Induced-Charge Electrokinetic Phenomena

Martin Z. BazantDepartment 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)

Induced-charge electrokinetics: MicrofluidicsCURRENTStudents: 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)

FORMERPhD: Jeremy Levitan, Kevin Chu (2005),Postodocs: Yuxing Ben (2004-06)Interns: Kapil Subramanian, Andrew Jones, Brian Wheeler, Matt Fishburn, Jacub KominiarczukCollaborators: 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

Outline

1. Electrokinetic microfluidics2. ICEO mixers3. AC electro-osmotic pumps

Electro-osmosis

Slip:

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

separation/detection

• 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

Porous Glass

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”

Outline

1. Electrokinetic microfluidics2. ICEO mixers3. AC electro-osmotic pumps

Induced-Charge Electro-osmosisGamayunov, Murtsovkin, Dukhin, Colloid J. USSR (1986) - flow around a metal sphereBazant & 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 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)

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)

Comparison of experimental (a,c) and calculated (b,d) results during steady flow of dyed and un-dyed solutions (2 l/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.

ICEO-based microfluidic mixing(C. K. Harnett, University of Louisville/M.P. Kanouff, Sandia National Laboratories)

Power Off:Incompletediffusionalmixing

Power On:CompleteICEO-basedmixing

experimental

experimental

calculated

calculated

“Fixed-Potential ICEO”

Example: metal cylinder grounded to an electrode supplying an AC field.

Fixed-potential ICEO mixer

Idea: Vary the induced total charge in phase with the local field.

Squires & Bazant, J. Fluid Mech. (2004)

Generalizes “Flow FET” ofGhowsi & Gale, J. Chromatogr. (1991)

QuickTime™ and aDV/DVCPRO - NTSC decompressor

are needed to see this picture.

Flow past a 20 micron electroplated gold post (J. Levitan, PhD Thesis 2005)

Outline

1. Electrokinetic microfluidics2. Induced-charge mixers3. AC electro-osmotic pumps

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

Circuit modelRamos et al. (1999)

Debye time:

“RC 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 pumpsAjdari (2000)

“Ratchet” concept inspired by molecular motors:

Broken local symmetry in a periodic structure with “shaking” causes pumpingwithout a global gradient.

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

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

More data for planar pumpsUrbanski et Appl Phys Lett (2006); Bazant et al, MicroTAS (2007)

KCl, 3 Vpp, loop chip 5x load

Puzzling features- flow reversal- decay with salt concentration- ion specificCan we improve performance?

Fast, robust “3D” pump designs

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

New design: electrode stepscreate a “fluid conveyor belt”

Theory: “3D” design is20x faster (>mm/sec at 3 Volts)and should not reverse

Bazant & Ben, Lab on a Chip (2006)

The Fluid Conveyor Belt

CQ Choi, “Big Lab on a Tiny Chip”, Scientific American, Oct. 2007.

3D ACEO pumping of water

QuickTime™ and aMPEG-4 Video decompressor

are needed to see this picture.

Movie of fast flows for voltagesteps 1,2,3,4 V (far from pump).

Max velocity 5x larger (+suboptimal design)

JP Urbanski, JA Levitan, MZB & T Thorsen, Appl. Phys. Lett. (2006)

Optimization of non-planar ACEO pumpsJP 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 3m KCl

Even faster, more robust pumpsDamian Burch & MZB, preprint arXiv:0709.1304

“plated”

“grooved”

grooved

plated

Grooved design amplifies thefluid conveyor belt

* 2x faster flow* less unlikely to reverse* wide operating conditions

Experiments coming soon…

AC vs. DC Electro-osmotic Pumps

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