1. Micro-fluidic Applications of Induced-Charge Electro-osmosis
Jeremy Levitan
Mechanical Engineering, MIT
Martin Bazant
Applied Mathematics, MIT
Todd Squires
Applied Mathematics, CalTech
Martin Schmidt
Electrical Engineering, MIT
Todd Thorsen
Mechanical Engineering, MIT

2. Pumping in Micro-Fluidics
Mechanical pumping
Robust
Poor scaling: U ~ h2 P/ 
Bulky external pressure source
Shear dispersion
Capillary electro-osmosis
Material sensitive
Plug flow: U = 100 um/sec in E = 100 V/cm
Linear: <U> = 0 in AC
DC requires Faradaic reactions => hydrolysis
Need large V for large E along channel

3. Mixing in Micro-Fluidics
Diffusion down a channel:
with EO
Jacobson, McKnight, Ramsey (1999)
Serpentine channels
Mengeaud et al (2002)
Geometric splitting
Schonfeld, Hessel, and Hofmann (2004), Wang et al (2002)
Passive recirculation
Chung et al (2004)
Pressure-driven flow with chaotic streamlines:
Johnson et al (2002), Stroock et al (2002)
AC Electro-osmosis
Studer, Pepin, Chen, Ajdari (2002)
Electrohydrodynamic Mixing
Oddy, Santiago and Mikkelsen (2001), Lin et al, Santiago (2001)
Micro peristaltic pumps (moving walls)
(Schilling 2001)
(Stroock 2002)

4. Induced-Charge Electro-Osmosis
Nonlinear slip at a polarizable surface
Example: An uncharged metal cylinder in a suddenly applied DC field
Metal sphere: V. Levich (1962); N. Gamayunov, V. Murtsovkin, A. Dukhin, Colloid J. USSR (1984).
E-field, t = 0
E-field, t » charging time
Steady ICEO flow
induced ~ E a
MZB & TMS, Phys, Rev. Lett. 92, (2004); TMS & MZB, J. Fluid. Mech. 509, 217 (2004).

5. A Simple Model System
100um dia. platinum wire transverse to PDMS polymer microchannel (200um tall, 1mm wide);
mM KCl with 0.01% by volume 0.5um fluorescent latex particles;
Sinusoidal voltage ( V) excitation, 0 DC offset; Applied 0.5cm away from center wire via gold and/or platinum wires; V
Cross-section of experiment

6. Simple Mathematical Model
1. Electrochemical problem for the induced zeta potential
Bazant, Thornton, Ajdari, Phys. Rev. E (2004)
Steady-state potential, electric field
after double layer charging
2. Stokes flow driven by ICEO slip
Steady-state Stokes flow
Simulation is of actual experimental geometry

7. Voltmeter
Function Generator
Viewing
Resistor
Platinum
Wire
Viewing Plane
KCl in
PDMS
Microchannel
Inverted Optics
Microscope
Bottom View
200 um X 1 mm X 1mm Channel

8. ICEO Around A 100 µm Pt Wire

9. Particle Image Velocimetry
500 nm seed particles
Slide used with permission of S. Devasenathipathy

10. PIV Mean Velocity Data
PIV measurement with 0.01% volume dielectric (fluorescent) tracer particles
Correct scaling, but inferred surface slip smaller from simple theory by 10
Metal colloids: Gamayunov, Mantrov, Murtsovkin (1992)

11. Frequency Dependence
At “fast” frequencies, double layer not fully charged;
Consistent with “RC” charging
U ~ U0/(1 + (/c)2)
c = 2  d a/D
= 1/c = 3 ms
Experiments in 1 mM KCl at 75 V

12. Extensions to Model Surface Capacitance/Contamination:
All reduce predicted velocities
Surface Capacitance/Contamination:
multi-step cleaning for metal surfaces;
Surface Conductance:
Visco-electric effect

13. Current Work Fixed potential posts; Post-array mixers;
Asymmetric objects;
Integration with microfluidic devices --
microchannels and valves;
DNA hybridization arrays;

14. Induced-Charge Electro-osmosis
Demonstrated non-linear electro-osmosis at polarizable (metal) surfaces
Sensitive to frequency, voltage, etc.
At low concentration (<1mM), no concentration dependence, but U decreases at higher c
Advantages in microfluidics:
Time-dependent local control of streamlines
Requires small AC voltages, transverse to channels
Compatible with silicon fabrication technology
Disadvantages:
Sensitive to surface contamination, solution chemistry
Relatively weak for long-range pumping
Additional movies/data:
Papers: