Credit: © 2006 APS

This might look like a gold-plated dome on an extravagant piece of architecture, but it is actually a droplet of liquid with an internal nanostructure. Most liquids form droplets with spherical caps if they cannot wet a surface, but other shapes — such as this terraced hyperbolic profile — are possible when the liquid possesses internal order or structure. Andrew Croll and colleagues at McMaster University in Canada and Reading University in the UK took this image of a diblock copolymer on a silicon surface with an atomic force microscope (Phys. Rev. Lett. 97, 204502; 2006). The layered structure is the result of interactions between the two different 'blocks' — polystyrene and PMMA — in the polymer. However, the layers, which look like rings when viewed from above, disappear if the temperature goes above a certain value or if the molecular weight is increased. Croll and co-workers show that two competing interactions are responsible for the hyperbolic shape: edge tension drives liquid from the smaller disks near the top to the larger disks below, while edge repulsion prevents two adjacent edges from getting too close to each. Understanding more about the behaviour of diblock copolymers should help researchers who are working to exploit the self-assembly properties of these materials in applications as diverse as nanoelectronics and synthetic muscle.