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Micro-fluidic Applications of
Induced-Charge Electro-osmosis
Jeremy Levitan
Mechanical Engineering, MIT
Martin Bazant
Applied Mathematics, MIT
Todd Squires
Applied Mathematics, CalTech
Todd Thorsen
Mechanical Engineering, MIT
Martin Schmidt
Electrical Engineering, MIT
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
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)
(Schilling 2001)
• 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)
(Stroock 2002)
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, 0066101 (2004); TMS & MZB, J. Fluid. Mech. 509, 217 (2004).
A Simple Model System
• 100um dia. platinum wire
transverse to PDMS polymer
microchannel (200um tall,
1mm wide);
• 0.1 - 1mM KCl with 0.01%
by volume 0.5um fluorescent
latex particles;
• Sinusoidal voltage (10 100V) excitation, 0 DC
offset; Applied 0.5cm away
from center wire via gold
and/or platinum wires;
V
Cross-section of experiment
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
Voltmeter
Function Generator
Viewing
Resistor
Platinum
Wire
Viewing Plane
KCl in
PDMS
Microchannel
Inverted Optics
Microscope
200 um X 1 mm X 1mm Channel
Bottom View
ICEO Around A 100 µm Pt Wire
QuickTime™ and a
DV/DVCPRO - NTSC decompressor
are needed to see this picture.
Particle Image Velocimetry
y
dp
df
x
z
500 nm seed particles
Seeded
Microflow
ICCD
Image at t
Microscope
Objective
Dichroic Mirror
Light Source
Optics
Eye
Image at t
1
2
Eyepiece
Color Filters
100X Oil
Immersion Objective
Microscope Body
df
de pt h of f i e l d
dp
pa r t i c l e di a m e t e r
di
t
i m a ge di a m e t e r
Mercury
tLamp
i me
Parabolic
Mirror
x'
di
y'
Translating
Stage
Flow
Direction
Interrogation
Spot I 1,k
Slide used with permission of S. Devasenathipathy
Interrogation
Spot I 2,k
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)
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
Extensions to Model
All reduce predicted velocities
• Surface Capacitance/Contamination:
multi-step cleaning for metal surfaces;
• Surface Conductance:
• Visco-electric effect
Current Work
• Fixed potential posts;
• Post-array mixers;
• Asymmetric objects;
• Integration with microfluidic
devices -• microchannels and
valves;
• DNA hybridization
arrays;
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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:
http://media.mit.edu/~jlevitan/iceo.html
http://math.mit.edu/~bazant