1. Induced-Charge Electrokinetic Phenomena
Paris-Sciences Chair Lecture Series, ESPCI
Induced-Charge Electrokinetic Phenomena
Martin Z. Bazant
Department of Mathematics, MIT
ESPCI-PCT & CNRS Gulliver
Introduction (7/1)
Induced-charge electrophoresis in colloids (10/1)
AC electro-osmosis in microfluidics (17/1)
Theory of electrokinetics at large voltages (14/2)

2. Research Interests Electrokinetics Microfluidics
= Electrically driven motion of particles and fluids
Microfluidics
Electrochemical systems
Granular flow
Applied mathematics

3. Induced-charge electrokinetics
Acknowledgments
Induced-charge electrokinetics
CURRENT
Students: Sabri Kilic, Damian Burch,
JP Urbanski (Thorsen)
Postdoc: Chien-Chih Huang
Faculty: Todd Thorsen (Mech Eng)
Collaborators: Armand Ajdari (St. Gobain)
Brian Storey (Olin College)
Orlin Velev (NC State), Henrik Bruus (DTU)
Antonio Ramos (Sevilla)
FORMER
PhD: Jeremy Levitan, Kevin Chu (2005),
Postodoc: Yuxing Ben ( )
Interns: Kapil Subramanian, Andrew Jones,
Brian Wheeler, Matt Fishburn
Collaborators: Todd Squires (UCSB),
Vincent Studer (ESPCI), Martin Schmidt (MIT)
Shankar Devasenathipathy (Stanford)
Funding:
Army Research Office
National Science Foundation
MIT-France Program
MIT-Spain Program

4. Outline Linear electrokinetics
Nonlinear “induced-charge” electrokinetics
Preview of upcoming lectures

5. Electrokinetic particle motion
Electrolyte (salt solution)
e.g. clay particles in water, Reuss 1808
Non-conducting viscous liquid,
e.g. oil drops in air, Millikan 1900 ?

6. The Electric Double Layer+
quasi-neutral
bulk liquid
electrolyte
solid
Electrostatic potential
Ion concentrations
Gouy 1910
continuum region

7. Electrokinetics in electrolytes- - - - - - - - - - - - - - - - -
Redo (a) tro show bulk flow
(a) Electro-osmosis = fluid slip across the double layer,
as an electric field pushes on the screening cloud
(b) Electrophoresis = particle motion due to electro-osmosis

8. Linear Electrokinetics: 1. Fluids
Response
Helmholtz: electro-osmotic flow
Onsager: streaming potential & current (inverse effects)
Ajdari: transverse couplings (anisotropic surfaces)
uniform zeta, thin double layers: potential flow (no vortices)
No net response to AC forcing

9. Application: DC EO pumps
Small channels (porous media) lead to large pressure (>10atm)
Disadvantages:
High voltage (kV)
Faradaic reactions
Gas management
Hard to miniaturize
Porous
Glass
Yau et al, JCIS (2003)

10. Linear Electrokinetics: 2. Particles
Smoluchowski: electrophoresis
Onsager: sedimentation potential, induced dipole
Dukhin, Deryaguin: surface conduction (large charge)
Anderson, Ajdari: transverse motion, rotation
uniform zeta, thin double layers: cannot separate particles!

11. Applications in microfluidics
Apply E across the chip
Advantages:
EO plug flow has low
hydrodynamic dispersion
Many uses of linear EP in separation/detection
Limitations:
High voltage (kV)
Often slow separations
No local flow control
“Table-top technology”
From Todd Thorsen

12. Outline Linear electrokinetics
Nonlinear “induced-charge” electrokinetics
Preview of upcoming lectures

13. Nonlinear Electrokinetic Phenomena
Some early examples
Dielectric liquids
dielectrophoresis (DEP): acts on induced dipole
Taylor (1966): deformation & flow in oil drops
Melcher (1960s): Traveling-wave AC pumping
Electrolytes
Shilov (1976): double-layer effects in DEP
Murtsovkin (1986): flows around polarizable particles
Dukhin (1986): 2nd kind EP at large current
Saville (1997): AC colloidal self-assembly on electrodes
Ramos (1999): AC electro-osmosis
Ajdari (2000): ACEO pumping with electrode arrays

