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Induced-Charge Electrokinetic Phenomena Martin Z. Bazant Department of Mathematics, MIT ESPCI-PCT & CNRS Gulliver Paris-Sciences Chair Lecture Series 2008, ESPCI 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)

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Induce-charge electrokinetics: Colloids 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, Hongwei Sun ( ) 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 Acknowledgments

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Outline 1.Linear electrophoresis 2.Induced-charge electrophoresis 3.Heterogeneous particles

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Electrophoresis in a dielectric liquid Particle frame Lab frame Long-range flow perturbationSize-dependent velocity

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Electrophoresis in an electrolyte force-free motion short-range flow perturbation Size-independent velocity Electro-osmotic slip Zeta potential Electrophoretic mobility Smoluchowski (1907)

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Electrophoresis of a colloid Morrison (1970) Solution for uniform mobility: potential flow No relative motion!

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Non-uniform surface charge (1) An inhomogeneous sphere rotates to align dipole with E Anderson 1984 Force-free ICEP enhances forced dielectrophoresis (DEP)

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Non-uniform surface charge (2) Transverse EP is possible, but only for certain orientations Anderson 1984 EP mobility is not related to total or average charge!

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Non-spherical & non-uniform particles A particle can exhibit transverse EP for any orientation: Ajdari 1995, Long & Ajdari 1998 Continuous rotation is also possible, but only around a special axis, with chiral, charged grooves

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Outline 1.Linear electrophoresis 2.Induced-charge electrophoresis 3.Heterogeneous particles

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Electrophoresis of Polarizable Particles Classical result, e.g. Levich (1956): Electrophoretic mobility depends on the total charge, but not on the induced dipole moment Induced dipole …BUT this only holds for linear response to small E

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Stotz-Wien-Dukhin Effect For nonlinear double-layer capacitance, the mobility depends on E, since induced charge must redistribute to maintain the same the total charge (AS Dukhin 1992) Can exploit for separation in “unbalanced” AC fields (SS Dukhin et al 1986, R Chimenti, patent 1986)

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Ion-specific mobility Bazant, Kilic, Storey, Ajdari, in preparation 2008 Dukhin effect in an asymmetric electrolyte Mobility must depend on ion charges, sizes, etc. Even an uncharged sphere can move, if it is polarizable Its apparent charge is that of ions which screen at higher density

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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 particle in a DC (or AC) field

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First studies of “ICEO” flow Vladimir A. Murtsokvin (work from 1983 to 1996) with Andrei Dukhin, Mantrov, Gamayunov Nonlinear flow induced around a tin particle (albeit in the “wrong” direction at large sizes…) Also studied interactions between particles

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Thin-DL, low-voltage theory Squires & Bazant, JFM 2004, 2006; Yariv 2005 “RC circuit” boundary condition Ohm’s law Total charge constraint ICEO slip Solve Stokes flow with for particle set by force=torque=0

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Dielectrophoresis Maxwell-Wagner induced dipole moment Time-averaged force and torque Velocity in Stokes flow Maxwell (1891), Pohl (1958)

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Dipolophoresis = DEP + ICEP In an electrolyte, ICEP opposes DEP for highly polarizable particles Both effects have the same scaling Shilov & Simonova, Colloid J. USSR (1981, 2001). Metal sphere “dipolophoresis” Squires & Bazant, J. Fluid Mech. (2006). General problem of DEP + ICEP Electric FieldFluid Streamlines

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General solution for any 2d shape in any non-uniform E field by complex analysis… Electric FieldFluid Streamlines

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Outline 1.Linear electrophoresis 2.Induced-charge electrophoresis 3.Heterogeneous particles

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Electrokinetic motion of rod-like metal particles Rose & Santiago, Phys Rev E (2006): Experiments on alignment of “nano-barcode” particles Saintillan, Darve & Shaqfeh, J Fluid Mech (2006): theory for spheroids + simulations Perturbation theory: Squires & Bazant, JFM (2006) Field offField on

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ICEP of Irregular Shapes ICEP can separate particles of the same material and same size by shape alone. Any direction is possible. Expts on quartz particles: Gamayunov & Murtsovkin (1992). Theory: Bazant & Squires (2004) Tensor relations: Yariv (2005); Perturbation analysis: Squires & Bazant (2006).

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ICEP of Asymmetric Shapes - long axis rotates to align with E - a “thin arrow” swims parallel to E, towards its “blunt” end - a “fat arrow” swims transverse to E towards its “pointed” end Squires & Bazant, J. Fluid Mech. (2006). ICEP can separate polarizable colloids by shape and size in a uniform DC or AC electric field, while normal (linear) electrophoresis cannot. An asymmetric metal post can pump fluid in any direction in a uniform DC or AC field, but ICEO flow has quadrupolar rolls, very different from normal EOF. Perturbation analysis E u FEMLAB finite-element simulation (Yuxing Ben)

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ICEP of Inhomogeneous Particles Bazant & Squires, Phys. Rev. Lett. (2004); Squires & Bazant, J. Fluid Mech (2006) StableUnstable 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. Example: Janus particle

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An Electrophoretic Pinwheel An even more surprising example (Squires & Bazant J Fluid Mech 2006): Responds to any electric field by rotating ( ) Could be used to apply torques to molecules or cells?

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ICEP Experiments S. Gangwal, O. Cayre, MZB, O.Velev, Phys Rev. Lett (2008). Gold/latex Janus spheres in NaCl

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Experimental data Good fit to theory for but particles move near the wall… why? 0.1 mM 300 V/cm

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ICEO wall interactions Zhao & Bau, Langmuir (2007) We expect repulsion from non-polarizable walls, but the Janus particles are attracted to the wall. Why?

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ICEP of Janus particles near a wall Kilic & Bazant, preprint, arXiv: Without tilting, the particle is indeed repelled, but it always rotates to meet the wall and can translate with stable angle.

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Simulations of Janus-Wall interaction Strong ICEO flows make particle face the wall and get stuck. Weaker ICEO flows produce tilted translation as observed (due to DEP).

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Conclusion Papers, slides… Induced-charge electrophoresis leads to many new phenomena in colloids with applications to separation, manipulation, self-assembly. Better theories needed. ACEO pumps in Lecture 3…

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