Laser + Critical Liquid = Micro-Engine

microsphere in laser trap, pursued by bubbles

In the micro-engine devised by the Gothenburg University–led team, a 2.48-micron-diameter silica sphere with iron-oxide inclusions is initially trapped in optical tweezers in a critical mixture of water and an organic liquid. Heating of the sphere causes demixing of the critical liquid; the resulting concentration gradients cause the sphere to rotate around the tweezer axis. [Image: Courtesy of F. Schmidt, University of Gothenburg]

A research team led by OSA Life Member Giovanni Volpe of the University of Gothenburg, Sweden, has unveiled a new approach to building laser-driven micro-engines (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.120.068004). The team’s creation uses optical tweezers to trap and rotate a 2.48-micron-diameter sphere immersed in a critical liquid mixture (a combination of two liquids at just below the temperature at which they separate) around the beam axis at rates of more than a thousand revolutions per minute (rpm).

Powering the particle motion, according to the researchers, is local demixing of the liquid due to irregular heating of the particle by the laser beam. The researchers believe that the setup, which requires low laser powers and which can be controlled by tweaking parameters such as temperature and laser power, could eventually find applications in many biological systems. It could even, they suggest, lead to “biocompatible engines” that might perform noninvasive kinds of microsurgeries.

Optical traps and micro “steam engines”

In the past few decades, numerous teams have reported a variety of methods for creating optically powered micro-engines. Those methods have included schemes that rely on the transfer of orbital or spin angular momentum to microparticles, and various microscopic “heat engines” involving time-dependent optical traps.

One researcher, in 2014, even created a microscopic “steam engine” (Nat. Commun., doi: 10.1038/ncomms6889). In that setup, a microparticle, heated in an optical trap in water, eventually caused the liquid nearby to boil; the particle was pushed out of the trap by the explosive force of cavitation bubbles in the boiling liquid, and then pulled back into the optical trap as the particle cooled. The result was a back-and-forth, piston-like motion of the particle, at rates ranging from a few tens of cycles to a thousand cycles per second.

Rotation by demixing

Volpe’s team—which included other researchers from Gothenburg and from Bilkent University, Turkey, and Leipzig University, Germany—used a variation on that theme. The team started with a mixture of water and 2,6-lutidine, a colorless, naturally occurring organic compound, with the lutidine making up 28.6 percent of the mixture by mass. At that concentration, lutidine and water form a critical mixture: the components behave as a homogenous fluid at temperatures below 34°C, but demix into separate water-rich and lutidine-rich phases at temperatures above that critical point.

The researchers then dispersed 2.48-micron-diameter silica spheres with iron-oxide inclusions into the mixture, and lowered the temperature to 26°C, eight degrees below the critical point. They set up optical tweezers in the mixture using a 976-nm infrared laser and an inverted microscope, and trapped one of the particles in the tweezers. They then tweaked the laser power, temperature and other parameters, monitoring the particle motion using 296-fps digital microphotography.

The team found that, with the microsphere trapped in the optical tweezers, the iron-oxide inclusions caused part of the light, on the side of the sphere closest to the focal spot, to be converted into heat. This, in turn, caused the fluid locally to rise above the critical demixing temperature, creating a concentration gradient that pushed the particle away to a new equilibrium position roughly a micron from center of the trap. And because of “small asymmetries” in the particle composition, the system also experienced a lateral force that caused the particle to rotate around the axis of the tweezers.

A 1000-rpm micromotor

The researchers discovered that a temperature of 26°C and a laser power of 2.7 mW constituted a sort of sweet spot for the micro-engine, with the microsphere rotating steadily around the optical-trap center at a rate of 1160 rpm. Power levels much below that would lead either to no motion at all or to less steady behavior (owing in part to Brownian motion in the fluid); higher powers caused the motion to become increasingly erratic.

Other experiments established that adjusting the criticality and temperature of the mixture could also be used to control the micro-engine’s speed and dynamics. And the team believes that the engine’s behavior might be further refined through the design of specific microparticles, such as chiral particles, that can provide a specific sense of rotation.

Those various control levers, coupled with the low laser powers required, suggest a number of possible applications for the micro-engine, according to the researchers. Such an engine could, for example, be used as a sort of micro-blender, to mix fluids on lab-on-chip devices. Perhaps more ambitiously, the authors point out that because “many natural and artificial systems are tuned near criticality,” micro-engines could conceivably be designed along similar lines to do useful work in a variety of places. They even suggest that “new biocompatible engines” based on the work could eventually “be able to perform medical surgeries noninvasively such as the treatment of arteriosclerosis.”

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