Phase behavior of a mixture of platelike colloids and nonadsorbing polymer

Suspensions of sterically stabilized colloidal gibbsite platelets have recently been found to exhibit both an isotropic-nematic (I-N) and a nematic-columnar (N-C) phase transition. In the present paper we show that depletion attraction, brought about by the addition of nonadsorbing polymer, enriches the phase behavior of these platelet suspensions even further. Pronounced broadening of the isotropic-nematic gap occurs, with purely nematic samples re-entering the biphasic state by the addition of nonadsorbing polymer. At the same time, depletion attraction enhances size fractionation between coexisting phases, which actually provides an effective means for reducing the polydispersity of the suspensions. An additional isotropic phase emerges, which leads to the appearance of several three-phase equilibria and even a four-phase equilibrium. We explain the observed topology of the phase diagram by the interplay between depletion attraction and the platelets' polydispersity.

Liquid crystal phase transitions in suspensions of polydisperse plate-like particles

Colloidal suspensions that form periodic self-assembling structures on sub-micrometre scales are of potential technological interest; for example, three-dimensional arrangements of spheres in colloidal crystals might serve as photonic materials, intended to manipulate light. Colloidal particles with non-spherical shapes (such as rods and plates) are of particular interest because of their ability to form liquid crystals. Nematic liquid crystals possess orientational order; smectic and columnar liquid crystals additionally exhibit positional order (in one or two dimensions respectively). However, such positional ordering may be inhibited in polydisperse colloidal suspensions. Here we describe a suspension of plate-like colloids that shows isotropic, nematic and columnar phases on increasing the particle concentration. We find that the columnar two-dimensional crystal persists for a polydispersity of up to 25%, with a cross-over to smectic-like ordering at very high particle concentrations. Our results imply that liquid crystalline order in synthetic mesoscopic materials may be easier to achieve than previously thought.