Nature 528, 392–395 (2015)

There are numerous ways to synthesize nanostructures, from top-down lithographic techniques to bottom-up chemical self-assembly. However, these methods struggle to provide the levels of control and sophistication that can be found in biological systems and new approaches are constantly sought. Stoyan Smoukov and colleagues from the University of Cambridge and Sofia University have now shown that complex structures can be formed by simply cooling droplets of oil.

The researchers used droplets of linear alkanes with 14–20 carbon atoms, which were dispersed in water with the help of surfactant molecules. By slowly cooling the samples, the spherical droplets transformed into a series of different micro- and nanoscale shapes, including regular octahedra, hexagonal platelets, triangular platelets and, ultimately, thin fibres. The shapes formed were dependent on the cooling rate, the initial droplet size, and the type of surfactant used, and each could be selectively frozen into the corresponding solid structure.

The droplet transformations are due to an internal phase-transition process. In particular, before freezing, the alkanes form a series of metastable plastic 'rotator' phases in which the molecules have long-range translational order, but rotate freely around their long axis; thin layers of these rotator phases are formed next to the surface of the drops and drive the transformations observed.