Princeton University, New Jersey, USA

A chemical engineer is struck by the strange properties of ‘patchy’ colloids.

A recent paper about the behaviour of colloids makes an intriguing prediction — suggesting that they can adopt an ‘empty’ liquid state.

I study disordered states of matter, such as liquids and glasses. I find colloids interesting because they make phenomena such as crystal nucleation and the glass transition amenable to direct observation. Nanometre- or micrometre-sized particles suspended in liquids are wonderful model atoms. They arrange themselves in the same way that atoms and simple molecules do into solids, liquids or gases.

But controlling the interactions between colloidal particles provides a window into structural and thermodynamic behaviour beyond that found in atomic systems, as this recent theoretical paper shows (E. Bianchi et al. Phys. Rev. Lett. 97, 168301; 2006).

It maps the phase diagrams of ‘patchy’ colloids. The particles in such colloids are decorated with sticky spots, which tend to bond them together. As the number of bonded neighbours per particle is reduced towards two, the phase diagrams predict liquid states with a vanishing packing fraction. This means the colloidal particles occupy a tiny fraction of the available space — but they still behave as a liquid that is distinct from the gas-like phase of still lower packing fraction.

The low-temperature behaviour of such ‘empty’ liquids is especially interesting. The calculations suggest that cooling the colloid can freeze in place the empty configuration to give a glassy state of arbitrarily low density.

These predictions have not been tested experimentally. But chemists have already developed techniques for making patchy particles, so the work of Bianchi et al. could guide experimentalists in their exploration of this fascinating form of matter.

Rensselaer Polytechnic Institute, Troy, New York

Childhood memories cause a nanotechnologist to go nuts for plant-derived nanomaterials.

As a child growing up in Kerala, southern India, I marvelled at the unusual cashew fruit, with its kidney-shaped nut dangling from a swollen apple.

Since then, nanotechnology has become my passion. So it was with a curious mix of scientific interest and childhood memories that I read a recent paper describing how nanomaterials could be derived from plant sources such as the cashew nut.

I had never thought of a cashew nut as anything more than a food item. However, a little research reveals that cashew-nut-shell liquid, rich in natural long-chain phenols, already has applications ranging from hydrophobic coatings to anti-ageing creams.

George John and Praveen Kumar Vemula at the City College of New York, in their recent article (G. John & P. K. Vemula Soft Matter 2, 909–914; 2006), show how cashew-nut-shell liquid can also serve as a starting material for a variety of nanostructures.

The oil contains molecules that have phenol groups for heads, and long hydrocarbon tails. These can form structures such as lipid nanotubes and twisted nanofibres.

To make this happen, the molecules’ structure is first modified by attaching water-loving sugar groups to the phenols. The cooperative effect of head groups hydrogen bonding and the hydrophobic interactions of the tails leads the molecules to self assemble into bilayers. These then further organize into the fibres and tubes.

Using a similar strategy, it should be possible to develop a wide range of novel soft nanomaterials from other plant resources. The breadth of precursors available in our plants and crops should inspire all nanotechnologists — not just those fond of cashew nuts.