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Induced-Charge Electrokinetic Phenomena

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

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Induced-Charge Electrokinetic Phenomena

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

  2. Research Interests • Electrokinetics = Electrically driven motion of particles and fluids • Microfluidics • Electrochemical systems • Granular flow • Applied mathematics

  3. Acknowledgments Induced-charge electrokinetics 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), Postodoc: Yuxing Ben (2004-06) Interns: Kapil Subramanian, Andrew Jones, Brian Wheeler, Matt Fishburn 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

  4. Outline • Linear electrokinetics • Nonlinear “induced-charge” electrokinetics • Preview of upcoming lectures

  5. Electrokinetic particle motion Electrolyte (salt solution) e.g. clay particles in water, Reuss 1808 Non-conducting viscous liquid, e.g. oil drops in air, Millikan 1900 ?

  6. The Electric Double Layer + + + quasi-neutral bulk liquid electrolyte solid Electrostatic potential Ion concentrations Gouy 1910 0 continuum region

  7. Electrokinetics in electrolytes - - - - - - - - - - - - - - - - - (a) Electro-osmosis = fluid slip across the double layer, as an electric field pushes on the screening cloud (b) Electrophoresis = particle motion due to electro-osmosis

  8. Linear Electrokinetics: 1. Fluids Linear Response • Helmholtz: electro-osmotic flow • Onsager: streaming potential & current (inverse effects) • Ajdari: transverse couplings (anisotropic surfaces) • uniform zeta, thin double layers: potential flow (no vortices) • No net response to AC forcing

  9. Application: DC EO pumps • Small channels (porous media) lead to large pressure (>10atm) • Disadvantages: • High voltage (kV) • Faradaic reactions • Gas management • Hard to miniaturize Porous Glass Yau et al, JCIS (2003)

  10. Linear Electrokinetics: 2. Particles • Smoluchowski: electrophoresis • Onsager: sedimentation potential, induced dipole • Dukhin, Deryaguin: surface conduction (large charge) • Anderson, Ajdari: transverse motion, rotation • uniform zeta, thin double layers: cannot separate particles!

  11. Applications in microfluidics • Apply E across the chip • Advantages: • EO plug flow has low hydrodynamic dispersion • Many uses of linear EP in separation/detection • Limitations: • High voltage (kV) • Often slow separations • No local flow control • “Table-top technology” From Todd Thorsen

  12. Outline • Linear electrokinetics • Nonlinear “induced-charge” electrokinetics • Preview of upcoming lectures

  13. Nonlinear Electrokinetic Phenomena Some early examples • Dielectric liquids • dielectrophoresis (DEP): acts on induced dipole • Taylor (1966): deformation & flow in oil drops • Melcher (1960s): Traveling-wave AC pumping • Electrolytes • Shilov (1976): double-layer effects in DEP • Murtsovkin (1986): flows around polarizable particles • Dukhin (1986): 2nd kind EP at large current • Saville (1997): AC colloidal self-assembly on electrodes • Ramos (1999): AC electro-osmosis • Ajdari (2000): ACEO pumping with electrode arrays

  14. “Induced-Charge Electro-osmosis” = nonlinear electro-osmotic slip at a polarizable surface Example: An uncharged metal cylinder in a suddenly applied DC field ICEO flow persists in an AC field (< charging frequency). Gamayunov, Murtsovkin, Dukhin, Colloid J. USSR (1986) - flow around a metal sphere Bazant & Squires, Phys, Rev. Lett. (2004) - general theory, broken symmetries, microfluidics

  15. Double-layer polarization and ICEO flow A conducting cylinder in a suddenly applied uniform E field. Electric field ICEO velocity Movies from a finite-element simulation by Yuxing Ben (2005) Solving the Poisson-Nernst-Planck/Navier-Stokes eqns l/a=0.005

  16. Experimental Observation of ICEO Jeremy Levitan, PhD Thesis in Mechanical Engineering, MIT (2005) 100 mm Pt wire on channel wall Viewing plane PDMS polymer microchannel Bottom view of optical slice Inverted optics microscope Micro-particle image velocimetry (mPIV) to map the velocity profile

  17. Simulated flow (side view) ICEO Experiments J. A. Levitan, S. Devasenathipathy, V. Studer, Y. Ben, T. Thorsen, T. M. Squires & M. Z. Bazant, Colloids and Surfaces (2005) Collapse of experimental data Movie: 5 m optical slice sweeping 100 m Pt cylinder (top view) 100 V/cm, 300 Hz, 0.1 mM KCl

  18. Examples of ICEO in Microfluidics Flow around a metal post Fixed-potential ICEO DC jet at a dielectric corner AC electro-osmosis Ramos et al (1999), Ajdari (2000) Thamida & Chang (2002)

  19. A pioneer, ahead of his time Vladimir A. Murtsokvin (work from 1983 to 1996) “ICEO” flow around an metal particle (courtesy of Andrei Dukhin)

  20. Induced-Charge Electrophoresis= ICEO swimming by broken symmetry Bazant & Squires, Phys. Rev. Lett. (2004); Yariv, Phys. Fluids (2005) Squires & Bazant, J. Fluid Mech (2006) Example: Janus particle A metal/dielectric sphere in a uniform E field always moves toward its dielectric face, which rotates to perpendicular to E. The particle swims sideways. Stable Unstable

  21. Experimental observation of induced-charge electrophoresis S. Gangwal, O. Cayre, MZB, O.Velev, Phys Rev. Lett. 100, 058302 (2008).

  22. Outline • Linear electrokinetics • Nonlinear “induced-charge” electrokinetics • Preview of upcoming lectures

  23. Lecture 2: ICEP in Colloids Thursday 10 Jan. 2pm • Theory • Field-dependent EO mobility • Shape-dependent ICEP motion • Wall interactions • ICEP & DEP in non-uniform fields • Applications in separation, assembly

  24. Some examples... E u The ICEO pinwheeel Non-uniform fields

  25. Lecture 3: ACEO in Microfluidics Thursday 17 Jan. 2pm • Theory • ICEO mixers • ACEO flow over electrode arrays • Fast (> mm/s), low-voltage (< 3V), high-pressure (10% atm) ACEO pumps • Applications to portable/implantable labs-on-a-chip, drug delivery

  26. Some examples... Scientific American, Oct. 2007. The “Fluid Conveyor Belt”

  27. Lecture 4: Theory at large voltages Postponed… • Experimental puzzles • Strongly nonlinear dynamics • Breakdown of dilute-solution theory • Modified theories for ion crowding • New phenomena and open questions

  28. Experimental puzzles in ACEO V. Studer et al., Analyst (2004) MZB et al., MicroTAS (2007) • High-frequency flow reversal • Concentration dependence • Ion specificity • Classical theory breaks down…

  29. Crucial new physics: Ion crowding at large voltages MZB, MS Kilic, B Storey, A Ajdari (2007) Poisson-Boltzman/ Dilute-solution theory ICEO experiments, New theory needed

  30. Conclusion Induced-charge electrokinetics provides many opportunities for new science and new applications Papers, slides… http://math.mit.edu/~bazant/ICEO

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