The assembly of complex structures out of simple colloidal building blocks is of practical interest for building materials with unique optical properties (for example photonic crystals and DNA biosensors) and is of fundamental importance in improving our understanding of self-assembly processes occurring on molecular to macroscopic length scales.

Scientists in the United States have demonstrated a new self-assembly principle that is capable of organizing a diverse set of colloidal particles into highly reproducible, rotationally symmetric arrangements [Erb, R. M., et al., Nature (2009) 457, 999].

Self-assembly mediated by Van der Waals forces and hydrophobic interactions leads primarily to close-packed structures such as colloidal crystals when the assembly is performed in the bulk fluid and to particle clusters when the assembly process is confined to the interior of an emulsion droplet or to a liquid–liquid interface.

Despite recent progress in colloidal self-assembly, there have been relatively few demonstrations of ordered structures in suspensions containing multiple different particle types, and so far there are no demonstrations of controlled assembly in suspensions containing three or more colloidal components.

Erb et al . have shown structures that are assembled using the magnetic interactions between effectively diamagnetic and paramagnetic particles within a magnetized ferro fluid (that is, Fe₃O₄ nanoparticles suspended in water). The resulting multi-polar geometries resemble electrostatic charge configurations such as axial quadrupoles (‘Saturn rings’), axial octupoles (‘flowers’), linear quadrupoles (‘poles’) and mixed multipole arrangements (‘two tones’), which represent just a few examples of the type of structure that can be built using this technique.

Essentially, the structures are produced by applying static, uniform magnetic fields to aqueous suspensions of paramagnetic and non-magnetic particles suspended in a ferro fluid. The ferro fluid concentration is an essential control parameter for this assembly technique. Particles with higher magnetization than their carrier fluid (for example paramagnetic particles) will exhibit a classical paramagnetic response with respect to the surrounding fluid. Conversely, particles less magnetizable than the carrier fluid (for example non-magnetic particles) will behave as non-magnetic cavities inside a magnetizable medium and will exhibit an effectively diamagnetic response with respect to the surrounding fluid.

Application of magnetically actuated self-assembly techniques to multi-particle colloidal mixtures provides a direct and general approach to the creation of complex colloidal super structures. A rich variety of different particle configurations is possible depending on the size, type and degree of magnetization of the different particle components. The scientists have successfully shown that non magnetic particles as small as 200 nm in diameter can be assembled into complex arrangements