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Electrodes for Microfluidic
Control and Sensing
C. K. Harnett

ECE Dept., University of Louisville, Louisville
KY USA
Thin-film vs. thick ―3D‖
electrodes
Thin-film (<1 micron)

Thick-film (>1 micron)

Induced-charge
electroosmotic mixing
 ...
Thin film: fine for counting droplets
Aqueous
disperse phase

conductive
droplet

Vmax

0
Center electrode voltage
vs drop...
Thin-film impedance sensing electrodes
can also detect particles in a flowing
ElectrodeElectrode
electrolyte
Flow

Cell (1...
But impedance-sensing
applications still benefit from 3D
electrodes
 Thick or cross-





channel electrodes
produce a ...
Induced-charge electroosmosis is
generally best with thick
electrodes
 Induced-charge
electro-osmosis
(ICEO) is a nonline...
How can 3D electrodes be made
without electroplating?
(Do the electrodes really need to be solid metal?)



Lithography o...
Lithography over topography:
isolated metal-coated posts in a
plastic chip

200 um
Harnett, C. K., Skulan, A. J., Hill, T....
Most streamlines are closed
loops—local mixing only

37 Hz
70 V p-p
1cm long channel
150 um post
diameter
Ion milling leaves metal on vertical
sidewalls, for isolated chargeable
pillars.
(a) Electrical and fluid feedthroughs pro...
Asymmetric posts can induce
pumping even in AC fields



Cross-channel pumping at triangular
obstacles can extend the bou...
A mixer with transverse electrodes
and triangular pillars was built and
tested

•(a) Simulation of dye loading in
the mixi...
Experiment and model show similar flow structures

Features in flow images (top row) are replicated in the model (bottom r...
Steady-state images of continuous mixing:
simulated and experimental

experimental

Power Off:
Incomplete
diffusional
mixi...
Global mixing at symmetric obstacles with ―blinking
vortex‖ splitting and recombination



Switching E-field direction p...
Global mixing by vortex splitting and
recombination
SEM: 250 um post diam

RMS Image

Starting from a crisp interface betw...



Meanwhile, asymmetric thin electrode pairs
can pump continuously using AC driving
signals.
Planar AC electroosmotic
(...
―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
...
Pop-up filaments can plate out
metal more efficiently than planar
ones

Planar device: plated
3D device: solution has
mate...
Rolled-up interdigitated electrodes
These tubes form
spontaneously from
surface stress when
released from the
substrate
But can these thin 3D structures
handle the lab-on-chip life?



Structures survive drying if comparable to or
shorter th...
Look at a different 3D improvement to
the ACEO pump: the ―fluid conveyor
belt‖ This 3-D ACEO pump is a relatively recent
d...
Shadow evaporation method makes
isolated, stepped conducting features
The tall feature casts a shadow
that creates two distinct circuits
100 micron
Voltage contrast electron
microscopy shows interdigitation
+Voltage

Charged electrodes
Uncharged electrodes

Ground

100 ...
Flow velocity was measured with
2 micron tracer particles in DI
water

2.5cm

PDMS

1cm
The resulting pump is
comparable to those made by
other methods
Electrode wrapping method

Shadow evaporation method
Elect...


Lithography over topography




Ion milling

Lifting up a thin-film
pattern


Shadow evaporation
Acknowledgments








Yehya Senousy, Evgeniya Moiseeva, Tom
Lucas, Jasmin Beharic, Rebecca Scott: students
who cont...
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AES2013 Harnett plenary talk: Electrodes for microfluidic applications

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Four ways to make metal 3D electrodes for microfluidics, without electroplating. The slides show examples from induced-charge electroosmosis and particle counting.

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AES2013 Harnett plenary talk: Electrodes for microfluidic applications

  1. 1. Electrodes for Microfluidic Control and Sensing C. K. Harnett ECE Dept., University of Louisville, Louisville KY USA
  2. 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. 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. 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. 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. 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. 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. 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. 9. Most streamlines are closed loops—local mixing only 37 Hz 70 V p-p 1cm long channel 150 um post diameter
  10. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 20. Rolled-up interdigitated electrodes These tubes form spontaneously from surface stress when released from the substrate
  21. 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. 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. 23. Shadow evaporation method makes isolated, stepped conducting features
  24. 24. The tall feature casts a shadow that creates two distinct circuits 100 micron
  25. 25. Voltage contrast electron microscopy shows interdigitation +Voltage Charged electrodes Uncharged electrodes Ground 100 micron
  26. 26. Flow velocity was measured with 2 micron tracer particles in DI water 2.5cm PDMS 1cm
  27. 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. 28.  Lithography over topography   Ion milling Lifting up a thin-film pattern  Shadow evaporation
  29. 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?

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