Micro-fluidic Applications of Induced-Charge Electro-osmosis

1. Micro-fluidic Applications of Induced-Charge Electro-osmosis Jeremy Levitan Mechanical Engineering, MIT Todd Squires Applied Mathematics, CalTech Martin Schmidt Electrical Engineering, MIT Martin Bazant Applied Mathematics, MIT Todd Thorsen Mechanical Engineering, MIT

2. Pumping in Micro-Fluidics • Mechanical pumping • Robust • Poor scaling: U ~ h2P/  • Bulky external pressure source • Shear dispersion • Capillary electro-osmosis • Material sensitive • Plug flow: U = 100 um/sec in E = 100 V/cm • Linear: <U> = 0 in AC • DC requires Faradaic reactions => hydrolysis • Need large V for large E along channel

3. Mixing in Micro-Fluidics • Diffusion down a channel: • with EO Jacobson, McKnight, Ramsey (1999) • Serpentine channels Mengeaud et al (2002) • Geometric splitting Schonfeld, Hessel, and Hofmann (2004), Wang et al (2002) • Passive recirculation Chung et al (2004) • Pressure-driven flow with chaotic streamlines: Johnson et al (2002), Stroock et al (2002) • AC Electro-osmosis Studer, Pepin, Chen, Ajdari (2002) • Electrohydrodynamic Mixing Oddy, Santiago and Mikkelsen (2001), Lin et al, Santiago (2001) • Micro peristaltic pumps (moving walls) (Schilling 2001) (Stroock 2002)

4. Induced-Charge Electro-Osmosis Nonlinear slip at a polarizable surface Example: An uncharged metal cylinder in a suddenly applied DC field Metal sphere: V. Levich (1962); N. Gamayunov, V. Murtsovkin, A. Dukhin, Colloid J. USSR (1984). E-field, t = 0 E-field, t » charging time Steady ICEO flow induced ~ E a MZB & TMS, Phys, Rev. Lett. 92, 0066101 (2004); TMS & MZB, J. Fluid. Mech. 509, 217 (2004).

5. A Simple Model System • 100um dia. platinum wire transverse to PDMS polymer microchannel (200um tall, 1mm wide); • 0.1 - 1mM KCl with 0.01% by volume 0.5um fluorescent latex particles; • Sinusoidal voltage (10 - 100V) excitation, 0 DC offset; Applied 0.5cm away from center wire via gold and/or platinum wires; V Cross-section of experiment

6. Simple Mathematical Model 1. Electrochemical problem for the induced zeta potential Bazant, Thornton, Ajdari, Phys. Rev. E (2004) Steady-state potential, electric field after double layer charging 2. Stokes flow driven by ICEO slip Steady-state Stokes flow Simulation is of actual experimental geometry

7. Voltmeter Function Generator Viewing Resistor Platinum Wire Viewing Plane KCl in PDMS Microchannel Inverted Optics Microscope Bottom View 200 um X 1 mm X 1mm Channel

8. ICEO Around A 100 µm Pt Wire

9. Particle Image Velocimetry 500 nm seed particles Slide used with permission of S. Devasenathipathy

10. PIV Mean Velocity Data • PIV measurement with 0.01% volume dielectric (fluorescent) tracer particles • Correct scaling, but inferred surface slip smaller from simple theory by 10 Metal colloids: Gamayunov, Mantrov, Murtsovkin (1992)

11. Frequency Dependence • At “fast” frequencies, double layer not fully charged; • Consistent with “RC” charging • U ~ U0/(1 + (/c)2) c = 2 d a/D = 1/c = 3 ms Experiments in 1 mM KCl at 75 V

12. Extensions to Model All reduce predicted velocities • Surface Capacitance/Contamination: multi-step cleaning for metal surfaces; • Surface Conductance: • Visco-electric effect

13. Current Work • Fixed potential posts; • Post-array mixers; • Asymmetric objects; • Integration with microfluidic devices -- • microchannels and valves; • DNA hybridization arrays;

14. Induced-Charge Electro-osmosis • Demonstrated non-linear electro-osmosis at polarizable (metal) surfaces • Sensitive to frequency, voltage, etc. • At low concentration (<1mM), no concentration dependence, but U decreases at higher c • Advantages in microfluidics: • Time-dependent local control of streamlines • Requires small AC voltages, transverse to channels • Compatible with silicon fabrication technology • Disadvantages: • Sensitive to surface contamination, solution chemistry • Relatively weak for long-range pumping • Additional movies/data: http://media.mit.edu/~jlevitan/iceo.html • Papers: http://math.mit.edu/~bazant