Bitty Beacon: Wee disks probe materials at microscales

Researchers charting the microscopic properties of materials now have tiny lighthouses to guide them. Illuminated by lasers, disks no larger than red blood cells can project rotating beams bright enough to create a light show in a darkened room, a team of university and industrial physicists has found.

WITH A TWIST. Sequence of frames shows rotation of a microscopic disk driven by a red laser beam. As the beam bounces off the disk, it forms back-to-back beacons. Z. Cheng et al./Physical Review Letters

Those shimmering disks may enable researchers to measure the elasticity and other characteristics of soft materials, including those inside living cells, says team member Paul M. Chaikin of Princeton University. The disks may also serve as components of micromachines such as pumps and optical switches.

Tiny plastic spheres have long been important laboratory tools for manipulating the microscopic realm. Attached to molecules, for instance, the beads act as handles that researchers can grip with laser beams in a technique known as optical tweezing (SN: 4/26/97, p. 256).

Theorists had predicted that disk-shape particles couldn’t be trapped by laser beams the way tiny beads can be. Yet the calculations didn’t consider disks that are thinner than the wavelength of the laser light, says Thomas G. Mason, a member of the research team at the ExxonMobil Research and Engineering Company in Annandale, N.J.

Mason had chanced upon such disks in a suspension of microspheres that he’d made from the wax known as alpha-eicosene. His ExxonMobil coworker Zhengdong Cheng then put a solution containing the disks under a microscope and illuminated the sample with laser tweezers. Cheng found that the light would not only grab a disk but also bounce off the disk in twin, opposite-facing beams, like those of a lighthouse.

When the researchers used so-called circularly polarized laser beams, whose electromagnetic fields corkscrew through space, the light caused the disks–and their twin beams–to spin. Reversing the orientation of the corkscrew made the disks spin the other way. A single rotating disk projects visible red streaks onto the ceiling of the unlit lab, Mason says.

David A. Weitz of Harvard University rates the work as “a really cool demonstration of a lot of things that you wouldn’t have expected.”

Microdisks can serve as probes of elasticity, viscosity, and other features of tiny regions of material because those characteristics affect the disks’ easily observed angular rotation, Chaikin says. Also, the disks could serve as rotating components in micromachines, he adds.

Chaikin, Mason, and Cheng, who is now at DiCon Fiberoptics in Richmond, Calif., describe their microdisk experiments in the Sept. 2 Physical Review Letters.

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