Liquid crystal phase transitions in suspensions of polydisperse plate-likeparticles

Colloidal suspensions that form periodic self-assembling structures on sub- micrometre scales are of potential technological interest; for example, thr ee-dimensional arrangements of spheres in colloidal crystals(1) might serve as photonic materials(2), intended to manipulate light. Colloidal particle s with non-spherical shapes (such as rods and plates) are of particular int erest because of their ability to form liquid crystals. Nematic liquid crys tals possess orientational order; smectic and columnar liquid crystals addi tionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering(3,4) may be inhibited in polydisperse col loidal suspensions. Here we describe a suspension of plate-like colloids th at shows isotropic, nematic and columnar phases on increasing the particle concentration. We find that the columnar two-dimensional crystal persists f or a polydispersity of up to 25%, with a cross-over to smectic-like orderin g at very high particle concentrations. Our results imply that liquid cryst alline order in synthetic mesoscopic materials may be easier to achieve tha n previously thought.