a.u.] light [a.u.] current [mA] voltage [V]

18. Time resolved current-voltage characteristics - breakdown sound [a.u.] light [a.u.] current [mA] voltage [V]

19. Time resolved current-voltage characteristics - discharge sound [a.u.] light [a.u.] current [mA] voltage [V]

20. Time resolved current-voltage characteristics - discharge sound [a.u.] light [a.u.] current [mA] voltage [V]

21. Mean current-voltage characteristics electrolyzis bubbles generation discharge electrolyzis breakdown discharge bubbles

22. Mean current-voltage characteristics

23. Effect of solution kind – breakdown voltage

24. Diaphragm/capillary – bubbling voltage d = 0.3 mm

25. Diaphragm/capillary – breakdown voltage d = 0.3 mm

26. Diaphragm/capillary – breakdown voltage l = 0.25 mm

27. Diaphragm/capillary – breakdown current d = 0.3 mm

28. Discharge running in bubbles plasma streamersbubbles

29. Discharge running in bubbles discharge inside bubbles but plasma streamers propagate into the liquid diaphragm 0.3/0.3 mm bubble at pin, diameter 2 mm

30. Conclusions Conclusions Pin-hole discharge is generated in bubbles (microbubbles) if non fast pulsing voltage is applied Voltage of bubbles generation as well as the breakdown voltage increase with the increase l/d parameter but there are some limits The breakdown current is more or less independent on the pin- hole length except very thin barriers Breakdown voltage decreases with the solution conductivity increase but it is independent of the water solution kind Bubbles can generate significant sound even without discharge Streamers (streamer like channels) propagates from the bubble into the liquid even at low voltage

31. Thank you for your attention!!! This work was supported by Czech Ministry of Culture project No. DF11P01OVV004