A novel model for smectic liquid crystals: Elastic anisotropy and response to a steady-state flow

Abstract

By means of a combination of equilibrium Monte Carlo and molecular dynamics simulations and nonequilibrium molecular dynamics we investigate the ordered, uniaxial phases (i.e., nematic and smectic A) of a model liquid crystal. We characterize equilibrium behavior through their diffusive behavior and elastic properties. As one approaches the equilibrium isotropic-nematic phase transition, diffusion becomes anisotropic in that self-diffusion D in the direction orthogonal to a molecule's long axis is more hindered than self-diffusion D in the direction parallel to that axis. Close to nematic-smectic A phase transition the opposite is true, D < D. The Frank elastic constants K, K, and K for the respective splay, twist, and bend deformations of the director field n̂ are no longer equal and exhibit a temperature dependence observed experimentally for cyanobiphenyls. Under nonequilibrium conditions, a pressure gradient applied to the smectic A phase generates Poiseuille-like or plug flow depending on whether the convective velocity is parallel or orthogonal to the plane of smectic layers. We find that in Poiseuille-like flow the viscosity of the smectic A phase is higher than in plug flow. This can be rationalized via the velocity-field component in the direction of the flow. In a sufficiently strong flow these smectic layers are not destroyed but significantly bent.

The Impact of Colloidal Surface-Anchoring on the Smectic A Phase

“…A parallel inspection of plots of the nematic order parameter S as a function of T reveals that T IN from finite-size scaling agrees very nicely with the point of inflection in the plot of S ( T ). By a similar token we estimate the temperature T NSma of the nematic–smectic A phase transition in our earlier work . However, we note in passing that in order to localize T NSmA with enhanced precision one would need to resort to a calculation of the free energy as suggested by Canabarro et al or by Zakharov and Sullivan .…”

“…Before we turn to a discussion of colloidal suspensions, we introduce properties of the host fluid in the bulk. In our previous work we have used various quantities to investigate the phase behavior, structural, dynamical, and elastic properties of the bulk phases formed by the mesogens at different temperatures. The mesogens are capable of undergoing an isotopic-nematic and nematic–smectic A phase transition at T IN ≃ 0.87 and T NSmA ≃ 0.78, respectively.…”

“…We emphasize that our model potential is attractive if the mesogens are in the side–side configuration ( û i · û j = ±1 and û i · r̂ ij = û j · r̂ ij = 0) and soft repulsive if the mesogens’ configuration is end–end ( û i · û j = û i · r̂ ij = û j · r̂ ij = ±1). The model potential introduced in eq is capable of forming various liquid-crystalline phases (i.e., nematic and smectic A phase) for a suitable choice of the thermodynamic state parameters …”

“…To the best of our knowledge, there are no studies of colloidal inclusions in smectic phases investigating the perturbations of smectic layers in the vicinity of a colloidal particle. To close this gap, we perform Monte Carlo (MC) simulations based on a novel interaction potential, which we use to model the liquid-crystalline host phase. We compute local quantities (i.e., density, nematic order parameter, director, etc.)…”

A biaxial nematic liquid crystal composed of matchbox-symmetric molecules

“…Systems composed of pseudo-shaped mesogens generally allow for an equilibration that is orders of magnitude faster than for models with explicit shape dependence. At the same time pseudo-shaped mesogens do exhibit physically sensible properties as we demonstrated in a number of earlier publications [51][52][53]. From our point of view purely entropic interactions have quite a few shortcomings as far as simulation studies are concerned.…”