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)

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 Brian Storey

(Olin College) Orlin Velev (NC State), Henrik Bruus Antonio Ramos (Sevilla) (St. Gobain) (DTU) 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

Outline 1. Electrokinetic microfluidics

2. ICEO mixers 3. 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 Porous Glass 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 • BUT – Must sustain direct current – Flow is set by geometry, not “tunable”

Outline

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 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)

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) experimental Power Off: Incomplete diffusional mixing calculated experimental Power On: Complete ICEO-based mixing calculated Comparison of experimental (a,c) and calculated (b,d) results during steady flow of dyed and un-dyed solutions (2 m l/min combined flow rate) without power (a,b) and with power (c,d). Flow is from left to right. 10 V pp , 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 decompressor are needed to see this picture.

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

Outline

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.

“RC time” Debye time:

Circuit model

Ramos et al. (1999)

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 - 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 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

Fast, robust “3D” pump designs

Bazant & Ben, Lab on a Chip (2006) Fastest planar ACEO pump Brown, Smith & Rennie (2001). Studer (2004) New design: electrode steps create a “fluid conveyor belt” Theory: “3D” design is 20x faster (>

mm/sec at 3 Volts

) 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™ and a MPEG-4 Video decompressor are needed to see this picture.

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 3 m m KCl

Even faster, more robust pumps

Damian Burch & MZB, preprint arXiv:0709.1304

grooved “plated” “grooved” plated Grooved design amplifies the fluid 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