A detailed simulation of the packing behaviour of deformable particles settles the debate about whether soft matter can adopt an unconventional crystal structure at high densities — it can. The hunt is now on for a real-world example.
Imagine the Rome metro at rush hour: passengers are squeezed into close contact with one another. But there is a physical limit beyond which they cannot go, because their bodies cannot occupy the same space. This common experience has an equivalent at the molecular scale, in what physicists call excluded volume — strong repulsive forces, of quantum-mechanical origin, that prevent atoms from occupying the same space. Because of this phenomenon, dense arrangements of atoms and molecules result in solids that have lattice structures, in which each particle excludes neighbours from its site in the lattice. It is therefore surprising to read Lenz and colleagues' paper1 in Physical Review Letters, which reports that the packing of soft particles may result in unusual crystals in which each lattice site is occupied not by a single particle, but by clumps of particles.
Soft particles are nanometre- or micrometre-sized macromolecules that have a deformable shape. Focusing on polymers, for example, one can envisage several different soft particles of increasing complexity (Fig. 1). These could be: linear chains; rings, in which the ends of a polymer chain are connected; stars, in which several polymer chains are joined at a common centre; and dendrimers, in which several stars are linked together. For a fairly small energy cost, the structures of soft particles can rearrange and interpenetrate to cope with excluded-volume constraints at the molecular scale. This allows the centres of mass of different soft particles to coincide, without any overlapping of the monomers (Fig. 1).