Colloids — a dispersion of small particles in a medium — can be used to mimic atoms, allowing phenomena in condensed-matter physics, such as glass transitions and crystal nucleation, to be visualized and studied. They also offer intriguing possibilities for building large-scale intricate and functional structures. However, colloids typically form crystals or aggregates, and the synthesis of precise 'molecules' has so far proved difficult. Now, Jérôme Bibette and colleagues from the ESPCI in Paris and New York University have developed colloidal 'atoms' that can self-assemble into a variety of controlled structures, including chiral clusters (Nature 455, 380–382; 2008).

The researchers began by synthesizing magnetic colloids (illustrated in the figure) that included an oil-based ferrofluid (blue) and micrometre-sized silica particles (yellow or red). By precisely controlling the conditions, they were able to create doublets of silica particles that had a solid magnetic ring located around the point of contact between the two particles. In the presence of a magnetic field, these magnetic 'belts' assembled into a chain (bottom middle). The researchers found that when the dimers had different-sized silica particles, the chain was forced to coil (bottom right), and if the size ratio between the particles was high enough, a single helicity developed.

Bibette and team expect that this colloidal chemistry could prove useful in creating optical and light-activated structures, and in modelling enantiomeric separation.