Scientists throw crystals a curve

Particles forced to assemble inside a sphere form ordered ribbons

CORRALLED CRYSTALS  Inside a droplet of water between about 11 and 35 micrometers across, particles (orange) assemble into strips of crystals rather than chunks to avoid stressful bending.

Courtesy of Guangnan Meng and V. Manoharan/Harvard Univ.  

Scientists don’t need a crystal ball to predict how crystalline solids grow on flat planes. But it might come in handy for curved surfaces.

By trapping particles in tiny balls of water, scientists got their first glimpse of crystal formation in rounded environments. Researchers found that the caged crystals formed arched ribbons inside the sphere rather than the compact clumps seen on flat surfaces. The findings appear in the Feb. 7 Science.

Curves offer a particular challenge for forming crystals, which are solids with molecules, atoms or other subunits assembled in ordered, symmetrical patterns. Chemical engineer Vinothan Manoharan of Harvard explains that growing crystals around a sphere “is kind of like trying to gift wrap a basketball.”

Scientists knew that encounters with curves cause crystals to develop defects — or pattern breaks — but researchers had not found a way to eye a growing crystal as it navigated bends.

Manoharan and colleagues came up with a technique using wee water droplets, suspended in oil and loaded with polymer particles with diameters of 80 nanometers and 1 micrometer. Using microscopy, the researchers monitored the enclosed particles, which attract each other, as they began forming crystals. The particles formed thin, branched crystalline ribbons along the inside of the drops.

On flat surfaces, the particles’ attraction to one another would make crystals form in a tight clump. “The behavior on curved surfaces is entirely different,” says biophysicist Gerhard Gompper of Forschungszentrum Jülich in Germany.

In this case, Manoharan and colleagues observed that the crystal’s shape is dictated by the drop’s curvature and by the crystal’s aversion to stressed arching shapes. Individual particles may not be affected by a curve, much like a person doesn’t feel the curvature of the Earth. But as the crystal grew larger, the team watched as the whole structure felt strain and grew into strips to avoid stretching and forming defects.

“It’s a nice piece of work,” says physicist William Irvine of the University of Chicago. “You can really watch this crystal grow.” These observations, he says, could further scientists’ understanding of how curved structures form in a wide variety of settings, including coatings on nanoparticles and the outer shells of viruses.

Editor’s Note: This story was updated on March 4, 2014, to correct the droplets’ diameters in the caption.

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