Thermodynamics of Microplasma Initiation in Liquids

Robert Geiger

Sagar Ghimire, Rei Kawashima

Advisor: Dr. David Staack

Texas A&M University- Mechanical Engineering

Plasma Engineering & Diagnostics Laboratory (PEDL)

Outline

• Motivation– Applications

– Control plasma parameters (ne, Te, d)

• Discharges in liquid– Near liquids– “In” liquids

• Experimental Setup• Transient discharges (OES)• Summary

plasma

wire

cells

Why Generate Plasma in Liquids?Chemical Applications:

Fuel Reforming

Bio/Medical

Physical Applications:

Shock Wave Generation MicrofluidicsSpecies Identification

Water Sterilization

Shock Wave

Discharges in Gases Near Liquids

Inside bubblesAbove liquid surface

Ref: Alyssa Wilson et al 2008 Plasma Sources Sci. Technol. 17 045001

In droplet containing gas

http://www.panoramio.com/photo/2843260

Discharges in Liquids – Steady State

Phase Instability

Steady state supercritical plasma

Ref:27.12 MHz Plasma Generation in Supercritical Carbon Dioxide

Ayato Kawashima et al, J. Appl. Phys.

Discharge in Liquids - Process

1) Initiation Low Density Region1) Electrolysis

2) Boiling (Joule Heating)

3) Electrostatic Cavitations

2) Breakdown1) Primary Streamer

2) Secondary Streamer

3) Spark

3) Thermalization

4) Relaxation

1950s-1980s thoroughly studied breakdown process in dielectrics

r

Discharges in Liquids - Initiation

Boiling Analysis (Energy Balance)

Electrolysis Analysis ( Faradays law of electrolysis)

Electrostatic Cavitation Analysis (Force Balance)

Assumptions: All initiation mechanism achieve a low density reduction n

Const (I) and (V)

Local Low Density Region (n)

Y = (Yeild of Fluid)

Electrode

Fluid

Cavitation

should be larger

Experimental Setup

V

Spark Gap 1R

CSpark Gap 2

Output

Circuit:

Electrode Configurations:

Point to Plane Point to Point Plane to Plane

Diagnostics

Discharges in Liquids – Transient & Plasma SizeSpark Streamer Corona

Water - Corona

Mineral Oil - Corona

Anode (+) Cathode (-)< 50 um

Discharges in Liquids – Transient & Thermalization

200 300 400 500 600 700 800-0.2

0

0.2

0.4

0.6

0.8

1

1.2Broadband spectra of Corona in Water\NaCl\KCl mixtures

Wavelength (nm)

Re

lativ

e In

tesi

ty

130uS310uS540uS780uS

Hα

Hβ ONaOH

Te,Tvib>Tgas Te,Tvib ≈ Tgas

𝛕 = f(ne, υen, Te, E/n, medium, …)

Discharges in Liquids - Transient

630 635 640 645 650 655 660 665 670 675 6800

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Hydrogen Alpha Peak for different conductivities

Wavelength (nm)

Re

lativ

e In

tesi

ty

130uS310uS540uS780uS

100 200 300 400 500 600 700 8001

2

3

4

5

6

7

8Hydrogen Alpha FWHM for different conductivities at the Anode

Conductivity (nm)

FW

HM

14W18W28W

100 200 300 400 500 600 700 8001

2

3

4

5

6

7

8Hydrogen Alpha FWHM for different conductivities at the cathode

Conductivity (nm)

FW

HM

14W18W28W

Discharges in Liquids - Transient

100 200 300 400 500 600 700 800

0.35

0.4

0.45

0.5

0.55

Conductivity (uS)

OH

/H)

Max Peak (OH/H) Cathode

14W18W28W

100 200 300 400 500 600 700 8000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Conductivity (uS)

OH

/H)

Max Peak (OH/H) Anode

14W18W28W

100 200 300 400 500 600 700 8000.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Conductivity (uS)

O/H

)

Max Peak (O/H) Cathode

14W18W28W

100 200 300 400 500 600 700 8000

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Conductivity (uS)

Na/

H)

Max Peak (Na/H) Cathode

14W18W28W

100 200 300 400 500 600 700 8000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Conductivity (uS)

Na/

H)

Max Peak (Na/H) Anode

14W18W28W

(OH/H)

(Na/H)

(O/H)

100 200 300 400 500 600 700 8000.05

0.1

0.15

0.2

0.25

0.3

Conductivity (uS)

O/H

)

Max Peak (O/H) Anode

14W18W28W

Anode Cathode

Summary• Control of plasma properties in liquids

• Characteristic times and initiation mechanisms

• Transient discharge breakdown development

• Experimental results– Discharge size

– Electron density

– Chemical Components

Future Work• Improve transient initiation model

• (dV/dt = const) instead of (V=const)

• ne = f(t) instead of (I = const)

• Dielectric Fluids

• Mechanical/Chemical Energy (Shockwaves vs. Radical Generation)

References

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.

Question?

Acknowledgements:

This material is based upon work suppoerted by the National Science Foundation Grant #1057175