Le résultat serait un motif défectueux contenant une zone de formes à sept et cinq côtés, plutôt que les hexagones à six côtés réguliers. Mais ce que Bowick et ses collègues ont prédit en utilisant des modèles informatiques, c'est que sur une surface courbe, une particule supplémentaire ajoutée à mi-chemin entre deux cicatrices créerait un défaut dans le motif qui se diviserait en deux parties.

Ils ont calculé que la contrainte sur la structure cristalline causée par ces deux défauts « s'écoulerait » loin du site, comme des ondulations sur un étang, au fur et à mesure que les particules réajustent leurs distances les unes par rapport aux autres. Finalement, les défauts migreraient vers les cicatrices opposées, où ils disparaîtraient.

Étonnamment, les scientifiques ont prédit que la masse de la particule originale resterait proche de l'endroit où elle a été placée, and large areas of the hexagonal pattern would have rotated slightly ­— about 30 degrees. But the original defect would be gone.

To prove this remarkable result experimentally, cependant, required a special instrument.

MAKING IT WORK

"William Irvine had already begun his beautiful experiments in my lab on colloidal crystals on curved surfaces, " recalled NYU's Chaikin. "The present study came from a conversation that Mark Bowick and I had on a plane coming back from a meeting several years ago. Mark's experiment was a natural extension of William's work."

"For this project, we had to figure out how to add a particle to the curved crystal, while imaging the particles as they shift around in three-dimensional space, " explained Irvine, who is now at UChicago's James Franck Institute. "This makes the experiment considerably more complicated."

Irvine planned to use optical tweezers to grab a microscopic particle from the surrounding emulsion and place it on the surface of a droplet using radiation pressure from a focused laser beam.

"In most experiments, you come in with the laser 'tweezers' using the same lens as you use for imaging the particle, and that's great, because you want to focus the beam on the same plane where you're looking, " Irvine said.

But for this experiment, the laser tweezers and the microscope had to be separated.

"A confocal microscope selects a very thin slice of the object to be imaged, so that one slice is in focus and the rest of the image (before and after) is out of focus, like a photo of a person with their face in focus and the background blurred, " he explained. "In order to create a full three-dimensional image, you move the objective up and down and bring the different slices into focus one at a time."

But moving the lens also moves the laser beam holding the particle.

"In order to hold onto a particle and watch what happens as you gradually bring it to the surface of the droplet, you have to essentially build a second microscope on top of the first one, " Irvine said. "Technically, that's not trivial—you have to get a lot of things to work at the same time."

But once Irvine had designed and built the instrument, the team tested Bowick's predictions and actually created video images showing the defects moving across the crystal surface and disappearing into the scars.

SELF-HEALING GRAPHENE

"The study of crystals on curved surfaces is interesting and important for systems that range from geodesic domes to viruses to Buckyballs, " said Chaikin, referring to symmetric molecules of carbon. "The defect structure and the 'healing' of defects are particularly important in the conductivity, heat and mechanical properties of carbon nanotubes, graphene and similar materials."

Graphène, a two-dimensional sheet of carbon molecules, is a very strong material and a good conductor of electricity.

"There are always going to be defects that will decrease the conductivity of graphene, " said Bowick. "Ultimately, for electronic devices, you want graphene with high conductivity and as pure as possible."

And that's where the researchers' discovery could prove an ideal solution. "You might be able to simply flex a piece of graphene, remove the defects, and improve the conductivity, " Bowick said.