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Physics 11, 16
A micrometer-sized sphere trapped by optical tweezers in a liquid, under the right conditions, orbits rapidly around the laser beam—creating a potential micromixing device.

Researchers have built a variety of tiny motors that convert various forms of energy into motion. The latest example is a spherical particle that rapidly orbits around a laser beam. The researchers say that future designs could allow the motion to be coupled to other microscale components and might be used, for example, to mix fluids in future lab-on-a-chip devices.

A few years ago, Pedro Quinto-Su of the National Autonomous University of Mexico created what he called a microscopic steam engine [1]. He trapped a small particle in a standard optical tweezers—a focused laser beam that can hold a particle at its most intense point. The particle was in water, and when heated with enough laser power, the adjacent water boiled and produced a vapor bubble that pushed the particle away from the beam axis. Then the particle cooled down and was pulled back by the optical tweezers when the bubble was reabsorbed. The result was a particle that continually hopped back and forth.

Giovanni Volpe of the University of Gothenburg, Sweden, and his colleagues set out to build what they thought would be an improved version of Quinto-Su’s engine, substituting a critical liquid mixture for the water. A critical mixture consists of two liquids that are just below a temperature at which they separate, or “demix.” With this substitution, the researchers found that instead of moving back and forth, the particle circled around the beam axis.

The mixture consisted of water and about 30% 2,6-lutidine, an organic compound. In these proportions, the two liquids mix when cold, but as they are heated above about $34{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C}$, they develop local concentration variations and then demix completely. The researchers immersed 2.48-micrometer-diameter spheres of silica with iron oxide inclusions in this liquid mixture at temperatures a few degrees below the critical value. They then trapped one of these spheres in an optical tweezers.

The iron oxide in the sphere allowed it to absorb light energy and heat up, and if it wandered a bit away from the center of the beam, the side closer to the beam center became hotter. This bit of heat increased the local concentration of lutidine relative to water on that side of the sphere, which nudged the sphere in the opposite direction because its surface was hydrophilic (attracted to water). So the sphere usually found a stable location with its center somewhat less than a micrometer away from the central axis of the laser beam.

In addition, irregular heating resulting from the nonuniform distribution of iron oxide within the sphere created a lateral force that pushed the sphere to rotate about the laser beam. The researchers conducted a series of tests in which they kept the liquid at $26{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C}$ and gradually increased the laser power up to a few milliwatts (mW). At low power, below about 2 mW, heating was not enough to cause the liquid to demix, and the sphere remained stationary. As the power rose and local heating increased, the sphere moved off-center and began circling the beam slowly but erratically. On occasion, the sphere drifted toward the axis and then started orbiting in the opposite direction. This happened, the researchers say, because Brownian motion disturbed the particle’s motion and orientation.

With laser power at 2.7 mW, the rotation became stable at 1160 revolutions per minute, with the center of the sphere 1 micrometer off-axis. This motion would continue indefinitely, says Volpe. At 3.2 mW, the motion became erratic again, and at still higher power the force due to demixing knocked the sphere out of the optical tweezers altogether. The team also performed numerical calculations that quantitatively agreed with the observed behavior, confirming their understanding of the forces involved.

Appropriate design of an asymmetric sphere should make it possible to force the particle to circle in a preferred direction, the researchers say. Volpe imagines the revolving particle could mix the tiny drops of fluid used in lab-on-a-chip devices, where natural mixing is limited because of the lack of turbulence found in such confined spaces. And it should not be difficult to couple the motion to another microscopic element, perhaps by a magnetic link or a polymer molecule, says Volpe.

Quinto-Su describes the new mechanism as “very ingenious.” He notes, as do Volpe and his colleagues, that it achieves high rotation speeds with small temperature changes and with just a few milliwatts of laser power, in contrast to other optically driven systems that require several watts and involve greater internal changes. These “remarkable characteristics” should encourage other researchers to explore additional microengine designs, says Quinto-Su.

This research is published in Physical Review Letters.

–David Lindley

David Lindley is a freelance science writer in Alexandria, Virginia.

## References

1. P. A. Quinto-Su, “A Microscopic Steam Engine Implemented in an Optical Tweezer,” Nat. Commun. 5, 5889 (2014).

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