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Induced-Charge Electrokinetic Phenomena

Paris-Sciences Chair Lecture Series 2008, ESPCI. Induced-Charge Electrokinetic Phenomena. Introduction ( 7/1) Induced-charge electrophoresis in colloids (10/1) AC electro-osmosis in microfluidics (17/1) Theory at large applied voltages (14/2). Martin Z. Bazant

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Induced-Charge Electrokinetic Phenomena

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  1. Paris-Sciences Chair Lecture Series 2008, ESPCI Induced-Charge Electrokinetic Phenomena Introduction (7/1) Induced-charge electrophoresis in colloids (10/1) AC electro-osmosis in microfluidics (17/1) Theory at large applied voltages (14/2) Martin Z. Bazant Department of Mathematics, MIT ESPCI-PCT & CNRS Gulliver

  2. Acknowledgments Induced-charge electrokinetics: Microfluidics 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), Postodocs:Yuxing Ben (2004-06) Interns: Kapil Subramanian, Andrew Jones, Brian Wheeler, Matt Fishburn, Jacub Kominiarczuk 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

  3. Outline • Electrokinetic microfluidics • ICEO mixers • AC electro-osmotic pumps

  4. Electro-osmosis Slip: Potential / plug flow for uniformly charged walls:

  5. Electro-osmotic Labs-on-a-Chip • Apply E across chip • Advantages • EO plug flow has low hydrodynamic dispersion • Standard uses of in separation/detection • Limitations: • High voltage (kV) • No local flow control • “Table-top technology”

  6. Pressure generation by slip Use small channels!

  7. 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 Porous Glass Yau et al, JCIS (2003) Juan Santiago’s group at Stanford

  8. 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 • BUT • Must sustain direct current • Flow is set by geometry, not “tunable”

  9. Outline • Electrokinetic microfluidics • ICEO mixers • AC electro-osmotic pumps

  10. 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

  11. ICEO Mixers, Switches, Pumps… • Advantages • tunable flow control • 0.1 mm/sec slip • low voltage (few V) • Disadvantages • small pressure (<< Pa) • low salt concentration

  12. 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 energized. • (d) Microfabricated device consisting of vertical gold-coated silicon posts and sidewall electrodes in an insulating channel. (Channel width 200 um, depth 300 um)

  13. 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).

  14. ICEO-based microfluidic mixing(C. K. Harnett, University of Louisville/M.P. Kanouff, Sandia National Laboratories) Power Off: Incomplete diffusional mixing experimental calculated Power On: Complete ICEO-based mixing experimental calculated 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.

  15. “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. Fixed-potential ICEO mixer Flow past a 20 micron electroplated gold post (J. Levitan, PhD Thesis 2005)

  16. Outline • Electrokinetic microfluidics • Induced-charge mixers • AC electro-osmotic pumps

  17. AC electro-osmosis A. Ramos, A. Gonzalez, A. Castellanos (Sevilla), N. Green, H. Morgan (Southampton), 1999.

  18. Circuit model Ramos et al. (1999) “RC time” Debye time:

  19. 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

  20. 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

  21. Experimental data • Brown et al (2001), water • straight channel • planar electrode array • similar to theory (0.2-1.2 Vrms) • Vincent Studer et al (2004), KCl • - microfluidic loop, same array • flow reversal at large V, freq • no flow for C > 10mM

  22. 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 • Can we improve performance? KCl, 3 Vpp, loop chip 5x load

  23. 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 20x faster (>mm/sec at 3 Volts) and should not reverse New design: electrode steps create a “fluid conveyor belt”

  24. The Fluid Conveyor Belt CQ Choi, “Big Lab on a Tiny Chip”, Scientific American, Oct. 2007.

  25. 3D ACEO pumping of water JP Urbanski, JA Levitan, MZB & T Thorsen, Appl. Phys. Lett. (2006) Movie of fast flows for voltage steps 1,2,3,4 V (far from pump). Max velocity 5x larger (+suboptimal design)

  26. 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 3m KCl

  27. Even faster, more robust pumps Damian Burch & MZB, preprintarXiv:0709.1304 grooved plated “plated” Grooved design amplifies the fluid conveyor belt * 2x faster flow * less unlikely to reverse * wide operating conditions Experiments coming soon… “grooved”

  28. AC vs. DC Electro-osmotic Pumps

  29. Conclusion * 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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