Transcript Paris-Sciences Chair Lecture Series, ESPCI Induced-Charge Electrokinetic Phenomena Martin Z. Bazant Department of Mathematics, MIT ESPCI-PCT & CNRS Gulliver 1.

Paris-Sciences Chair Lecture Series, ESPCI
Induced-Charge Electrokinetic Phenomena
Martin Z. Bazant
Department of Mathematics, MIT
ESPCI-PCT & CNRS Gulliver
1. Introduction (7/1)
2. Induced-charge electrophoresis in colloids (10/1)
3. AC electro-osmosis in microfluidics (17/1)
4. Theory of electrokinetics at large voltages (14/2)
Research Interests
• Electrokinetics
= Electrically driven motion of particles and fluids
• Microfluidics
• Electrochemical systems
• Granular flow
• Applied mathematics
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 (2004-06)
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
Outline
1. Linear electrokinetics
2. Nonlinear “induced-charge” electrokinetics
3. Preview of upcoming lectures
Electrokinetic particle motion
Non-conducting viscous liquid,
e.g. oil drops in air, Millikan 1900
Electrolyte (salt
solution)
e.g. clay particles in
water, Reuss 1808
?
The Electric Double Layer
+
solid
quasi-neutral
bulk liquid
electrolyte
+
+
Electrostatic potential
Ion concentrations
0
continuum region
Gouy 1910
Electrokinetics in electrolytes
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(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
Linear Electrokinetics: 1. Fluids
Linear
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
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)
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!
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:
–
–
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–
High voltage (kV)
Often slow separations
No local flow control
“Table-top technology”
From Todd Thorsen
Outline
1. Linear electrokinetics
2. Nonlinear “induced-charge” electrokinetics
3. Preview of upcoming lectures
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
“Induced-Charge Electro-osmosis”
= nonlinear electro-osmotic slip at a polarizable surface
Example: An uncharged metal cylinder in a suddenly applied DC field
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
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
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
Inverted optics
microscope
Micro-particle image
velocimetry (mPIV) to
map the velocity profile
Bottom view
of optical slice
ICEO Experiments
Simulated flow (side view)
J. A. Levitan, S. Devasenathipathy, V. Studer,
Y. Ben, T. Thorsen, T. M. Squires & M. Z. Bazant,
Colloids and Surfaces (2005)
QuickTime™ and a
H.263 decompressor
are needed to see this picture.
Movie: 5 mm optical slice sweeping
100 mm Pt cylinder (top view)
100 V/cm, 300 Hz, 0.1 mM KCl
Collapse of experimental data
Examples of ICEO in Microfluidics
QuickTime™ and a
DV/DVCPRO - NTSC decompressor
are needed to see this picture.
Flow around a metal post
DC jet at a dielectric corner
Thamida & Chang (2002)
QuickTime™ and a
DV/DVCPRO - NTSC decompressor
are needed to see this picture.
Fixed-potential ICEO
AC electro-osmosis
Ramos et al (1999), Ajdari (2000)
A pioneer, ahead of his time
Vladimir A. Murtsokvin (work from 1983 to 1996)
“ICEO” flow around an metal particle (courtesy of Andrei Dukhin)
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
Stable
Unstable
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.
Experimental observation of
induced-charge
electrophoresis
S. Gangwal, O. Cayre, MZB, O.Velev,
Phys Rev. Lett. 100, 058302 (2008).
Outline
1. Linear electrokinetics
2. Nonlinear “induced-charge” electrokinetics
3. Preview of upcoming lectures
Lecture 2: ICEP in Colloids
Thursday 10 Jan. 2pm
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Theory
Field-dependent EO mobility
Shape-dependent ICEP motion
Wall interactions
ICEP & DEP in non-uniform fields
Applications in separation, assembly
Some examples...
E
u
The ICEO pinwheeel
Non-uniform fields
Lecture 3: ACEO in Microfluidics
Thursday 17 Jan. 2pm
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Theory
ICEO mixers
ACEO flow over electrode arrays
Fast (> mm/s), low-voltage (< 3V), highpressure (10% atm) ACEO pumps
• Applications to portable/implantable
labs-on-a-chip, drug delivery
Some examples...
Scientific American, Oct. 2007.
The “Fluid Conveyor Belt”
Lecture 4: Theory at large voltages
Postponed…
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Experimental puzzles
Strongly nonlinear dynamics
Breakdown of dilute-solution theory
Modified theories for ion crowding
New phenomena and open questions
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…
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
Conclusion
Induced-charge electrokinetics
provides many opportunities for
new science and new applications
Papers, slides… http://math.mit.edu/~bazant/ICEO