Anisotropic magnetic colloids: a strategy to form complex structures using nonspherical building blocks.

Microscopic magnetic particles dispersed in a solvent—or dipolar colloidal fluids—commonly assemble into chains due to a directional attractive interparticle potential. These chained structures can impart optical anisotropy (i.e., birefringence) to dipolar fluids, and have been demonstrated as effective matrix materials in the rapid separation of DNA using microfluidic electrophoresis. In the special case of magnetorheological (MR) systems, which require external fields to induce dipoles in polarizable colloids, an abrupt microstructural transition from isolated (unpolarized) particles into oriented chains can produce dramatic changes in the viscous behavior. This responsive property makes MR fluids attractive as field-controllable damping fluids in hydraulic valves, shock absorbers, brakes, and so on. Dipolar fluids also display interesting ordering phenomena beyond the 1D case. For instance, nickel-coated glass microspheres were shown to pack onto square, oblique, triangular, and even quasicrystalline lattices with five-fold symmetry in 2D. Binary particle mixtures assembled into ‘‘flower’’-like aggregates and superlattices in a planar configuration. In 3D, dipolar fluids were predicted to form rich mesophases as a function of the dipole moment strength, particle volume fraction, and the relative ‘‘hard’’ or ‘‘soft’’ nature of the colloidal interactions. Examples include the body-centered-tetragonal (bct) and body-centered-orthorhombic (bco) crystals, along with a broad fluid–bct coexistence phase. Yethiraj and van Blaaderen demonstrated these phases experimentally using a system with tunable long-range