Controlling the behaviour of particles suspended in a fluid is critical, whether you are trying to mix paint or separate crude oil. A group at the Hong Kong University of Science and Technology are taking the control of particle properties in solution to a new level, in the hope of creating useful photonic materials (the optical analogues of semiconductors).

Ping Sheng and his colleagues (Phys. Rev. Lett. 82, 4248-4251; 1999) coated micrometre-sized glass spheres with a layer of nickel, followed by PZT (lead zirconate titanate, a quartz-like ceramic), another layer of nickel and finally titanium oxide. The four coatings (left side of figure) are intended to give a large response to applied electric or magnetic fields.

The coated spheres are suspended in silicon oil. In zero applied field, the spheres are randomly dispersed. As the external electric field is increased, the particles form columns, and other changes emerge when a magnetic field is applied. By freezing the system at different field strengths and taking cross-sectional micrographs, Sheng and co-workers have snapshots of the changing structure (right side of figure).

figure 1

Figure 1

They discover that as the magnetic field increases, the spheres change from a body-centred-tetragonal (bct) structure to a face-centred-cubic (fcc) structure. Small movements of the spheres produce the structural transition, without any long-range diffusion. Such a transition can be achieved — simply by varying the relative strengths of the external fields — because the difference in energy between the bct and fcc structures is very small.

A tunable crystal structure might offer new ways to make photonic materials that permit the passage of photons of certain energies, while excluding others. The difficulty in finding photonic crystals with the right properties (lattice dimensions from a few hundred nanometres to tens of micrometres) could be solved by these made-to-order crystals.