A team of scientists led by Syracuse University unravel 100-year-old puzzle in condensed matter physics

A team of scientists led by Syracuse University unravel 100-year-old puzzle in condensed matter physicsMarch 13, 2003Judy Holmesjlholmes@syr.edu

A discovery of how nature arranges charged particles on the surface of a sphere is likely to impact fields as diverse as nanomanufacture and medicine and could help researchers find new ways to attack some bacteria and viruses.

In a groundbreaking National Science Foundation-supported study to be published in the March 14 issue of Science, researchers from Syracuse University and Harvard and Iowa State universities have found that spherical crystals compensate for the curved surface on which they form by developing “scars,” defects that allow the particles to pack into place.

In contrast, on flat surfaces, the particles will arrange themselves at the vertices of a perfect lattice of identical triangles. It has long been known that such a lattice cannot be simply wrapped around a sphere; the perfection of the lattice is lost. Yet, as early as 1904, when Nobel prize-winning physicist J.J. Thomson theorized about electron shells in atoms, researchers have wondered what structure the thin web of particles would choose from among the myriad of possibilities. This deceptively simple problem has resisted solution for 100 years.

“The theoretical work from our laboratories, and others, suggested that crystals on a curved surface would pack unusually, in a way not found in flat crystals, if the crystal was bigger than a critical radius,” says physicist Mark Bowick of Syracuse University. Bowick and Andres Bausch, of Technische Universitat in Munich, led the team of researchers who participated in the experiments.

In their Science paper, Bowick and Bausch describe the experiment in which they combined water droplets with tiny, self-assembling polystyrene beads that were about one micron (1/100 the diameter of a human hair) in size in an oily mixture of toluene and chlorobenzene. The beads were coaxed to congregate and form crystals around the balls of water. The resulting lattice structure, observed under powerful microscopes, contained areas that were disrupted and squeezed due to defects (spots where a bead had five or seven close neighbors rather than the six that it would normally have on the perfect lattice of a flat surface).

The experiment proved the researchers’ 1999 theory, published in Physics Review by Bowick, David Nelson of Harvard and Alex Travesset of Iowa State, that crystals on a curved surface would pack in a manner that resulted in “scars,” each containing alternate chains of five and seven particles, and that the larger the radius of the sphere, the longer the scars.

“Twelve fissures formed on the water droplets, just as we had predicted,” Bowick says. “As the crystals got larger, the scars got longer. But the scars end inside the medium-as we had also predicted-like roads that stop and don’t go anywhere. More importantly, these scars are a signature of the curved geometry and do not depend on the details of the particle interactions on the surface. The scars should appear in any type of spherical packing or crystallization.”

The experiment to prove the theory was designed by Bowick, Nelson and Travesset and carried out by Bausch along with Harvard researchers David Weitz and Harvard graduate students Ming Hsu and Anthony Dinsmore. Other researchers involved in the project were Michael Nikolaides of Technische Universitat, Angelo Cacciuto of the FOM Institute for Atomic and Molecular Physics in the Netherlands, and the Ames National Laboratory.

“The study’s interplay between theory and experiment reveals fascinating insights,” says Daryl Hess, the program officer who oversees NSF support for the project. “This is curiosity driven research-from the structure of biological systems to the venerable old problem framed before quantum mechanics, these findings will likely have impact across many fields of science.”

In addition to confirming the researchers’ theory, the new findings shed light on how such structures form and persist in nature. Some viruses and bacteria have similar spherical structures, and knowledge of scar formation on the structures may reveal how to target chemical reactions at those sites and potentially lead to new drug treatments.

The research also sheds light on some of the most prevalent structures in nanoscale science and engineering-the buckminsterfullerenes, or “buckyballs.” Buckyballs are spheres of carbon molecules that are produced in the laboratory and used in numerous experiments. Knowing how defects can progress across these nanostructures will help researchers devise better methods to create buckyballs with the most desirable characteristics.