Micro-fluidic Applications of

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Micro-fluidic Applications of Induced-Charge
Electro-osmosis
Jeremy Levitan Mechanical Engineering, MIT
Todd Squires Applied Mathematics, CalTech Martin
Schmidt Electrical Engineering, MIT
Martin Bazant Applied Mathematics, MIT
Todd Thorsen Mechanical Engineering, MIT
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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 ltUgt 0 in AC
• DC requires Faradaic reactions gt hydrolysis
• Need large V for large E along channel

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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)
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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).
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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
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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
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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
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ICEO Around A 100 µm Pt Wire
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Particle Image Velocimetry
500 nm seed particles
Slide used with permission of S. Devasenathipathy
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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)
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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
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Extensions to Model
All reduce predicted velocities

• Surface Capacitance/Contamination
• multi-step cleaning for metal surfaces
• Surface Conductance
• Visco-electric effect

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Current Work

• Fixed potential posts
• Post-array mixers
• Asymmetric objects
• Integration with microfluidic devices --
• microchannels and valves
• DNA hybridization arrays

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Induced-Charge Electro-osmosis

• Demonstrated non-linear electro-osmosis at
  polarizable (metal) surfaces
• Sensitive to frequency, voltage, etc.
• At low concentration (lt1mM), 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 http//media.mit.edu/j
levitan/iceo.html
Papers http//math.mit.edu/bazant