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Induced-Charge Electro-osmosis and Electrophoresis. Nonlinear Electrokinetics @ MIT Students: Jeremy Levitan (ME PhD’05), Kevin Chu (Math PhD’05), JP Urbanski (ME), Mustafa Sabri Kilic , Sergiy Sidenko (Math) Postdocs : Yuxing Ben , Hongwei Sun (Math)

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induced charge electro osmosis and electrophoresis

Induced-Charge Electro-osmosis and Electrophoresis

Nonlinear Electrokinetics @ MIT

Students:Jeremy Levitan (ME PhD’05),

Kevin Chu (Math PhD’05), JP Urbanski (ME),

Mustafa Sabri Kilic, Sergiy Sidenko (Math)

Postdocs: Yuxing Ben, Hongwei Sun (Math)

Faculty: Todd Thorsen (ME), Martin Schmidt (EE)

Visitors: Armand Ajdari, Vincent Studer (ESPCI)

Collaborators: Todd Squires (UCSB),

Shankar Devasenathipathy (Stanford)

Howard Stone (Harvard)

Martin Z. Bazant

Department of Mathematics & Institute for Soldier Nanotechnologies, MIT

Funding: US Army Research Office

(Contract DAAD-19-02-002) and

MIT-France Program

ICEO in a microfluidic device.

the electrochemical double layer
The Electrochemical Double Layer








Electrostatic potential

Ion concentrations


continuum region

electrokinetic phenomena
Electrokinetic Phenomena

Helmholtz-Smoluchowski fluid “slip” formula:



The classical theory assumes that the “zeta potential” z (or charge density q) is a constant material property, but what happens at a polarizable (e.g. electrode) surface?

diffuse charge dynamics
Diffuse-Charge Dynamics

Bazant, Thornton, Ajdari, Phys. Rev. E. (2004).

Analysis of the Poisson-Nernst-Planck equations

by time-dependent matched asymptotic expansions.

Model Problem

Classical “equivalent circuit” in

the thin-double-layer approximation

Time scales

ac electro osmosis
AC Electro-osmosis

Ramos et al., JCIS (1999); Ajdari, Phys. Rev. E (2000)

Steady flow for

AC period =

How general is this phenomenon?

Need electrode arrays? Need “AC”?

induced charge electro osmosis
“Induced-Charge Electro-osmosis”

= nonlinear electro-osmotic slip at a polarizable surface

Bazant & Squires, Phys, Rev. Lett. 92, 0066101 (2004).

Example: An uncharged metal cylinder in a suddenly applied DC field

Same effect for metals & dielectrics, DC & AC fields…

double layer polarization and iceo flow
Double-layer polarization and ICEO flow

A conducting cylinder in a suddenly applied uniform E field.

Electric field ICEO velocity

FEMLAB simulation by Yuxing Ben

Poisson-Nernst-Planck/Navier-Stokes eqns


experimental observation of iceo
Experimental Observation of ICEO

J. A. Levitan, S. Devasenathipathy, V. Studer, Y. Ben, T. Thorsen, T. M. Squires, & M. Z. Bazant,

Colloids and Surfaces (2005)

100 mm Pt wire

on channel wall

Viewing plane




Bottom view

of optical slice

Inverted optics


Micro-particle image

velocimetry (mPIV) to

map the velocity profile

induced charge electrokinetic phenomena
“Induced-Charge Electrokinetic Phenomena”

1. Prior examples of “ICEO”

  • Electro-osmotic flows around metal particles
  • Dielectrophoresis of spheres in electrolytes (“dipolophoresis”)
  • AC electro-osmosis & colloidal aggregation at electrodes
  • DC “electrokinetic jet” at a microchannel corner

Gamayunov, Murtsovkin, Dukhin, Colloid J. USSR (1986); Levich (1960)

Simonova, Shilov, Colloid J. USSR (1981, 1998)

Ramos et al. (1998); Ajdari (2000); “EHD” Ristenpart, Saville (2004)…

Thamida & Chang (2002)

2. Some new examples - breaking symmetries

  • ICEO pumps and mixers in microfluidics
  • “Fixed-potential ICEO”
  • “Induced-charge electrophoresis” (ICEP) particle motion

Bazant & Squires, PRL (2004); Levitan et al. Colloids & Surfaces (2005).

Squires & Bazant, JFM (2004); Levitan, PhD thesis MIT (2005).

Bazant & Squires, PRL (2004); Yariv, Phys. Fluids (2005);

Squires & Bazant, JFM (2006); Saintillon, Darve & Shaqfeh, preprint.

fixed potential iceo
“Fixed-Potential ICEO”

Squires & Bazant, J. Fluid Mech. (2004)

Idea: Vary the induced

total charge in phase

with the local field.

Generalizes “Flow FET” of

Ghowsi & Gale, J. Chromatogr. (1991)

Example: metal cylinder grounded to an electrode supplying an AC field.

