Thomas B. Jones
Professor of Electrical Engineering (Emeritus)
Ph.D., Massachusetts Institute of Technology, 1970
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Research

Laser Target Fabrication using Microfluidics   Publications

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.

DEP Microactuation & Droplet Dispensing   Publications

Dielectrophoresis (DEP) and electrowetting-based (EWOD) are of interest in the lab on a chip. One droplet transport and dispensing scheme uses an open geometry of co-planar electrodes patterned on a substrate. The system moves the liquid, typically water, very fast: >10 cm/sec. We have demonstrated dispensing of linear arrays of uniformly sized droplets of volumes from hundreds of nanoliters down to tens of picoliters. Scale-up to droplet arrays of >1000 droplets is possible. We have also employed optical sensing and feedback to control the dispensing process.

             

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

             

You may download a PDF file of the above (left) video showing operation of a DEP micronsiphon. When 60 kHz voltage is applied to the parallel co-planar strip electrodes, a narrow finger of water (de-ionized) emerges from the ~10 microliter droplet at lower left, travels quickly along the 100 micron gap between them, turns two corners and then stops at the end of the structure. This mechanism is not true pumping, but better thought of as a mechanism akin to capillary action.

Capillary instability divides the finger into uniform droplets when voltage is removed. Placing a 45 degree angle mirror beside the electrodes facilitates simultaneous observation of the motion of the finger from above and from a low angle. The video at the above right reveals considerable information about the finger dynamics and shows the droplet formation when the voltage is removed.

The video at right shows a two-stage droplet dispenser using four individually addressable electrodes. When voltage is first applied, an intermediate droplet of volume ~80 nanoliters collects at the upper left. Next, the electrode connections are changed, and rf voltage is again applied. Three separate droplets, each ~7 nanoliters, form rapidly in line and to the right of the intermediate droplet. The volume of the initial droplet at lower left, dispensed by a micropipette, is ~10 microliters.

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.

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Papers on E-field Induced Droplet Centering for Laser Target Fabrication


Papers on DEP Microactuation & Electrowetting


Microfluidics Patents


Papers on Ink Jet Physics


Papers on Electromechanics of Liquids


For J. R. Melcher's notes concerning electrical forces on dielectrics, CLICK HERE.

Books


Other research and educational activities

Electromechanics of Particles, Biological Dielectrophoresis, Levitation

Lecture Demonstrations on Electrostatics

On-line Interactive Nomograms


I don't do Facebook or Twitter.  CLICK HERE to learn one reason why. Buzz Aldrin at least seems to agree with me.

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Last modified on Thursday, 17-Apr-2014 11:06:20 EDT