Microparticles in Laser Light Unlock Movement Mysteries

The universe is filled with spinning, orbiting, and dancing things. Eddies in the atmosphere come toward each other, tango, and fly apart. Algae cells wander randomly while they rotate, until they find themselves pulled together in a twirling partnership, and then they go off again on their own. Now a team of researchers at New York University has replicated the phenomenon by rotating microparticles with light, shedding some light on why so much of the world dances the way it does.  

But the question they first set out to answer was both more narrow and more complex. “We wanted to make particles that are rotating, say, clockwise, and other particles that are rotating counterclockwise,” said Matan Yah Ben Zion, an NYU graduate student in physics. “There were all sorts of predictions, theoretical simulations, saying there should be interesting phases, material phases, and all sorts of edge modes. And there was no system like that.” 

Researchers have rotated tiny particles already, but they used magnetic particles in a magnetic field. That means those particles dance in synchronized patterns, with their magnetic poles facing the same direction. To get the particles to spin freely as separate units, Ben Zion and his colleagues turned to lasers.  

Photons have the power to push—it’s what powers a solar sail and allowed the recent creation of optical tweezers that can capture a single particle with a focused beam. But photons also have angular momentum and the NYU researchers hoped to use that with a non-focused laser to send a slew of particles spinning.   

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But for that to happen, they needed particles with the ability to absorb a photon’s angular momentum but not dissolve within the liquid in which they were suspended. The momentum-absorbing particles they wanted to use were vaterite, a form of calcium carbonate that dissolves quickly. To prevent the particles’ speedy disintegration, the team created a process to coat them in several layers of silica, which proved the most difficult and longest part of the project. 

Once the researchers fabricated long-lasting, momentum-absorbing particles, they hit them with a polarized columnated beam of laser light. That is, a single beam was separated into multiple strands for omni-directionality. 

“I probably spent maybe 50-odd hours tinkering with and aligning the laser,” said Alvin Modin, an undergraduate researcher at NYU and co-author of the paper, “Hydrodynamic spin-orbit coupling in asynchronous optically driven micro-rotors,” which appeared in Nature Communications this summer. “And, finally, you see these things spinning—you’re like, ‘Wow, this actually works.’”  

And the particles didn’t just spin in place, like so many turbines in a wind farm. They also roamed, found themselves in pairs, circled each other, and wandered off again. The researchers were able to see just what they were doing thanks to the fact that the particles blink as they turn in the polarized light.

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“They’re undergoing a Brownian walk in 2D,” said Ben Zion. “As they approach each other, they slow down their rotation. Their spinning rate can decrease by something like thirty percent. And as they get farther, they rotate faster. This is qualitatively different than anything that was done so far with synthetic systems.” 

In natural systems, the spin-orbit coupling behavior was familiar. In fact, soon after their breakthrough, Raymond Goldstein, a professor of complex physical systems at Cambridge, contacted the NYU team to report that he’d seen the same thing with Volvox, a rotating algae, on a scale some 50-times larger than the particles. 

“The algae behaves like our spinners. They’re courting each other in this mating ritual,” said Modin. “And the dynamics are very, very similar.” 

In addition to offering a model of the complexities of rotating systems in the natural world, the swirling particles may have some applications. “There’s a lot of interest right now in designing robots that can interact with each other, either autonomously or through some decentralized network,” said Modin. “To do that, you need to understand the dynamics of their interaction and how they collectively behave.”  

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A similar set of particles could be used to create self-assembling materials that are more complex than the sum of their parts, for enhanced strength or specific optical properties. “You could create spinning materials that selectively reflect light for adaptive camouflage or for use in photonics,” Modin added. 

But before they start developing any such advanced materials, Modin and Ben Zion still want to answer the question they originally set out to answer: How do particles rotating in opposite directions interact? Their breakthrough occurred mid-experiment, when their particles were all spinning in the same direction after being hit with a single beam of light.

To get another set of particles spinning the other way, they had hoped to mix in magnetic particles with the vaterite particles and rotate them simultaneously with a magnetic field—a setup that is trickier than it might seem. The force of the laser beam lifts the nonmagnetic particles slightly, whereas the magnetic particles rotate where they are, essentially creating two planes of non-interacting rotation. But it’s a difficulty they are confident they can overcome, possibly by creating “left-handed particles” that spin the opposite way in the same light. And then, maybe, they’ll have a model for even more complex systems in the world.   

“I think nature tends to be advanced in ways that we maybe don't even understand,” said Modin.  

Michael Abrams is a writer in Westfield, N.J.