Fixed-potential ICEO mixer

iceo microfluidic elements
ICEO Microfluidic Elements

J. A. Levitan, Ph.D. Thesis (2005).

Fixed-potential ICEO “pump”

(u = 3 mm/sec)

ICEO “mixer” or “trap”

(u = 0.2 mm/sec)

E = 100V/cm (< 10 Volt), 300 Hz AC, 0.1 mM KCl, 0.5 mm fluorescent tracers

50-250 mm electroplated gold posts, PDMS polymer microchannels

A promising platform for portable microfluidics…

induced charge electrophoresis iceo swimming via broken symmetries
“Induced-Charge Electrophoresis”= ICEO swimming via broken symmetries

Bazant & Squires, Phys. Rev. Lett. (2004); Yariv, Phys. Fluids (2005).

I. Heterogeneous Surfaces

Squires & Bazant, J. Fluid Mech. (2006).

A metal sphere with a partial dielectric

coating swims toward its coated end,

which rotates to align perpendicular to E.

An “ICEO pinwheel” rotates to align and

spins continuously in a uniform AC field!



icep ii asymmetric shapes
ICEP II. Asymmetric Shapes

Squires & Bazant, J. Fluid Mech. (2006).

ICEP can separate polarizable colloids by shape

and size in a uniform DC or AC electric field,

while normal (linear) electrophoresis cannot.

  • long axis rotates to align with E
  • a “thin arrow” swims parallel to E,
  • towards its “blunt” end
  • a “fat arrow” swims transverse to E
  • towards its “pointed” end

Perturbation analysis

E u

An asymmetric metal post

can pump fluid in any direction

in a uniform DC or AC field, but

ICEO flow has quadrupolar rolls,

very different from normal EOF.

FEMLAB finite-element simulation (Yuxing Ben)

icep iii non uniform fields
ICEP III. Non-uniform Fields

Shilov & Simonova, Colloid J. USSR (1981, 2001). Metal sphere “dipolophoresis”

Squires & Bazant, J. Fluid Mech. (2006). General problem of DEP + ICEP

  • Must include electrostatic force and torque (Maxwell stress tensor)
  • Dielectrophoresis (DEP) + ICEP
  • For metals, ICEP points up, and DEP down, an electric field gradient
  • ICEP cancels DEP for a metal sphere (but not a cylinder or other shapes)

Electric Field

Fluid Streamlines


General solution for any 2d shape in any non-uniform E field bycomplex analysis…

Electric Field

Fluid Streamlines

weakly nonlinear theory of iceo
“Weakly Nonlinear” Theory of ICEO

Gamayunov et al. (1986); Ramos et al. (1998); Ajdari (2000); Squires & Bazant (2004).

1. Equivalent-circuit modelfor the induced zeta potential

Bulk resistor (Ohm’s law):

Double-layer BC:

Double-layer circuit elements:

Gouy-Chapman capacitor

Stern model

Constant-phase-angle impedance

2. Stokes flow driven by ICEO slip


Dimensionless BC for AC forcing

Green et al, Phys Rev E (2002)

Levitan et al. Colloids & Surf. (2005)

femlab simulation of our first experiment iceo around a 100 micron platinum wire in 0 1 mm kcl
FEMLAB simulation of our first experiment:ICEO around a 100 micron platinum wire in 0.1 mM KCl

Levitan, ... Y. Ben,… Colloids and Surfaces (2005).

Low frequency DC limit

At the “RC” frequency

Electric field lines:

Electric Field lines

Electric field lines

Electric field lines

Velocity fields

Velocity fields


Comparision of Simulation and PIV Data:Velocity Profiles

Raw data from a slice

0-10 mm above the wire

Data collapse when scaled to

characteristic ICEO velocity

  • Scaling and flow profile consistent with ICEO theory
  • Flow magnitude roughly 2 times smaller than in simple theory
  • Need better theories for large voltages and varying solution chemistry…
theory of strongly nonlinear electrokinetics
Theory of “strongly nonlinear” electrokinetics?

Use the basic methods of applied mathematics:

(Analysis) Solve the existing equations in a new regime.

This leads to some interesting new effects, but does not explain all

the experimental data (e.g. decrease in ICEO flow for C > 10 mM).

More importantly, the solutions contain physical nonsense!

(Modeling) Postulate new equations, solve & compare to experiments.

This is now the only choice, and progress is underway.

classical equations of dilute solution theory
Classical Equations of “Dilute Solution Theory”

Poisson-Nernst-Planck ion transport equations

Singular perturbation

Navier-Stokes fluid equations with electrostatic stresses

strongly nonlinear solutions to the classical equations
Strongly Nonlinear Solutions to the Classical Equations

1. Breakdown of circuit models: Surface adsorption and bulk diffusion

Bazant, Thornton, Ajdari, PRE (2004).

2. Tangential transport of ions in the double layer

Bikerman (1933), SS Dukhin & Deryaguin (1969, 1974)

Linear theory for small E, highly charged surfaces

Kevin Chu, Ph.D. thesis (2005).

