Science 343, 634–637 (2014)

Crystallization is a ubiquitous phenomenon that has been studied for centuries. However, the effect of substrate curvature on the process is not well understood, despite the fact that it could provide insight into the growth of curved nanostructures such as functionalized nanoparticles and viral capsids. To address this issue, Vinothan Manoharan and colleagues at Harvard University have now explored the growth of two-dimensional colloidal crystals on the inside walls of spherical water droplets.

Using confocal microscopy, the researchers examined the assembly of polystyrene microspheres in water-in-oil droplets that had various different curvatures. The polystyrene spheres were mixed with smaller polymer nanoparticles, which induce short-range depletion attractions between the spheres. This depletion attraction is an entropic force that results from an exclusion of the nanoparticles from the intersphere region. The force also confines the spheres to the interface of the droplets.

Over the course of a few hours, the spheres were found to form branched, ribbon-like domains, which left large empty spaces on the droplets. If the crystals had been grown on a flat surface these voids would have been filled with other spheres. However, on curved surfaces elastic stress is present, which is minimized by forming such broken morphologies.