1. Discharges Between Charged Particles in Oil
Robert Geiger
Advisor: Dr. David Staack
Texas A&M University- Mechanical Engineering
Plasma Engineering & Diagnostics Laboratory (PEDL)

2. Outline Motivation Liquid Discharges Particle Dynamics
Single Charge Carrier
Multiple Charge Carriers
Modes of Operation
Bubble
Microspark
Long Chain Spark
Chemistry
Summary

3. Why Generate Plasma in Liquids?
Chemical Applications:
plasma
wire
cells
Bio/Medical
Water Sterilization
Fuel Reforming
Physical Applications:
Species Identification
Microfluidics
Shock Wave Generation
Shock Wave

4. Discharge in Liquids - Process
Initiation  Low Density Region
Electrolysis
Boiling (Joule Heating)
Electrostatic Cavitations
Breakdown
Primary Streamer
Secondary Streamer
Spark
Thermalization
Relaxation
1950s-1980s thoroughly studied breakdown process in dielectrics

5. Bubble Formation in Liquids
Making a Bubble is easy: Especially when you put 1 mJ in 8 μm3

6. Discharges in Liquids – Thermalization
Streamer Corona
Spark
Anode (+)
Cathode (-)
< 50 um
Streamer growth occurs ~30 ns which is the same duration as the current pulses observed in the microsparks. Energy input during this time is ~10^-3 eV/mol which is equivalent to about 10K of heating (nonthermal). Are microsparks nonthermal? Spectra doesn’t show nonthermal behavior… why? What is the size of discharge? On the order of microns… how much does the particle move during the discharge? Current pulse width/Velocity ~ 30ns/Velocity ~ 0.5 um (half the distance of the discharge length).
Is it possible to limit current enough while still having enough voltage for breakdown and generate only nonthermal plasma? Particle voltage depends on applied voltage and beta as well as tau_relax.
Water - Corona
Mineral Oil - Corona

7. Low Energy Input – Double Spark GapV
Spark Gap 1 R C
Spark Gap 2
Output
Charcteristics
Energy Per Pulse ~ C
Stray Capacitance ~ 5 pF
Lowest Energy ~ 1 mJ

8. Low Energy Input – Charge Carrier Method
ball
Discharge
Electrode HV
GND
(V ~ 5 – 30 kV)
Spherical Capacitor
C = 4πε0R
R ~ 0.5 – 5 mm
C ~ pF
E ~ 0.5 – 200 μJ 2R

9. Particle Dynamics – Contact Charging
Field Enhancement Factor
Lift off Voltage
mg = qE
g = qαV/d
q = α CV
α = (mgd/CV2)1/2
α ~ 1/β
Experimental
α ≈ 0.3
Charge relaxation
τ = ε/σ
For Mineral Oil ~ 0.5 s
Liu, T. M.-C. (2010). The Design of a Micro/Nano-Particle Electrostatic Propulsion System.

10. Particle Dynamics - Motion
Fvis
Fel HV
GND
1) Position
2) Velocity
3) Stokes Approximation
4) Charge Relaxation
5) Electrical Breakdown Condition ,
Mineral Oil Dielectric Strength:
200 kV/cm
Ref. 1: Jones, Thomas B. Electromechanics of Particles. Cambridge University Press 1995
Ref. 2:Melcher, James R. Continuum Electromechanics. Cambridge, MA: MIT Press, 1981

11. Experimental Setup – Single Charge Carrier
Power Supply
Resistor
Lens
ICCD
ICCD

12. Expermental Light intensity decreases linearly with energy Charge ~ pC
α ~ 0.3 (Same as before)

13. Microsparks – Emission SpectraHα

14. Multiple Charge Carrier - Batch Reactor
Voltage on electrodes transferred to charge carriers during contact.
Charge carriers move back and forth transferring charge.
Discharge power limited (~pC per discharge event).
Size, material, voltage control discharge parameters and chemistry.

15. Discharge Modes: Multiple charge carriers
1) Gas Bubble Chain Formation
High temperature gas phase
DC Mode Glow Discharge
Duration ~ 5 s
Ballasted
Energy Determined by discharge current
2) Spark Chain Formation
High temperature liquid phase
Transient Spark
Energy determine by Capacitance
Duration ~ ns
3) Microplasma Mode
Low temperature liquid phase
Power Density ~100 W/l
0 s
4 s
6 s
12 s
14 s
20 s

16. Chemistry – Gas Chromatography of Hexadecane

17. Summary Future Work Control of plasma properties in liquids
Create non-thermal discharges in liquid
Interesting way to initiate nanosecond microsparks
Great control over discharge energy
Spark Gaps (mJ  J)
Charge Carriers (uJ  mJ )
Scaling is possible
Cracking to lighter hydrocarbons
Future Work
Self-organization of particles
Effect of energy per pulse on chemistry
Deforming charged particles

18. Charged Deforming Particles

19. PPPL for the use of High Speed Video Camera
References
Question?
Acknowledgements:
This material is based upon work suppoerted by the National Science Foundation Grant #
PPPL for the use of High Speed Video Camera
References:
Alyssa Wilson et al 2008 Plasma Sources Sci. Technol
Ayato Kawashima et al, J. Appl. Phys.
D. Staack, A. Fridman, A. Gutsol et al., Angewandte Chemie-International Edition, vol. 47, no. 42, pp , 2008.

20. Discharges in Liquids - Initiation
Assumptions: All initiation mechanism achieve a low density reduction  n
Const (I) and (V) r
Local Low Density Region (n)
Electrolysis Analysis ( Faradays law of electrolysis)
Boiling Analysis (Energy Balance)
Y = (Yeild of Fluid)
Electrostatic Cavitation Analysis (Force Balance)
Cavitation
Electrode
Should be larger than
Fluid