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Message in a bottle

For years, engineers have relied on physicist Daniel Ou-Yang’s pioneering work with a laser technique called “optical tweezers” to explore the complexity of cellular biomechanics. Now, he’s developing a new technique that promises to shed light on the particle- to-particle interactions that underlie a host of engineering applications.

Inside Lehigh’s biophotonics laboratory, Daniel Ou-Yang shines a green laser on tiny, fluorescently-labeled spheres of polystyrene suspended in a glass of water, and observes behavior that in humans would be called antisocial.

The spheres, measuring 60–100 nanometers, show no attachment for one another, no inclination to interact. Each particle is content to keep to itself.

Now Ou-Yang takes a stronger, infrared laser, aligns it collinearly with the green laser and shines it on the solution. The scene changes dramatically. Hundreds of particles that a moment earlier had shown no interest in each other now swarm into the micron-sized ellipse formed by the laser’s focal point.

Ou-Yang switches the infrared laser off and just as quickly the particles disperse. On and off he switches the laser, several times more, to watch the migration into, and the exodus from, the focal point of the laser.

Ou-Yang, professor of physics and associate director of Lehigh’s bioengineering program, calls the laser’s football-shaped focal point an “optical bottle.” He believes the bottle offers scientists and engineers a promising window to study the behavior of particles, and he has applied for a patent on his technique.

“By using light to create mechanical forces that can manipulate the physical behaviors of particles in surrounding media,” says Ou-Yang, “we think we have acquired a new and improved tool for studying particle-to-particle interactions in media. We can now calculate these interactions in situ, without needing to interrupt a process or remove the solution from a process.

“A better understanding of these interactions, coupled with the ability to manipulate the motions of the particles, is vital to industries that use nanoparticle-sized objects such as proteins, carbon nanotubes, pigments, gold particles. If we can attach these objects to the polystyrene spheres, or if we can manipulate them like we control the spheres, the processes of these industries could be refined to improve efficiency.

“Another potential application is diagnostics. If we can bind proteins, which are smaller than the latex particles, to those particles, we might achieve greater sensitivity in detecting those proteins.”

Starting with tweezers
Ou-Yang began a dozen years ago to explore the applications that become possible when light is harnessed to produce mechanical forces. Using a laser tool known as optical tweezers, he learned that, by focusing a laser beam through an objective lens in an optical microscope, he could manipulate individual micron-sized dielectric objects. Using mirrors to steer the laser beam before it entered the microscope, he was able to oscillate the objects and measure the micromechanical properties of the materials in which the objects were embedded.

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“We think we have acquired a new and improved tool for studying particle-to-particle interactions in media.” —Daniel Ou-Yang
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