Electromagnetic Field Guides Flow through Microfluidic Networks
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zero. In this way, the information in the probe pulse is “mapped” into the atomic coherence of the material, and can be read out subsequently by turning the coupling laser on again. The ability to store and recover the coherent light wave depends upon the spin coherence of the system as well as decoherence effects. Since the initial “ultraslow” light observations in “cold” and “hot” atomic gases and subsequent extensions to “trapping” light, there has been considerable speculation regarding potential applications such as quantum computing, novel optical devices, and advanced measurement techniques. Nevertheless, the gaseous nature of the light-trapping medium has posed an obvious practical limitation for a variety of these uses. The researchers believe that their use of a solid-state medium opens the door to potential practical applications that had previously not been feasible from a materials perspective. However, all of the work reported here has been performed on samples held at a temperature of 5 K. Practical applications may require operation at much higher temperatures. EMILY JARVIS
hydrophobic, respectively. The pressure difference across the fluid surface set the liquid-wall contact angle, θ, and surface curvature and was readily controlled through fluid flow velocity, gas pressure, or channel width. For fluid contact angles θ > 90°, the interface bulged upward in the center, creating a local electromagnetic potential minimum where the particles accumulated. Conversely, for θ < 90°, the interface lowered in the center, and particles moved toward the channel walls. Rapid particle separation of the ongoing stream was possible by switching from θ > 90° to θ < 90°. The researchers considered an annular
ring with a large radius to prove their claims both numerically and experimentally. They modeled the fluid surface profile by solving a differential equation that included the effects of the difference in density of the fluids, the fluid surface tension, and gravitational effects. They then overlaid the profile with a density plot of applied magnetic field. To test the theory, experiments were conducted with a transparent poly(dimethylsiloxane) ring channel (4-mm width) and 4.5-µm-diameter superparamagnetic beads. The results showed excellent agreement with theoretical predictions, with the same beads flowing at the channel center and around the
Electromagnetic Field Guides Flow through Microfluidic Networks Nonspecific binding to channel walls and particle clogging of microchannels are key obstacles that limit the application of microfluidic lab-on-a-chip technologies. In the February 25 issue of Applied Physics Letter, researchers G. Zabow and colleagues at Harvard University described a technique that allowed guided flow of particles through microchannels, even through those with arbitrary geometries, without particle adhesion to channel walls. The technique was an electromagnetic guided approach to attract transported particles toward the channel centers and eliminate particle adhesion to channel walls, c
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