In collaboration with the Laboratory for Laser Energetics, we are exploiting electrowetting and liquid dielectrophoresis to fabricate and process targets for laser-driven inertial fusion. The three main thrusts are (i) electric field-mediated droplet centering; (ii) assembly line formation of double-emulsion droplets; (iii) manipulation & metering of cryogenic deuterium for target fueling.
Forming Concentric Double-Emulsion Droplets Click images to view video
The videos above demonstrate centering one liquid droplet inside another using a uniform AC electric field. Centering requires that
the relative dielectric constant of the suspending liquid is lower than that of the outer shell. An application of this technique is
in the formation of highly concentric polymer foam shells for laser targets. For 3 to 6 mm diameter droplets, centering occurs in ~60 seconds, with E ~ 10E4 V/m.
A stronger field achieves more rapid centering
at the expense of increased ellipsoidal distortion. The required AC frequency depends on the electrical conductivity of the outer shell
and its thickness. For the polymer chemistries of interest in foam shell fabrication, f ~ 20 MHz.
The videos above shows various microfluidic structures with electrodes patterned on one of the pair of parallel, transparent, InSn oxide-coated glass substrates. Electrode spacing ~ 100 microns & width ~ 200 microns. The liquid is DI water, the AC voltage frequency is ~100 kHz, and the applied voltage is ~200 V-rms. The structures are viewed normally through the pair of transparent electrodes. The spiral and stepped structures (middle and on right) nicely visualize the largely uncoupled roles exhibited by capillarity and liquid DEP. With voltage on, the water is drawn along the narrow patterned electrodes by the DEP force and then stops when it gets to the end. When the voltage is removed, the electrical force is no longer present and so now wetting causes the liquid to spread out and capillarity takes over. It is best not to think of liquid DEP as a pumping mechanism. Rather, the non-uniform electric field estabishes an electric field-coupled hydrostatic equilibrium. When the field is on, capillarity only influences the liquid meniscus locally between the parallel, transparent electrodes; it is the DEP force due to the non-uniform E field that contains the liquid.
Basics of coplanar DEP microactuation Click on image to view video
Precision droplet dispensing Click on image to view video
Other interesting phenomena Click on image to view video
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.
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.
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.
Electromechanics of Particles, Biological Dielectrophoresis, Levitation
Lecture Demonstrations on Electrostatics
On-line Interactive Nomograms
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