Windmills have provided power to humanity for centuries. Now scientists from the University of California, Berkeley have created a tiny gold "light mill."
"It's the smallest motor so far," said Ming Liu, a scientist at the University of California, Berkeley and a co-author of a recent paper in Nature Nanotechnology. "It may be used in winding/unwinding DNA, actuators for nanoelectromechanical systems and solar energy harvesting."
The new light mills are made of gold and shaped like a tetra-gammadion. They are also tiny —about 200 nanometers in diameter.
A light mill might be inspired by windmills, but the two work very differently, said Liu. The blades of a windmill are tilted, to channel the physical impact of air into angular movement in one direction. The blades of the light mills, by contrast, are completely flat and can rotate either clockwise or counterclockwise.
Although the blades of a light mill appear flat, at two specific wavelengths of light, 810 nanometers and 1,700 nanometers, the light mills look very different. To those two wavelengths of light, the mill is bent, twisted one direction or another, just like a windmill.
"For the light, the shape of the sails are changed," said Liu. "In other words, to the light, a light mill looks no longer planar but more like a real windmill. That is why a light mill can work."
The research is a "breakthrough," according to Erez Hasman, a professor at Technion in Israel who reviewed the work.
"In the past there have been several observations of rotating nanoparticles by exploiting the incident intrinsic movement of the light," said Hasman.
"This is the first observation, to my knowledge, that you can induce the rotation of a nanoparticle without exploiting the intrinsic angular momentum of light," he added.
This new ability gives scientists a powerful new tool to manipulate tiny objects, from DNA to nanomachines.
The new light mills are quite powerful. They create enough torque to rotate an object 4,000 times their size in water. Attached to a nanomachine, the light mill could act as a propeller, driving the tiny device through water, blood or some other medium to deliver drugs or assemble larger nanomachines.
One of the most likely uses of the light mill is in biology, however. Using light, not chemicals, would give scientists better control when it comes to winding or unwinding DNA, said Hasman and Liu. That could speed up the sequencing of DNA, among other applications.
The light mills could also indirectly do work by generating electricity. Multiple light mills of different sizes would be turned by various wavelengths, generating electricity from a wide swath of the electromagnetic spectrum.
These applications are still years away, however. The next step in the research, says Liu, is to improve the efficiency of the light mills and scale them up for easier production.