Phase behaviour of hard board-like particles

Abstract

We examine the phase behaviour of colloidal suspensions of hard board-like particles (HBPs) as a function of their shape anisotropy, and observe a fascinating spectrum of nematic, smectic, and columnar liquid-crystalline phases, whose formation is entirely driven by excluded volume effects. We map out the phase diagram of short and long HBPs by gradually modifying their shape from prolate to oblate and investigate the long-range order of the resulting morphologies along the phase directors and perpendicularly to them. The intrinsic biaxial nature of these particles promotes the formation of translationally ordered biaxial phases, but does not show solid evidence that it would, per se, promote the formation of the biaxial nematic phase. Our simulations shed light on the controversial existence of the discotic smectic phase, whose layers are as thick as the minor particle dimension, which is stable in a relatively large portion of our phase diagrams. Additionally, we modify the Onsager theory to describe the isotropic-nematic phase transition of freely rotating biaxial particles as a function of the particle width, and find a relatively strong first-order signature, in excellent agreement with our simulations. In an attempt to shed light on the elusive formation of the biaxial nematic phase, we apply this theory to predict the uniaxial-biaxial nematic phase transition and confirm, again in agreement with simulations, the prevailing stability of the translationally ordered smectic phase over the orientationally ordered biaxial nematic phase.

Monte Carlo simulation of binary mixtures of hard colloidal cuboids

“…While our particles are identical to those designed by Belli, they are completely free to assume any possible orientation, rather than only the six allowed by the Zwanzig model. This difference is particularly relevant when assessing the formation of uniaxial and biaxial phases, as we have recently noticed in systems of monodisperse boardlike particles [10]. Figure 1.…”

“…Our recent work on a wide range of oblate and prolate monodisperse HBPs highlighted the existence of a rich variety of LC phases, including the long-debated discotic smectic (Sm) phase, consisting of layers as thick as the particle minor axis [10]. However, no evidence of the existence of the N B phase could be provided, even at the so-called self-dual shape, a particle geometry almost exactly in between prolate and oblate.…”

“…The intriguing prospect of manufacturing biaxial LCDs has been enfeebled by the difficulty of obtaining a stable molecular N B phase, especially at convenient temperatures for display applications. The experimental findings by Vroege and coworkers, who observed a remarkably stable N B phase in systems of polydisperse goethite particles, have provided renewed expectations and perhaps an indication on how, at the molecular scale, the stability of the N B phase might be enhanced [8].Our recent work on a wide range of oblate and prolate monodisperse HBPs highlighted the existence of a rich variety of LC phases, including the long-debated discotic smectic (Sm) phase, consisting of layers as thick as the particle minor axis [10]. However, no evidence of the existence of the N B phase could be provided, even at the so-called self-dual shape, a particle geometry almost exactly in between prolate and oblate.…”

Hard Competition: Stabilizing the Elusive Biaxial Nematic Phase in Suspensions of Colloidal Particles with Extreme Lengths

“…smectic phase either [30]. Interestingly, however, recent simulations of hard spheroplatelets (with a stable smectic phase in their phase diagram) also revealed a stable N b phase [31], whereas ostensibly similarly shaped cuboidal particles do not [32]. On top of this confusing situation comes an unsettled issue regarding the topology of the phase diagram, in particular whether a prolate (N + ) or oblate (N − ) uniaxial nematic phase intervenes the isotropic (I) and N b phase or whether a direct I − N b phase transition is possible.…”

“…Interestingly, for dual-shaped spheroplatelets that closely resemble our cuboids at first sight, recent simulations [31,42] revealed an N b phase for L * > 9 rather than L * > 23 for cuboids. This trend in which particle shapes with triangular, rounded, or rhombic cross-sections have smaller L * min than cuboids suggests that the 2D packing efficiency of the particle cross-section in a smectic layer largely determines L * min [32,34,43,44]. Rhombic platelets with their minimal L * min are indeed ideal candidates for observing a N b phase.…”