PowerPoint Presentation - Electrokinetics & Granular Flow by XWNj40k


									     Paris-Sciences Chair Lecture Series 2008, 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 at large applied voltages (14/2)
   Induced-charge electrokinetics: Microfluidics
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)

PhD: Jeremy Levitan, Kevin Chu (2005),
Postodocs: Yuxing Ben (2004-06)
Interns: Kapil Subramanian, Andrew Jones,
     Brian Wheeler, Matt Fishburn,                Funding:
     Jacub Kominiarczuk                           • Army Research Office
Collaborators: Todd Squires (UCSB),               • National Science Foundation
Vincent Studer (ESPCI), Martin Schmidt (MIT),     • MIT-France Program
Shankar Devasenathipathy (Stanford)               • MIT-Spain Program
1. Electrokinetic microfluidics
2. ICEO mixers
3. AC electro-osmotic pumps


Potential / plug flow for uniformly charged walls:
   Electro-osmotic Labs-on-a-Chip
• Apply E across chip

• Advantages
  – EO plug flow has low
    hydrodynamic dispersion
  – Standard uses of in

• Limitations:
  – High voltage (kV)
  – No local flow control
  – “Table-top technology”
Pressure generation by slip

                   Use small channels!
         DC Electro-osmotic Pumps
• Nanochannels or porous media
  can produce large pressures
  (0.1-50 atm)

• Disadvantages:
  –   High voltage (kV)
  –   Faradaic reactions
  –   Gas management
  –   Hard to miniaturize

                                     Yau et al, JCIS (2003)
                                     Juan Santiago’s group at Stanford
     Electro-osmotic mixing
• Non-uniform zeta produces vorticity
• Patterned charge + grooves can also
  drive transverse flows (Ajdari 2001) which
  allow lower voltage across a channel
  – Must sustain direct current
  – Flow is set by geometry, not “tunable”
1. Electrokinetic microfluidics
2. ICEO mixers
3. AC electro-osmotic pumps
Induced-Charge Electro-osmosis
Gamayunov, Murtsovkin, Dukhin, Colloid J. USSR (1986) - flow around a metal sphere
Bazant & Squires, Phys, Rev. Lett. (2004) - general theory, broken symmetries, microfluidics

   Example: An uncharged metal cylinder in a DC (or AC) field

 Can generate vorticity and pressure with AC fields
ICEO Mixers, Switches, Pumps…

               • Advantages
                   • tunable flow control
                   • 0.1 mm/sec slip
                   • low voltage (few V)
               • Disadvantages
                   • small pressure (<< Pa)
                   • low salt concentration
            ICEO-based microfluidic mixing
(C. K. Harnett, University of Louisville/M.P. Kanouff, Sandia National Laboratories)

                                               •(a) Simulation of dye loading in
                                               the mixing channel by pressure-
                                               driven flow. Some slow
                                               diffusional mixing is seen.
                                               •(b) Simulation of fast mixing
                                               after loading, when sidewall
                                               electrodes are energized.
                                               •(c) Simulated velocity field
                                               surrounding the triangular posts
                                               when sidewall electrodes are
                                               •(d) Microfabricated device
                                               consisting of vertical gold-coated
                                               silicon posts and sidewall
                                               electrodes in an insulating
                                               channel. (Channel width 200 um,
                                               depth 300 um)
              ICEO-based microfluidic mixing
   (C. K. Harnett, University of Louisville/M.P. Kanouff, Sandia National Laboratories)

Features in flow images (top row) are replicated in the model (bottom row)
•without electric field (a) (b)
•and with electric field applied between channel sidewalls (c), (d).
               ICEO-based microfluidic mixing
  (C. K. Harnett, University of Louisville/M.P. Kanouff, Sandia National Laboratories)

                                                                                   Power Off:
   experimental                                                                    diffusional


                                                                                  Power On:

Comparison of experimental (a,c) and calculated (b,d) results during steady
flow of dyed and un-dyed solutions (2 ml/min combined flow rate) without
power (a,b) and with power (c,d). Flow is from left to right. 10 Vpp, 37 Hz
square wave applied across 200 um wide channel. Left-right transit time ~2 s.
               “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.

                   QuickTime™ and a
            DV/DVCPRO - NTSC d ecompressor
             are neede d to see this picture.

                                                                              Fixed-potential ICEO mixer

                                                               Flow past a 20 micron electroplated gold post
                                                               (J. Levitan, PhD Thesis 2005)
1. Electrokinetic microfluidics
2. Induced-charge mixers
3. AC electro-osmotic pumps
              AC electro-osmosis
A. Ramos, A. Gonzalez, A. Castellanos (Sevilla), N. Green, H. Morgan (Southampton), 1999.
              Circuit model
                Ramos et al. (1999)

“RC time”

Debye time:
  ICEO flow over electrodes

• Example: response to a sudden DC voltage
• ACEO flow peaks if period = charging time
• Maximizes flow/voltage due to large field
AC electro-osmotic pumps
        Ajdari (2000)

                  “Ratchet” concept inspired
                  by molecular motors:

                  Broken local symmetry in a
                  periodic structure with
                  “shaking” causes pumping
                  without a global gradient.

                  Brown, Smith, Rennie (2001):
                  asymmetric planar electrodes
               Experimental data

Brown et al (2001), water            Vincent Studer et al (2004), KCl
- straight channel                   - microfluidic loop, same array
- planar electrode array             - flow reversal at large V, freq
- similar to theory (0.2-1.2 Vrms)   - no flow for C > 10mM
      More data for planar pumps
        Urbanski et Appl Phys Lett (2006); Bazant et al, MicroTAS (2007)

Puzzling features
- flow reversal
- decay with salt concentration
- ion specific                                KCl, 3 Vpp, loop chip 5x load
Can we improve performance?
        Fast, robust “3D” pump designs
                              Bazant & Ben, Lab on a Chip (2006)

Fastest planar ACEO pump
Brown, Smith & Rennie (2001). Studer (2004)

                                                    Theory: “3D” design is
New design: electrode steps                         20x faster (>mm/sec at 3 Volts)
create a “fluid conveyor belt”                      and should not reverse
The Fluid Conveyor Belt

CQ Choi, “Big Lab on a Tiny Chip”, Scientific American, Oct. 2007.
          3D ACEO pumping of water
                 JP Urbanski, JA Levitan, MZB & T Thorsen, Appl. Phys. Lett. (2006)

                 QuickTime™ an d a
            MPEG-4 Video decompressor
           are need ed to see this p icture .

Movie of fast flows for voltage
steps 1,2,3,4 V (far from pump).
                                                Max velocity 5x larger (+suboptimal design)
Optimization of non-planar ACEO pumps
  JP Urbanski, JA Levitan, D Burch, T Thorsen & MZB, J Colloid Interface Science (2007)

                                         • Electroplated Au steps on Au/Cr/glass
                                         • Robust mm/sec max flow in 3mm KCl
Even faster, more robust pumps
           Damian Burch & MZB, preprint arXiv:0709.1304


                          Grooved design amplifies the
                          fluid conveyor belt
                                  * 2x faster flow
                                  * less unlikely to reverse
                                  * wide operating conditions

“grooved”                 Experiments coming soon…
AC vs. DC Electro-osmotic Pumps
* Induced-charge electro-osmotic flows driven by
    AC voltages offer new opportunities for
    mixers, switches, pumps, droplet
    manipulation, etc. in microfluidics
* Better theories needed…. (Lecture 4 14/2/08)

  Papers, slides… http://math.mit.edu/~bazant/ICEO

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