Nonlinear theory for large E, uncharged conductors

3. Diffusio-osmosis (= flow due to gradients

in bulk salt concentration)

Deryaguin (1964)

Bulk diffusion around an

uncharged metal sphere

in a uniform E field.

modified equations for electrokinetics
Modified Equations for Electrokinetics

Sabri Kilic, Bazant, Ajdari, in preparation.

1. Steric effects (finite ion size) on equilibrium:

Modified Poisson-Boltzmann equation

PB = Poisson-Boltzmann theory

Borukhov et al. Phys. Rev. Lett. (1997).

2. Steric effects on dynamics:

Modified Nerst-Planck equations

Steric & viscoelectric effects on electro-osmosis:

Modified Helmholtz-Smoluchowski slip formula

4. Steric & viscoelectric effects on ICEO…

New prediction: An uncharged metal sphere will move by ICEP

in a large uniform field, if the electrolyte is asymmetric.

engineering of microfluidic pumps
Engineering of Microfluidic Pumps

JP Urbanski, Levitan, Bazant, Thorsen, in preparation

  • Exploit fixed-potential ICEO, and standard ACEO
  • Electroplated interdigitated & recessed gold electrodes on glass
  • PDMS soft lithography for microchannels
fast ac electrokinetic pumps
Fast AC Electrokinetic Pumps

Bazant, Ben (2006)

The “conveyor belt principle”: Raised pumping surfaces, recess reverse rolls.

Apply to periodic array of electrodes in existing ACEO pumps

Raise half of each electrode to make a fast pump

Ramos et al (1999), Ajdari (2000)

optimization of iceo aceo pumps
Optimization of ICEO/ACEO pumps

Bazant, Yuxing Ben (2005)

Fastest existing ACEO pump

Green et al. (2003) theory;

Studer et al. (2004) expt.

New design:

10 times faster!

iceo a platform for portable micro fluidics
ICEO: a platform for portable microfluidics?
  • State-of-the-art “table-top microfluidics”
    • Pressure-driven microfluidics (e.g. K. Jensen)
    • Capillary electro-osmosis (e.g. J. Santiago)
    • Soft microfluidic networks (e.g S. Quake)
  • Possible advantages of ICEO:
    • Low voltage (< 10 Volt), low power (< 1 mW)
    • AC (< kHz) reduces unwanted reactions / bubbles in linear EOF
    • Time-dependent local flow control for mixing, trapping, switching,…
    • Excellent scaling with miniaturization
    • Standard “hard” microfabrication methods
  • Possible disadvantages:
    • Requires low ionic strength (< 10 mM)
    • Sensitive to solution chemistry, surface contamination

commercial applications

Engineering Applications of ICEO

Commercial Applications

1. Battery-powered microfluidics

  • Portable/implantable devices for medical or chemical monitoring
  • Localized drug delivery
  • Pressure control (e.g. glaucoma)
  • Cooling portable electronics

Example: on-field detection of exposure to biowarfare agents for the dismounted soldier by monitoring nanoliters of blood.

(T. Thorsen @ MIT Mech Eng)

  • 2. Polarizable colloids
  • ICEO flows in dielectrophoresis
  • ICEO manipulation of nanobarcodes(Santiago, Shaqfeh @ Stanford Mech Eng)

iceo icep

From mathematical theory….

to scientific experiments and engineering applications.


ICEO microfluidic pumps without moving parts

Jeremy Levitan, Ph.D. thesis, Mechanical Engineering MIT (2005)

  • Experimental fabrication: soft lithography for micro-channels (50-200 mm) and electroplating for gold structures (25-200 mm wide, 5-50 mm tall) on glass

Deposit and pattern gold

on glass wafer

Electroplate gold

Strip resist; cap with PDMS

to form micro-channel

Deposit and pattern

thick resist mold


Comparision of Simulation and PIV Data:Scaling with Voltage and Frequency

Similar ”ICEO flow” observed around mercury drops

(without any quantitative analysis):

Gamayunov, Mantrov, Murtsovkin, Colloid J. USSR (1992)

strongly nonlinear solutions as required by the experimental parameters
“Strongly Nonlinear” Solutions(as required by the experimental parameters)
  • Breakdown of circuit models at “large” voltages
  • when V > 2 kT/e = 0.05 V (z=V)

“Transient Dukhin number”

Bazant, Thornton & Ajdari, Phys. Rev. E 70, 021506 (2004).

1d model problem

(PNP equations)

V = 4 kT/e

potential charge density salt concentration

Neutral salt adsorption by the diffuse charge layer and bulk diffusion

towards a new mathematical model
Towards a new mathematical model…

1. Anolmalous “constant phase angle” double-layer impedance

Data suggests BC for power-law

“fractional relaxation”:

Hypothesis: long waiting times

for Stern-layer adsorption

(not fractal surface roughness)

KCl/Au expt

By J. Levitan

2. Strong dependence on surface and solution chemistry

ICEO flow decreases with concentration

and depends on ion valence, size,…

Hypothesis: steric effects +

variable viscosity in the Stern layer

Borukhov et al

Phys Rev Lett (1997)