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Register a new userDynamics of Uniaxial-to-Biaxial Nematics Switching in Suspensions of Hard Cuboids
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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.
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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.
Understanding how colloidal suspensions behave in confined environments has a striking relevance in practical applications. Despite the fact that the behaviour of colloids in the bulk is key to identify the main elements affecting their equilibrium and dynamics, it is only by studying their response under confinement that one can ponder the use of colloids in formulation technology. In particular, confining fluids of anisotropic particles in nanopores provides the opportunity to control their phase behaviour and stabilise a spectrum of morphologies that cannot form in the bulk. By properly selecting pore geometry, particle architecture and system packing, it is possible to tune thermodynamic, structural and dynamical properties for ad hoc applications. In the present contribution, we report Grand Canonical and Dynamic Monte Carlo simulations of suspensions of colloidal cubes and cuboids constrained into cylindrical nanopores of different size. We first study their phase behaviour, calculate the chemical potential vs density equation of state and characterise the effect of the pore walls on particle anchoring and layering. In particular, at large enough concentrations, we observe the formation of concentric nematic-like coronas of oblate or prolate particles surrounding an isotropic core, whose features resemble those typically detected in the bulk. We then analyse the main characteristics of their dynamics and discover that these are dramatically determined by the ability of particles to diffuse in the longitudinal and radial direction of the nanopore.
Equilibrium director structures in two thin hybrid planar films of biaxial nematics are investigated through Markov chain Monte Carlo simulations based on a lattice Hamiltonian model within the London dispersion approximation. While the substrates of the two films induce similar anchoring influences on the long axes of the liquid crystal molecules (viz. planar orientation at one end and perpendicular, or homeotropic, orientations at the other), they differ in their coupling with the minor axes of the molecules. In Type-A film the substrates do not interact with the minor axes at all (which is experimentally relatively more amenable), while in Type-B, the orientations of the molecular axes at the surface layer are influenced as well by their biaxial coupling with the surface. Both films exhibit expected bending of the director associated with ordering of the molecular long axes due to surface anchoring. Simulation results indicate that the Type-A film hosts stable and noise free director structures in the biaxial nematic phase of the LC medium, resulting from dominant ordering of one of the minor axes in the plane of the substrates. High degree of this stable order thus developed could be of practical interest for in-plane switching applications with an external field. Type-B film, on the other hand, experiences competing interactions among the minor axes, due to incompatible anchoring influences at the bounding substrates, apparently leading to frustration, and hence to noisy equilibrium director structures.
Within the framework of liquid crystal flows, the Qian & Sheng (QS) model for Q-tensor dynamics is compared to the Volovik & Kats (VK) theory of biaxial nematics by using Hamiltons variational principle. Under the assumption of rotational dynamics for the Q-tensor, the variational principles underling the two theories are equivalent and the conservative VK theory emerges as a specialization of the QS model. Also, after presenting a micropolar variant of the VK model, Rayleigh dissipation is included in the treatment. Finally, the treatment is extended to account for nontrivial eigenvalue dynamics in the VK model and this is done by considering the effect of scaling factors in the evolution of the Q-tensor.
We report on a large scale computer simulation study of crystal nucleation in hard spheres. Through a combined analysis of real and reciprocal space data, a picture of a two-step crystallization process is supported: First dense, amorphous clusters form which then act as precursors for the nucleation of well-ordered crystallites. This kind of crystallization process has been previously observed in systems that interact via potentials that have an attractive as well as a repulsive part, most prominently in protein solutions. In this context the effect has been attributed to the presence of metastable fluid-fluid demixing. Our simulations, however, show that a purely repulsive system (that has no metastable fluid-fluid coexistence) crystallizes via the same mechanism.
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