Light drives microscopic metal gearsBy Kimberly Patch, Technology Research News
An international group of researchers has made a light-powered micromachine part that bears at least a passing resemblance to a cellular-sized washing machine.
The researchers are are using light to indirectly power gears made of silicon dioxide.
The tiny gears are nine microns in diameter, or about twice the size of a red blood cell. They move when nearby calcite crystals are pummeled with photons from a laser, causing the crystals to spin. This agitates the water the crystals and gears are suspended in, causing the gears to spin as well.
In order to get the crystals and gears close enough to transfer the action, the researchers used a pair of laser traps -- one to hold and spin the crystal, and the other to hold the gear. Laser traps, or tweezers use the energy of photons to hold tiny objects in place or move them around.
"The two traps are steerable, so we can bring the crystal and the machine element together, and the torque is transferred to the machine element via the fluid interface by friction," said Marlies Friese, a physics research fellow at the University of Queensland in Australia.
The calcite crystals spin because calcite is a birefringent material, meaning it changes the polarization of light passing through. Unpolarized lightwaves vibrate in all directions within the plane perpendicular to direction the light is traveling. Polarized light vibrates in only one direction within the perpendicular plane. In circularly polarized light, the polarization direction rotates like a corkscrew as the light travels.
In the course of changing the angle of the light, the crystal absorbs the difference in angular momentum. When a spinning baseball deflects off a person's arm, the forces the person feels are both directional momentum and angular momentum from the baseball, said Friese. The angular momentum portion of the force is similar to what moves the crystal. "When a ball hits an object and changes direction, its momentum changes [and] the object gains the difference in momentum. If [the ball is] spinning... and afterwards the ball spins in a different direction or faster or slower, the object gains the difference in angular momentum," he said.
This difference in angular momentum translates to enough force to make the crystal spin about 350 hertz, or times a second. "The transfer of angular momentum from light to the material results in a constant torque on the material, causing it to spin at a constant rate in a viscous medium [like] water," said Friese.
The torque, or moment of force transferred to the tiny gears is on the order of one millionth of one billionth of a newton meter, according to Friese. A newton is about the force of falling apple. A newton meter is the force required to accelerate that apple an additional meter per second. The torque would be enough to easily spin a red blood cell, he added.
The researchers made the gears in bulk using double-liftoff photolithography. The method allows them to make the parts by the thousands relatively quickly, Friese said.
To make the parts, the researchers first made a glass photolithography mask of the six-tooth gears using electron beam lithography. The teeth are not needed for mechanical action, but make it easier to see that the gear is spinning.
They baked two layers of photoresist polymers onto a silicon wafer, exposed the first layer to ultraviolet light through the mask, and dissolved the exposed material, leaving gear-shaped depressions the depth of the 1.5-micron top layer.
They deposited a layer of silicon oxide onto the pattern resist layers using electron beam evaporation, then dissolved the rest of the top layer of polymer, leaving silicon oxide in the pattern of the mask on the second layer of polymer. They released the silicon oxide shapes into a liquid suspension by dissolving the second layer of polymer.
The devices could eventually be used as parts of micromachines that stir and pump materials, said Friese.
In a similar research effort, researchers from the Hungarian Academy of Sciences to near infrared light to spin resin rotors that ranged in size from five microns to half a micron. (See Light Spins Resin Rotors, TRN, January 17, 2001.) The resin parts were made by shining a laser into liquid resin, which hardened when it came into contact with the light.
The methods the two groups are using to spin objects are also different. The Hungarian group's method uses the energy of the photons themselves to spin complicated shapes, similar to wind spinning a windmill. "Our experiment differs... in that the torque does not result from the asymmetry of the elements, [but] from the birefringence of the material," said Friese.
The international researchers plan next to look into spinning micromachine parts made of polymers directly using the birefringence method, which would allow the researchers to manipulate objects in air.
The group has also used birefringent spinning particles to move chloroplasts under a microscope in order to study them from different angles, Friese said. Chloroplasts are the organelles within plant cells that are key to photosynthesis, the biological reaction that extracts energy from light.
The research could be applied practically in two to five years, said Friese.
Friese's research colleagues were Halina Rubinsztein-Dunlop of the University of Queensland, and Julie Gold, Petter Hagberg and Dag Hanstorp of the Chalmers Institute of Technology in Göteborg, Sweden. They published the research in the January 22, 2001 issue of Applied Physics Letters.
The research was funded by the Australian Research Council, the Australian Department of Training, Education and Youth Affairs, the Swedish Board of Technical Development, and the Swedish Nanometer Laboratory.
Timeline: 2-5 years
TRN Categories: MicroElectroMechanical Systems (MEMS)
Story Type: News
Related Elements: Technical paper, "Optically Driven Micromachine Elements," Applied Physics Letters, January 22, 2001.
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