Lasers are one of the iconic breakthroughs of 20th century science. They produce coherent photons in tight beams of specific energy. They can transmit data, detect molecules, and burn through metal. The photons they produce also have significant momentum.
And that raises an interesting question. Is it possible to transfer this momentum to generate matter rays such as a liquid flow? Not until now.
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Jiming Bao at the University of Houston in Texas and a few pals say they have discovered an entirely new optofluidics process that makes it possible to use a laser beam to create a stream of liquid. The technique has widespread applications in microfluidics, biochemistry, microfabrication, and any process that depends on lab-on-chip technology.
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What Bao and co have discovered is a way of generating tightly focused liquid streams inside a fluid. This discovery is something of a surprise.
Laser light does not usually interact with water, except at an interface with another medium, such as air. Photons can “push” against such an interface, although the momentum transfer is small and certainly too weak to drive fluid flow.
However, Bao and co found they could generate a stream of water inside a bigger volume of water if it contained gold nanoparticles. They shined a pulsed green laser through the glass wall of the container and, after a few minutes, observed a current of liquid flowing rapidly along the direction of the beam.
“The flows appear as liquid analogues of laser beams and move in the same directions of the refracted beams as if they are directly driven by photons of laser beams,” say Bao and co. “We call this phenomenon laser streaming.”
This is something of a surprise, and the nanoparticles are key. If the water is pure—with no added nanoparticles—the laser beam passes through unhindered with no streaming at all.
Bao and co have to work hard to determine what’s going on. It turns out that the nanoparticles significantly absorb green light, which is close to the resonant frequency of the electrons they contain.
This causes the particles to heat up and cool down with each pulse of light, expanding and contracting in the process. That generates acoustic wavs in the water. This kind of ultrasound has long been known to move liquid in a process called acoustic streaming.
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But ultrasound by itself doesn’t guarantee liquid motion. So something else must be going on. Bao and co say the heating and cooling of nanoparticles near the container wall causes them to bond with the glass. Over time, the nanoparticles become encrusted around the point where the laser enters the liquid and this creates a kind of nanocavity on the glass.
The nanocavity is the key to this phenomenon. By wondrous coincidence, the cavity is just the right size and shape to focus the ultrasound being generated by the encrusted nanoparticles. In other words, the cavity becomes a resonant chamber—a loudspeaker—that generates a beam of ultrasound. Bao and co say the liquid stream is driven by this focused, directional ultrasound.
That’s a fascinating discovery that links nanophotonics, microfluidics, acoustics, and materials science. And it has significant implications. The ability to move liquids on a microscopic scale is crucial for all kinds of lab-on-a-chip experiments. It is also useful for nanofabrication and even for laser propulsion.
Bao and co are optimistic about the future. “Laser streaming will find applications in optically controlled or activated devices such as microfluidics, laser propulsion, laser surgery and cleaning, mass transport or mixing, to name just a few,” they say.
