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.
AES2013 Harnett plenary talk: Electrodes for microfluidic applications
Electrodes for Microfluidic
Control and Sensing
C. K. Harnett
ECE Dept., University of Louisville, Louisville
Thin-film vs. thick ―3D‖
Thin-film (<1 micron)
Thick-film (>1 micron)
pumping using metal
Creating ion pulses
Thin film: fine for counting droplets
Center electrode voltage
vs droplet position
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,
Thin-film impedance sensing electrodes
can also detect particles in a flowing
Cell (12 micron
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
applications still benefit from 3D
Thick or cross-
produce a more
field than planar
This reduces peakheight dependence
on vertical location
Then you can
histograms of Ph.D. Thesis: C. Bernabini, U. Southampton
particle sizes (2010) and K. Cheung, U. Seger, A. Bertsch, and P.
Renaud. Dielectric spectroscopy in a
micromachined flow cytometer: theoretical and
Induced-charge electroosmosis is
generally best with thick
(ICEO) is a nonlinear
near a polarized
metal object and are
moved by the
field, dragging the
The same flow
when the field
direction is reversed.
Illustration of ICEO phenomenon
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).
How can 3D electrodes be made
(Do the electrodes really need to be solid metal?)
Lithography over topography
Lifting up a thin-film
Lithography over topography:
isolated metal-coated posts in a
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
Most streamlines are closed
loops—local mixing only
70 V p-p
1cm long channel
150 um post
Ion milling leaves metal on vertical
sidewalls, for isolated chargeable
(a) Electrical and fluid feedthroughs produced by
chemical etching in low-conductivity silicon.
(b) Through-wafer metal contacts made to high
(c). Posts cut into high-conductivity silicon by
reactive ion etching, then conformally coated with
(d) Ion milling leaves metal only on the post
(e) The channel seals with an interlocking
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).
A mixer with transverse electrodes
and triangular pillars was built and
•(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)
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).
Steady-state images of continuous mixing:
simulated and experimental
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.
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
We saw that the vortices around
symmetric posts were closed
loops, only good for local stirring.
Most of the fluid stays trapped in its
•Horizontal electric field
produces four triangular
vortices at each post.
•Diagonal electric field
produces peanutshaped, shared vortices at
Global mixing by vortex splitting and
SEM: 250 um post diam
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
Meanwhile, asymmetric thin electrode pairs
can pump continuously using AC driving
Planar AC electroosmotic
(ACEO) pump1 based on
• Net forward pumping
• 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
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
―Pop-up‖ method lifts electrodes out of
plane. Structures can have contact
Moiseeva, E., Senousy, Y. M., McNamara, S., and
Harnett, C. K., "Single-mask microfabrication of threedimensional objects from strained bimorphs," J. Micromech.
Pop-up filaments can plate out
metal more efficiently than planar
Planar device: plated
3D device: solution has
material shows diffusion- access to electrodes
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
Rolled-up interdigitated electrodes
These tubes form
surface stress when
released from the
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
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
―Fluid Conveyor Belt‖ concept: Cooperating vortices at stepped
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
1 C.Huang,M. Z. Bazant and T.Thorsen , Lab on a Chip
Can we build this by depositing metal on a polymer
substrate, even an injection molded substrate?
Shadow evaporation method makes
isolated, stepped conducting features
The tall feature casts a shadow
that creates two distinct circuits
Voltage contrast electron
microscopy shows interdigitation
Flow velocity was measured with
2 micron tracer particles in DI
The resulting pump is
comparable to those made by
Electrode wrapping method
Shadow evaporation 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
Lithography over topography
Lifting up a thin-film
Yehya Senousy, Evgeniya Moiseeva, Tom
Lucas, Jasmin Beharic, Rebecca Scott: students
who contributed to this work at the University of
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