Scientific Method / Science & Exploration

Synthetic particles that flock like birds

A blue light triggers a chemical reaction that lets the material self-organize.

A flock of starlings is called a murmuration.

Scientists have built a self-organizing system of synthetic particles that assemble into clusters in a way that mimics the complicated organization of flocks of birds or colonies of bacteria. The particles form a “living crystal” that moves, swirls, and adjusts to heal cracks.

Self-assembly is a common way to build materials. Often, individual building blocks stick together due to inherent attractions, like bases of DNA bonding to form a nanotube, proteins gathering to form a helical virus coat, or nanospheres gathering to form a photonic crystal.

But what draws flocks of starlings, schools of fish, or rafts of ants together? Flocking or schooling can be a social behavior. However, the similarities among these phenomena, regardless of the creatures involved, led NYU's Jérémie Palacci and his colleagues to wonder if an underlying physical principle could also govern the organization process.

Moving flocks are considered out-of-equilibrium because the movement of each individual in the group causes the equivalent of a flux of energy in the flock. Self-assembling DNA, on the other hand, snaps together under equilibrium conditions—just mix complementary bases in solution and they'll find each other as they drift through the liquid.

The scientists wondered if they could recreate an out-of-equilibrium system using synthetic particles, ensuring that any aggregation could not be due to social interactions. They built synthetic particles using nanoscale cubes of hematite surrounded by a polymer shell. Each cube moves due to a light-triggered chemical reaction. When the researchers shine blue light on these particles, hydrogen peroxide decomposes on an exposed portion of the metallic cubes. That sets up a gradient of peroxide in the solution, with lower concentrations of peroxide next to the cube and higher concentrations further away.

The metallic particles follow that gradient with essentially random motion. But when two particles get close to each other, their induced gradients interact and draw the particles together. With enough particles in the solution, they cluster into crystals.

These crystals rotate, crack, and reorganize to heal defects. This “living crystal” behavior was quite striking and surprising, Palacci says. And it’s completely due to the out-of-equilibrium nature of the system. When the scientists turn off the blue light, the chemical reaction at the surface of the particles stops, the crystal disintegrates, and individual particles disperse in the solution.

Switching on a blue light causes the particles to spontaneously form complex aggregates. (Courtesy Science/AAAS)

The scientists can also control the movement of the particles and the living crystals with an external magnetic field. Now Palacci says they’re studying how the shape of the particles influences the aggregation.

Self-assembly could be a way to build microscale devices like solar cells. Palacci says this “living crystal” could find interesting applications in materials science—in self-healing materials, for example, or ones that change their physical properties on demand.

Science, 2013. DOI: 10.1126/science.1230020  (About DOIs).

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