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Register a new userSelf-assembly of Freely-rotating Polydisperse Cuboids: Unveiling the Boundaries of the Biaxial Nematic Phase
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Colloidal cuboids have the potential to self-assemble into biaxial liquid crystal phases, which exhibit two independent optical axes. Over the last few decades, several theoretical works predicted the existence of a wide region of the phase diagram where the biaxial nematic phase would be stable, but imposed rather strong constraints on the particle rotational degrees of freedom. In this work, we employ molecular simulation to investigate the impact of size dispersity on the phase behaviour of freely-rotating hard cuboids, here modelled as self-dual-shaped nanoboards. This peculiar anisotropy, exactly in between oblate and prolate geometry, has been proposed as the most appropriate to promote phase biaxiality. We observe that size dispersity radically changes the phase behaviour of monodisperse systems and leads to the formation of the elusive biaxial nematic phase, being found in an large region of the packing fraction vs polydispersity phase diagram. Although our results confirm the tendencies reported in past experimental observations on colloidal dispersions of slightly prolate goethite particles, they cannot reproduce the direct isotropic-to-biaxial nematic phase transition observed in these experiments.
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We report small-angle x-ray scattering (SAXS) experiments on aqueous dispersions of colloidal silica with a broad monomodal size distribution (polydispersity 18%, size 8 nm). Over a range of volume fractions the silica particles segregate to build first one, then two distinct sets of colloidal crystals. These dispersions thus demonstrate fractional crystallization and multiple-phase (bcc, Laves AB$_2$, liquid) coexistence. Their remarkable ability to build complex crystal structures from a polydisperse population originates from the intermediate-range nature of interparticle forces, and suggests routes for designing self-assembling colloidal crystals from the bottom-up.
Field-induced reorientation of colloidal particles is especially relevant to manipulate the optical properties of a nanomaterial for target applications. We have recently shown that surprisingly feeble external stimuli are able to transform uniaxial nematic liquid crystals (LCs) of cuboidal particles into biaxial nematic LCs. In the light of these results, here we apply an external field that forces the reorientation of colloidal cuboids in nematic LCs and sparks a uniaxial-to-biaxial texture switching. By Dynamic Monte Carlo simulation, we investigate the unsteady-state reorientation dynamics at the particle scale when the field is applied (uniaxial-to-biaxial switching) and then removed (biaxial-to-uniaxial switching). We detect a strong correlation between the response time, being the time taken for the system to reorient, and particle anisotropy, which spans from rod-like to plate-like geometries. Interestingly, self-dual shaped cuboids, theoretically considered as the most suitable to promote phase biaxiality for being exactly in between prolate and oblate particles, exhibit surprisingly slow response times, especially if compared to prolate cuboids.
Investigations of the phase diagram of biaxial liquid crystal systems through analyses of general Hamiltonian models within the simplifications of mean-field theory (MFT), as well as by computer simulations based on microscopic models, are directed towards an appreciation of the role of the underlying molecular-level interactions to facilitate its spontaneous condensation into a nematic phase with biaxial symmetry. Continuing experimental challenges in realising such a system unambiguously, despite encouraging predictions from MFT for example, are requiring more versatile simulational methodologies capable of providing insights into possible hindering barriers within the system, typically gleaned through its free energy dependences on relevant observables as the system is driven through the transitions. The recent brief report from this group [B. Kamala Latha, et. al., Phys. Rev. E 89, 050501 (R), 2014] summarizing the outcome of detailed Monte Carlo simulations carried out employing entropic sampling technique, suggested a qualitative modification of the MFT phase diagram as the Hamiltonian is asymptotically driven towards the so-called partly-repulsive regions. It was argued that the degree of the (cross) coupling between the uniaxial and biaxial tensor components of neighbouring molecules plays a crucial role in facilitating, or otherwise, a ready condensation of the biaxial phase, suggesting that this could be a plausible f actor in explaining the experimental difficulties. In this paper, we elaborate this point further, providing additional evidences from curious variations of free-energy profiles with respect to the relevant orientational order parameters, at different temperatures bracketing the phase transitions.
Phase sequences of the biaxial nematic liquid crystal in the interior of the essential triangle are studied with Wang Landau sampling. The evidence points to the existence of an intermediate unixial phase with low biaxiality in the isotropic to biaxial nematic phase sequence.
The biaxial smectic-A* (Sm-A_B*) phase, appearing in the phase sequence Sm-A*--Sm-A*_B--Sm-C*, is analyzed using Landau theory. It is found to possess a helical superstructure with a pitch that is significantly shorter than the pitch of the Sm-C* helical superstructure. The Sm-A_B*--Sm-C* transition can be either 1st or 2nd order, and correspondingly there will be either a jump or continuous variation in the pitch. The behaviors of the birefringence and electroclinic effect are analyzed and found to be similar to those of a Sm-C*_alpha phase. As such, it is possible that the Sm-A_B* phase could be misidentified as a Sm-C*alpha phase. Ways to distinguish the two phases are discussed.
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