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AES2013 Harnett plenary talk: Electrodes for microfluidic applications
- 1. Electrodes for Microfluidic Control and Sensing C. K. Harnett ECE Dept., University of Louisville, Louisville KY USA
- 2. Thin-film vs. thick ―3D‖ electrodes Thin-film (<1 micron) Thick-film (>1 micron) Induced-charge electroosmotic mixing AC electroosmotic pumping using metal sidewalls Impedance based particle sizing Sample stacking Impedance based particle detection Metering droplets Creating ion pulses
- 3. Thin film: fine for counting droplets Aqueous disperse phase conductive droplet Vmax 0 Center electrode voltage vs droplet position Oil continuous phase Center electrode V 0V max Moiseeva, E. V. and Harnett, C. K., ―Shear-Based Droplet Production for Biomaterial Printing,‖ Proceedings of Digital Fabrication 2009, Louisville, KY September 21-25, 2009,
- 4. Thin-film impedance sensing electrodes can also detect particles in a flowing ElectrodeElectrode electrolyte Flow Cell (12 micron diameter) An insulating particle interrupts the electric field and produces a resistance spike. Spike height is related to particle volume. Scott, R., Sethu, P., and Harnett, C. K., Review of Scientific Instruments 79, 046
- 5. But impedance-sensing applications still benefit from 3D electrodes Thick or cross- channel electrodes produce a more uniform electric field than planar electrodes This reduces peakheight dependence on vertical location Then you can make better histograms of Ph.D. Thesis: C. Bernabini, U. Southampton particle sizes (2010) and K. Cheung, U. Seger, A. Bertsch, and P. S. Gawad, Renaud. Dielectric spectroscopy in a micromachined flow cytometer: theoretical and
- 6. Induced-charge electroosmosis is generally best with thick electrodes Induced-charge electro-osmosis (ICEO) is a nonlinear electrokinetic effect. Charges separate near a polarized metal object and are moved by the field, dragging the surrounding fluid. The same flow pattern appears when the field direction is reversed. Illustration of ICEO phenomenon References: 1) M. Z. Bazant and T. M. Squires, Phys. Rev. Lett. 92, 066101/14 (2004). 2) T. M. Squires and M. Z. Bazant, J. Fluid Mech. 509, 217 (2004).
- 7. How can 3D electrodes be made without electroplating? (Do the electrodes really need to be solid metal?) Lithography over topography Ion milling Lifting up a thin-film pattern Shadow evaporation
- 8. Lithography over topography: isolated metal-coated posts in a plastic chip 200 um Harnett, C. K., Skulan, A. J., Hill, T. F., L.M. Barrett, G.J. Fiechtner, and E.B. Cummings, ―Microparticle mixing and separation by nonlinear electrokinetic effects in microfluidic channels,‖ Proceedings of Ninth International Conference on Micro Total
- 9. Most streamlines are closed loops—local mixing only 37 Hz 70 V p-p 1cm long channel 150 um post diameter
- 10. Ion milling leaves metal on vertical sidewalls, for isolated chargeable pillars. (a) Electrical and fluid feedthroughs produced by chemical etching in low-conductivity silicon. (b) Through-wafer metal contacts made to high conductivity silicon. (c). Posts cut into high-conductivity silicon by reactive ion etching, then conformally coated with metal by sputtering. (d) Ion milling leaves metal only on the post sidewalls. (e) The channel seals with an interlocking elastomer lid.
- 11. Asymmetric posts can induce pumping even in AC fields Cross-channel pumping at triangular obstacles can extend the boundary between co-flowing fluids M. Z. Bazant and T. M. Squires, Phys. Rev. Lett. 92, 066101/1-4 (2004).
- 12. A mixer with transverse electrodes and triangular pillars was built and tested •(a) Simulation of dye loading in the mixing channel by pressuredriven flow. 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. • (d) Microfabricated device consisting of vertical gold-coated silicon posts and sidewall electrodes in an insulating channel. (Channel width 200 um, depth 300 um)
- 13. Experiment and model show similar flow structures 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).
- 14. Steady-state images of continuous mixing: simulated and experimental 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 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.
- 15. Global mixing at symmetric obstacles with ―blinking vortex‖ splitting and recombination Switching E-field direction periodically will create new vortex array A particle’s path depends greatly on its position when switching occurs We saw that the vortices around symmetric posts were closed loops, only good for local stirring. Most of the fluid stays trapped in its original vortex. •Horizontal electric field produces four triangular vortices at each post. •Diagonal electric field produces peanutshaped, shared vortices at each post
- 16. Global mixing by vortex splitting and recombination SEM: 250 um post diam RMS Image Starting from a crisp interface between beads and electrolyte solution, the 70V, 54 Hz electric field is switched from horizontal to diagonal every 2.5 s. Beads are ―mixed‖ and able to escape their original vortex.
- 17. Meanwhile, asymmetric thin electrode pairs can pump continuously using AC driving signals. Planar AC electroosmotic (ACEO) pump1 based on asymmetric inter-digitated electrode arrays2 • Net forward pumping over frequency range(0.5-100 KHz). • Working fluid is DI water. • Maximum speed of flow is120 um/sec at Vrms=1.2 V and f=1khz. 1 A. Ramos, H. Morgan, N. G. Green, and A. Castellanos, J. Colloid Interface Sci. 217, 420 (1999). 2 A. B. D. Brown, C. G. Smith and A. R. Rennie, Phys. Rev. E Stat,2000,63,016305 Can we wrap the walls of a channel with this asymmetric pattern so that all surfaces are pumping surfaces?
