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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‖
electrodesThin-film (<1 micron) Thick-film (>1 micron)
Sample stacking
Impedance based
particle detection
Metering droplets
Creating ion pulses
Induced-charge electroosmotic mixing
AC electroosmoticpumping using metal sidewalls
Impedance based particle sizing
Center
electrode
Aqueous
disperse phase
Oil
continuous
phase
V 0V
Cen
ter e
lectro
de
volta
ge
vs d
rople
t positio
n
max
Vm
ax
conductive
droplet
0Moiseeva, E. V. and Harnett, C. K., ―Shear-Based Droplet
Production for Biomaterial Printing,‖ Proceedings of Digital
Fabrication 2009, Louisville, KY September 21-25, 2009,
pages 806-809.
Thin film: fine for counting droplets
An insulating particle interrupts the electric field and produces a
resistance spike. Spike height is related to particle volume.
Cell (12 micron
diameter)
ElectrodeElectrode
Flow
Thin-film impedance sensing electrodes can also detect particles in a flowing electrolyte
Scott, R., Sethu, P., and Harnett, C. K., Review of Scientific Instruments 79, 046104, 2008
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 peak-height dependence on vertical location
Then you can make better histograms of particle sizes
Ph.D. Thesis: C. Bernabini, U. Southampton (2010) andS. Gawad, K. Cheung, U. Seger, A. Bertsch, and P. Renaud. Dielectric spectroscopy in a micromachined flow cytometer: theoretical and practical considerations.Also Lab on a Chip, 4(3):241–251, 2004.
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, 066101/1-
4 (2004).
2) T. M. Squires and M. Z. Bazant, J.
Fluid Mech. 509, 217 (2004).
Induced-charge electroosmosis is
generally best with thick
electrodes
How can 3D electrodes be made
without electroplating?
Lithography over topography
Ion milling
Lifting up a thin-film
pattern
Shadow evaporation
(Do the electrodes really need to be solid metal?)
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
Analysis Systems vol. 1 82-85, 2005
200 um
Lithography over topography: isolated metal-coated posts in a plastic chip
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 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.
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) Simulation of dye loading in
the mixing channel by pressure-
driven 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)
A mixer with transverse electrodes
and triangular pillars was built and
tested
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).
Experiment and model show similar flow structures
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.
Steady-state images of continuous mixing:
simulated and experimental
Power Off:
Incomplete
diffusional
mixing
Power On:
Complete
ICEO-based
mixing
experimental
experimental
calculated
calculated
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 peanut-
shaped, shared vortices at
each post
Global mixing by vortex splitting and
recombination
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.
RMS Image
SEM: 250 um post diam
Planar AC electroosmotic
(ACEO) pump1 based on asymmetric inter-digitatedelectrode 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
Meanwhile, asymmetric thin electrode pairs
can pump continuously using AC driving
signals.
Can we wrap the walls of a channel with this
asymmetric pattern so that all surfaces are pumping
surfaces?
―Pop-up‖ method lifts electrodes out of
plane. Structures can have contact
pads.
Moiseeva, E., Senousy, Y. M., McNamara, S., and
Harnett, C. K., "Single-mask microfabrication of three-
dimensional objects from strained bimorphs," J. Micromech.
Microeng. 17, N63-68, 2007
a b c
atm atm+4.5psi
atm+8.5psi
300 mm
Pop-up filaments can plate out
metal more efficiently than planar
ones
Planar device: plated
material shows diffusion-
limited dendrites
3D device: solution has
access to electrodes
from a larger solid
angle, no dendritesHarnett, C. K., Lucas, T. M., Moiseeva, E.
V., Casper, B., and Wilson, L., Proc IEEE
I2MTC 2010, pages 328-331, DOI
10.1109/IMTC.2010.5488211
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 than the elastocapillary length. The above
structures at 300 microns are about 2x the
elastocapillary length. They clump together upon
drying.
1 C.Huang,M. Z. Bazant and T.Thorsen , Lab on a Chip 2010,6,80-85
Look at a different 3D improvement to the ACEO pump: the ―fluid conveyor belt‖
• ―Fluid Conveyor Belt‖ concept: Co-operating 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
This 3-D ACEO pump is a relatively recent design1 that is about 10x faster than the the planar version.
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
100 micron
+Voltage
Ground
Charged electrodes
Uncharged electrodes
Voltage contrast electron
microscopy shows interdigitation
100 micron
PDMS
1cm
2.5cm
Flow velocity was measured with
2 micron tracer particles in DI
water
Comparison between the velocity of flow of the planar and 3D ACEO pumps at 2Vpp
Electrode wrapping method
Shadow evaporation method
Electroplating method
Planar ACEO pump
Senousy, Y. M. and Harnett, C. K. (2010)
Biomicrofluidics 4 036501, DOI: 10.1063/1.3463719
The resulting pump is
comparable to those made by
other methods
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 contributed to this work at the University of Louisville
University of Louisville cleanroom staff
Martin Bazant, MIT: ICEO discussions
Mike Kanouff, Katherine Dunphy-Guzman, 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?