When it comes to particles, we usually think of opposites attracting - north and south, positive and negative. But, somehow particles that one might expect to mutually repel somehow manage to form clusters in solution. How this can be was the subject of a research project undertaken by Gerhard Kahl of the Institute of Theoretical Physics at the Vienna University of Technology, and colleagues. Their finding could be important for understanding how polymers become organized and improve the prospects of the burgeoning field of soft matter research.

Milk and mayonnaise, paints and inks, proteins and DNA, are all examples of what is known as "soft matter". It is only recently that their physical characteristics have been systematically investigated, often with surprising outcomes. "Intuitively, you would only expect particles in a fluid to aggregate when they attract each other," explains Kahl, "but we have been able to show that this needn't always be the case." He and his colleagues have found that even particles that completely repel each other can form clusters in so-called colloidal dispersions in which relatively large particles are dissolved in a solvent composed of much smaller particles.

Working with colleagues at the University of Dusseldorf, the researchers have computed the physical structure of colloidal solutions and shown that repulsive particles can aggregate if they can "overlap" and if an increasing separation is associated with a rapid fall-off in repulsive strength.

The team also studied the behavior of particles under pressure. "Under high pressure," Kahl explains, "the clusters arrange themselves into crystals. We were even more surprised with the findings of further investigations which showed that the spacing between the ordered crystalline clusters remains constant when compressed further - a characteristic which is made possible by the aggregation of more and more particles in the clusters." These findings contrast with the behavior of other ordered systems, such as ordinary crystalline metals, where the lattice spacing decreases under pressure.

Phys Rev Lett, 2006, 96, 45701; http://dx.doi.org/10.1103/PhysRevLett.96.045701

http://tph.tuwien.ac.at/smt/kahl.htm