- 18. ―Pop-up‖ method lifts electrodes out of plane. Structures can have contact pads. atm 300 mm a atm+4.5 psi b atm+8.5 psi c Moiseeva, E., Senousy, Y. M., McNamara, S., and Harnett, C. K., "Single-mask microfabrication of threedimensional objects from strained bimorphs," J. Micromech.
- 19. Pop-up filaments can plate out metal more efficiently than planar ones Planar device: plated 3D device: solution has material shows diffusion- access to electrodes limited dendrites from a larger solid angle, no dendrites Harnett, C. K., Lucas, T. M., Moiseeva, E. V., Casper, B., and Wilson, L., Proc IEEE I2MTC 2010, pages 328-331, DOI
- 20. Rolled-up interdigitated electrodes These tubes form spontaneously from surface stress when released from the substrate
- 21. But can these thin 3D structures handle the lab-on-chip life? Structures survive drying if comparable to or shorter than the elastocapillary length. The above structures at 300 microns are about 2x the elastocapillary length. They clump together upon
- 22. Look at a different 3D improvement to the ACEO pump: the ―fluid conveyor belt‖ This 3-D ACEO pump is a relatively recent design1 that is about 10x faster than the the planar version. • ―Fluid Conveyor Belt‖ concept: Cooperating vortices at stepped electrode pairs. • Net forward pumping occurs over the frequency range 0.5-100 KHz • Peak flowspeed (≈1.3 mm/sec) at 1.06 Vrms and f=1kHz using DI water 1 C.Huang,M. Z. Bazant and T.Thorsen , Lab on a Chip 2010,6,80-85 Can we build this by depositing metal on a polymer substrate, even an injection molded substrate?
- 23. Shadow evaporation method makes isolated, stepped conducting features
- 24. The tall feature casts a shadow that creates two distinct circuits 100 micron
- 25. Voltage contrast electron microscopy shows interdigitation +Voltage Charged electrodes Uncharged electrodes Ground 100 micron
- 26. Flow velocity was measured with 2 micron tracer particles in DI water 2.5cm PDMS 1cm
- 27. The resulting pump is comparable to those made by other methods Electrode wrapping method Shadow evaporation method Electroplating method Planar ACEO pump Comparison between the velocity of flow of the planar and 3D ACEO pumps at 2 Senousy, Y. M. and Harnett, C. K. (2010) Biomicrofluidics 4 036501, DOI: 10.1063/1.3463719
- 28. Lithography over topography Ion milling Lifting up a thin-film pattern Shadow evaporation
- 29. Acknowledgments Yehya Senousy, Evgeniya Moiseeva, Tom Lucas, Jasmin Beharic, Rebecca Scott: students who contributed to this work at the University of Louisville University of Louisville cleanroom staff Martin Bazant, MIT: ICEO discussions Mike Kanouff, Katherine DunphyGuzman, Jeremy Templeton,Tyrone Hill, Andrew Skulan, Eric Cummings, Chris Moen, Jim Van de Vreugde, Dan Yee at Sandia National Laboratories contributed to simulations, microfluidics, and electronics Jerry Drumheller and Rob Ilic at the Cornell Nanoscale Science and Technology Facility for ion milling and fabrication discussions Questions?
Editor's Notes
- I am talking about chemically inert solid electrodes that come into contact with the sample. Other approaches not covered here are using electrolyte(plus a barrier) or liquid metal or polymers stuffed with carbon particles.Electrode-equipped glass chip from Harnett, C. K., Mosier, B. P., Caton, P. F., Wiedenman, B., and Crocker, R. W., Conductivity pulse time-of-flight flow sensor for sub-microliter/minute flow rates. Proceedings of Seventh International Conference on Micro Total Analysis Systems vol. 1 139-142, 2003
- Also, the flow is 3D, it’s harder to analyze and simulate because of the tapering volcano shaped pillars
- Dye: 70,000 MW Dextrans with Texas Red fluorophore (neutral charge)Low-conductivity electrolyte: 0.1 mMKClA single syringe pump sent dyed and undyedKCl through the channel at 1-10 microliters per minuteDye concentration before and after mixing was evaluated by video microscopy through the optically transparent PDMS lid Particle imaging velocimetry experiments were also carried out, using fluorescent microparticles
- Previous ion milling method won’t make metal surfaces with different orientations
- Elastocapillary length is sqrt (B/surface tension) where b is Eh^3/12(1-nu^2) where nu is Poisson’s ratio h is material thickness and E is Young’s modulus of the pop up structure.For these, the elastocapillary length is about 120 microns.We were able to use popped-up electrodes to plate out metal from solutionHarnett, C. K., Lucas, T. M., Moiseeva, E. V., Casper, B., and Wilson, L., "Microscopic containers for sample archiving in environmental and biomedical sensors," Proceedings of IEEE International Instrumentation and Measurement Technology Conference, Austin, Texas, May 3-5, 2010, pages 328-331, DOI 10.1109/IMTC.2010.5488211
- If you made your features by injection molding, congratulations, start at step f
- At 100x lower cost than electroplatingThe top curve is from Lab Chip – Huang Bazant & Thorsen 2010, Ultrafast –they used lithography to wrap SU-8 bars with a coating. Then you get the back sidewall coated too.