Precision droplet dispensing Click on image to view video
DEP can be used to dispense large numbers of very small droplets very rapidly from a
single larger parent deposited manually with a micropipette. CLICK HERE to see an SEM
and photomicrograph of a structure (R. Ahmed) that
produced an array of 21 droplets of volume ~13 picoliter each. The three-electrode structure shown in the video at the left just above (R. Ahmed)
operates by (i) filling the entire length of the electrodes with liquid, (ii) trapping the static liquid rivulet to the left of
the T junction, and (iii) forming droplets at each bump by removal of the voltage. The video at the right, obtained by K. L. Wang, shows a similar structure fabricated
on a <100> Si wafer.
Other interesting phenomena Click on image to view video
In the video clip just above, ethylene glycol was actuated on strip electrodes in an insulating mineral oil bath.
The relative interfacial tension between these two liquids is quite low. As a result, when voltage is removed
the finger retracts back into the parent droplet to the left without forming droplets.
The siphon and two-stage droplet generator videos above were taken in 2000 by M. Gunji at Kyoto University.
Electromechanics of Liquids Publications
We have investigated the hydrostatics and dynamics of liquids under the influence of variable frequency electric
fields. EWOD and DEP liquid microactuation are, respectively, the low and high frequency limits of the electromechanical
response of conductive, dielectric liquids. A simple RC circuit model successfully predicts these limits and the critical frequency
that deliniates them. It may be shown that changes to the contact angle are not responsible for the motions
exploited in microfluidic applications. CLICK HERE to download an updated lecture presentation (2007)
that argues the case for an interpretation of EWOD as an electromechanical phenomenon.
Transient Motion Click images to view video
Transient E-field driven microfluidic flows have been investigated intensively at Rochester. One important finding is that for aqueous liquids the so-called
dynamic frictional force per unit length of the contact line seems to dominate viscous wall shear.
Surface Waves Click images to view video
We are also studying the effects of AC time-varying electric fields upon contact angle and displacement using the classic experimental geometry of Pellat:
parallel, vertical electrodes dipped into liquid. The high-speed videos above
(taken by K-L. Wang) show the motion of DI water when AC voltage is suddenly applied to Parylene-coated electrodes.
The liquid rises rapidly to approach the static equilibrium height. The applied frequency is 2 kHz for the video at the left
and 100 Hz for the one at the right. Some interesting surface dynamics, most evident at lower frequencies, are revealed in the videos below.
In these four videos, the upward direction is to the left. The surface waves motion depends strongly on the frequency of the applied voltage. At DC, no sloshing motion is
evident. For AC, strong surface vibrations and sloshing are evident. The sloshing is presumably due to parametric
EHD surface instability. At an electrode spacing of ~2 mm, virtually all surface wave dynamics are suppressed above ~1 kHz. Furthermore, at 10 kHz the contact angle is ~90 degrees
and the surface is almost flat.
Over the years, our research has been supported by the National Science Foundation (USA), the Japan Society for the Promotion of Science,
the National Institutes of Health, the Center for Future Health (Univ. of Rochester),
the Infotonics Technology Center, Inc., the Engineering and Physical Science Research Council (UK), NexPress Solutions, Inc.,
the Center for Electronic Imaging Science (Univ. of Rochester), Corning, Inc., the Laboratory for Laser Energetics
(Univ. of Rochester), General Atomics, Eastman Kodak, Inc., and Cypress Semiconductors, Inc.
