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?