Physicists shine a light on particle assembly (w/ video)

January 31, 2013

New York University physicists have developed a method for moving microscopic particles with the flick of a light switch. Their work, reported in the journal Science, relies on a blue light to prompt colloids to move and then assemble—much like birds flock and move together in flight.

The method offers the potential to enhance the design of a range of industrial products, including the architecture of electronics.

The study's authors were: Jeremie Palacci and Stefano Sacanna, post-doctoral fellows in NYU's Center for Research who devised the research; David Pine and Paul Chaikin, professors in NYU's Department of Physics; and Asher Preska Steinberg, an undergraduate at Brandeis University who was a summer research program participant at NYU.

This real-time movie shows the light activated motion of the self-propelled particles (diameter D = 2:2μm). The hematite cube is visible as the darker spot on the particles. When the blue light is shut, the particle exhibits random thermal motion, when the light is on the cube faces the cell surface and exhibits an active motion. The propulsion stops when the light is shut. Credit: Science, DOI: 10.1126/science.1230020

The work addresses a fundamental question in nature—what causes flocks and swarms to form and move in a particular way? Schools of fish, colony formations of bacteria, or flocks of birds are examples of how this occurs in living matter. In this inquiry, the researchers focused on making artificial systems exhibit similar activity. They used colloids—small suspended within a fluid medium—and discovered the basic organizing principles in natural flocking and how to use this to organize inorganic matter.

This exploration is a significant one. Colloidal dispersions are composed of such everyday items such as paint, milk, gelatin, glass, and porcelain. By better understanding driven colloidal self-organization, scientists have the potential to harness these particles and create new and enhanced materials—possibilities that are now largely untapped.

This movie, accelerated 15x, shows the formation and the dynamics of the living crystals. The active particles spontaneously assemble in crystals which merge, break apart and reform. The crystals dissolve when the light is shut and reform when the light is on again. Credit: Science, DOI: 10.1126/science.1230020

This movie, accelerated 35x, shows the dynamics of the living crystals under an external magnetic field (B ~ 1mT). The field orientation is externally varied in time. Under the imposed magnetic field, the composite particle slightly tilts and the direction of self-propulsion aligns with the direction of the field. The direction of B is indicated in the movie by the white arrow and is changed externally in orthogonal directions. The use of the magnetic field allows us to steer the living crystals. Moreover, as visible on the movie, forcing the particles to go in the same direction suppresses the crystal breakup. Credit: Science, DOI: 10.1126/science.1230020
To explore this, the research team developed -activated self-propelled particles, "swimmers," from the micro-meter-sized particles in solution. To separate the effects of swimming from simple , they created a system where the particles turn on and off with application of blue light. With the light on, the self-propelled random swimmers collide and cluster. The light also triggers a slight chemical attraction and leads the clusters to crystallize and grow until the swimmers turn in separate directions and splinter the crystals. The "living" crystals continually form, swirl, and split. When the light is extinguished, the swimmers stop and the structures dissolve into individual diffusing colloidal particles.

Using the slight magnetism of the particles allows direction of the individual swimmers as well as the crystals. With control of light, magnets, and chemical attraction, these active particles bring biological organization to the materials world.

Explore further: Researchers create new microparticles that self-assemble like atoms into molecules

More information: Living Crystals of Light-Activated Colloidal Surfers, Science, DOI: 10.1126/science.1230020

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