- A sphere (micrometer-scale) is trapped in a liquid mixture using optical tweezers, and under specific conditions it rotates around the laser beam.
- The sphere’s rotational speed increases with the increase in laser power.
- The behavior can also be controlled by adjusting the temperature and criticality of the mixture.
In the last couple years, scientists have developed different type of tiny engines for converting numerous forms of energy into motion. One of those recent inventions is a spherical particle orbiting around a laser beam.
In 2014, researchers at National Autonomous University of Mexico developed a microscopic steam engine by trapping a tiny particle in a standard optical tweezers. The particle was placed in a water and they used laser power to heat the water and create vapor bubbles. These bubbles pushed the small particle away from the axis of a light beam. Then the temperature of the particle dropped and was pulled back (once the bubble was reabsorbed) by optical tweezers, thus giving particle continuous back and forth motion.
Now, researchers at University of Gothenburg in Sweden developed an enhanced version this microscopic steam engine, by replacing the water with critical liquid mixture. This critical mixture consists of two liquids that are slightly below the temperature at which they get separated. This mixture change the back and forth motion of the particle to circular motion (around the axis of the light beam).
How Does It Work?
The mixture contains 30% of organic compound (2,6-lutidine) and water. These two liquids mix when the temperature falls below 34 degree Celsius. The scientists put tiny spheres in this mixture just below the critical temperature. The spheres were 2.48 micrometers in diameter made of silica and iron oxide. Then they trapped one of these spheres in an optical tweezers.
Tiny sphere with irregular iron oxide trapped by optical tweezers | F. Schmidt/Univ. of Gothenburg
Since the sphere is made of iron oxide, it absorbed light energy and heat raised the local concentration of organic compound relative to water on the side closer to the light beam, and this pushed the sphere in the opposite direction (due to its hydrophilic surface).
Therefore, the tiny sphere found a stable region with its center slightly less than a micrometer away from the beam’s central axis. The iron oxide has been distributed irregularly within the sphere which caused a lateral force. This force made the sphere rotate around the laser beam.
Reference: Physics Review Letter| doi.org:10.1103/PhysRevLett.120.068004 | University of Gothenburg
As the laser power increases, the heat rises and so does the sphere’s rotational motion. The sphere begins rotating around the beam gradually but erratically. It often drifted towards the axis and then began orbiting the opposite direction, because Brownian motion influenced the motion and orientation of the particle.
Motion of microsphere under different laser power | Physics Review Letter
In the experiment, the rotational speed became stable at 1,160 revolutions per minute, with sphere center one micrometer away from the axis, when laser power is set to 2.7 milliwatt. The motion again became inconsistent when laser power reached to 3.2 milliwatt.
In future, the design can be tweaked to couple motion to other microscale modules and may be used, for instance, to mix fluids in next generation lab instruments.
According to the researchers, an efficient design of an asymmetric sphere would make the particle rotate in circular motions in a desired direction. It could also be possible to link other microscopic components, perhaps by a polymer molecule or magnetic link.
Because several naturals and artificial systems are configured near criticality, the working principle of the critical engine could be exploited in a wide range of application, and used to explain how natural mechanisms work, for example, molecular motors acting within a cellular membrane.
It could also exploit any order parameter configured near criticality, for example, particle concentration or pH-value. The critical engine operation can also arise in the absence of temperature gradients, as long as other gradients capable of configuring the system critically are present.
Phase separation have been already discovered inside the human body and some of them are sources of diseases like protein condensation. New biocompatible engines can be developed based on this critical engine that could perform medical surgeries noninvasively, like arteriosclerosis treatment.