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

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

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

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

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

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

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

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 ~ 0.05 - 0.5 pF E ~ 0.5 – 200 μJ 2R

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.

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

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

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

Microsparks – Emission Spectra Hα

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.

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 ~ 10-100 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

Chemistry – Gas Chromatography of Hexadecane

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

Charged Deforming Particles

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 #1057175 PPPL for the use of High Speed Video Camera References: Alyssa Wilson et al 2008 Plasma Sources Sci. Technol. 17 045001 Ayato Kawashima et al, J. Appl. Phys. D. Staack, A. Fridman, A. Gutsol et al., Angewandte Chemie-International Edition, vol. 47, no. 42, pp. 8020-8024, 2008.

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