14. “Induced-Charge Electro-osmosis”
= nonlinear electro-osmotic slip at a polarizable surface
Example: An uncharged metal cylinder in a suddenly applied DC field
Refs at end
ICEO flow persists in an AC field (< charging frequency).
Gamayunov, Murtsovkin, Dukhin, Colloid J. USSR (1986) - flow around a metal sphere
Bazant & Squires, Phys, Rev. Lett. (2004) - general theory, broken symmetries, microfluidics

15. Double-layer polarization and ICEO flow
A conducting cylinder in a suddenly applied uniform E field.
Electric field ICEO velocity
Movies from a finite-element simulation by Yuxing Ben (2005)
Solving the Poisson-Nernst-Planck/Navier-Stokes eqns
l/a=0.005

16. Experimental Observation of ICEO
Jeremy Levitan, PhD Thesis in Mechanical Engineering, MIT (2005)
100 mm Pt wire
on channel wall
Viewing plane
PDMS
polymer
microchannel
Bottom view
of optical slice
Inverted optics
microscope
Micro-particle image
velocimetry (mPIV) to
map the velocity profile

17. Simulated flow (side view)
ICEO Experiments
J. A. Levitan, S. Devasenathipathy, V. Studer,
Y. Ben, T. Thorsen, T. M. Squires & M. Z. Bazant,
Colloids and Surfaces (2005)
Collapse of experimental data
Movie: 5 m optical slice sweeping 100 m Pt cylinder (top view)
100 V/cm, 300 Hz, 0.1 mM KCl

18. Examples of ICEO in Microfluidics
Flow around a metal post
Fixed-potential ICEO
DC jet at a dielectric corner
AC electro-osmosis
Thamida & Chang (2002)
Ramos et al (1999), Ajdari (2000)

19. A pioneer, ahead of his time
Vladimir A. Murtsokvin (work from 1983 to 1996)
“ICEO” flow around an metal particle (courtesy of Andrei Dukhin)

20. Induced-Charge Electrophoresis = ICEO swimming by broken symmetry
Bazant & Squires, Phys. Rev. Lett. (2004); Yariv, Phys. Fluids (2005)
Squires & Bazant, J. Fluid Mech (2006)
Example: Janus particle
A metal/dielectric sphere in a uniform E field always moves
toward its dielectric face, which
rotates to perpendicular to E.
The particle swims sideways.
Stable
Unstable

21. Experimental observation of induced-charge electrophoresis
S. Gangwal, O. Cayre, MZB, O.Velev,
Phys Rev. Lett. 100, (2008).

22. Outline Linear electrokinetics
Nonlinear “induced-charge” electrokinetics
Preview of upcoming lectures

23. Lecture 2: ICEP in Colloids
Thursday 10 Jan. 2pm
Theory
Field-dependent EO mobility
Shape-dependent ICEP motion
Wall interactions
ICEP & DEP in non-uniform fields
Applications in separation, assembly

24. Some examples... E u
The ICEO pinwheeel
Non-uniform fields

25. Lecture 3: ACEO in Microfluidics
Thursday 17 Jan. 2pm
Theory
ICEO mixers
ACEO flow over electrode arrays
Fast (> mm/s), low-voltage (< 3V), high-pressure (10% atm) ACEO pumps
Applications to portable/implantable labs-on-a-chip, drug delivery

26. Some examples... The “Fluid Conveyor Belt”
Scientific American, Oct
The “Fluid Conveyor Belt”

27. Lecture 4: Theory at large voltages
Postponed…
Experimental puzzles
Strongly nonlinear dynamics
Breakdown of dilute-solution theory
Modified theories for ion crowding
New phenomena and open questions

28. Experimental puzzles in ACEO
V. Studer et al., Analyst (2004)
MZB et al., MicroTAS (2007)
High-frequency flow reversal
Concentration dependence
Ion specificity
Classical theory breaks down…

29. Ion crowding at large voltages
Crucial new physics:
Ion crowding at large voltages
MZB, MS Kilic, B Storey, A Ajdari (2007)
Poisson-Boltzman/
Dilute-solution theory
ICEO experiments,
New theory needed

30. Conclusion Induced-charge electrokinetics
provides many opportunities for
new science and new applications
Papers, slides…