Nematic–isotropic transition in polydisperse systems of infinitely thin hard platelets

Abstract

We study the phase behavior of model colloidal systems composed of infinitely thin hard platelets, with polydispersity in the size of the particles. Semi-grand Gibbs ensemble simulations are used to study the coexisting nematic and isotropic phases for a range of systems with varying polydispersity. Particle size segregation is observed in the two coexisting phases, with the larger particles tending to be found in the nematic phase. This fractionation becomes more evident with increasing polydispersity. We examine the relationship between the size of a particle and its orientation in the nematic phase and find that the larger particles tend to be more orientationally ordered than the smaller ones. The coexistence densities determined from the simulations are compared to those obtained from recent experiments on colloidal platelets.

Isotropic−Nematic Phase Separation in Suspensions of Polydisperse Colloidal Platelets

“…Computer simulations of monodisperse, infinitely thin platelets have shown that the coexisting densities and pressure at the I-N transition are 16 where n I and n N are the number densities of platelets in the isotropic and the nematic phase, respectively, and Π ) ΠD 3 / kT is the reduced osmotic pressure. On the other hand, in the case of platelets with a finite thickness (D/L ) 10) these quantities are…”

“…The computer simulation data showed that the width of the coexistence region depends quadratically on the polydispersity in diameter. 16 Yet, size segregation between the coexisting phases is weak in both experiment and simulation.…”

“…For instance, little is known about the effect of polydispersity on the I-N transition. If we define the polydispersity, σ, as the standard deviation of the size distribution divided by the mean, the main effect of polydispersity on the I-N transition, that has come to light from experiments on the aforementioned gibbsite platelets (σ = 0.25) 14 and from computer simulations (0 < σ < 0.25), 16 is a broadening of the I-N coexistence region. The computer simulation data showed that the width of the coexistence region depends quadratically on the polydispersity in diameter.…”

“…The paucity of experimental platelet systems which exhibit the I-N transition, 3,14 and the (probably related) small number of simulation studies that have been conducted in this area, 15,16 leaves some elemental questions unanswered. For instance, little is known about the effect of polydispersity on the I-N transition.…”

Evidence of the hexagonal columnar liquid-crystal phase of hard colloidal platelets by high-resolution SAXS

“…In general, phase-separating colloidal systems, of spheres [46,[61][62][63] or platelets [59,60,64,65], deal with this by fractionation of the particles between the phases, leading to sub-phases with a (slightly) smaller polydispersity than the parent suspension. In sample C, where non-adsorbing polymer enhances fractionation even more, this lowering of the polydispersity facilitates the formation of the single columnar crystal.…”