Defect topologies in chiral liquid crystals confined to mesoscopic channels

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 142, 194704 (2015) and may be found at https://doi.org/10.1063/1.4920979.We present Monte Carlo simulations in the grand canonical and canonical ensembles of a chiral liquid crystal confined to mesochannels of variable sizes and geometries. The mesochannels are taken to be quasi-infinite in one dimension but finite in the two other directions. Under thermodynamic conditions chosen and for a selected value of the chirality coupling constant, the bulk liquid crystal exhibits structural characteristics of a blue phase II. This is established through the tetrahedral symmetry of disclination lines and the characteristic simple-cubic arrangement of double-twist helices formed by the liquid-crystal molecules along all three axes of a Cartesian coordinate system. If the blue phase II is then exposed to confinement, the interplay between its helical structure, various anchoring conditions at the walls of the mesochannels, and the shape of the mesochannels gives rise to a broad variety of novel, qualitative disclination-line structures that are reported here for the first time.DFG, 65143814, GRK 1524: Self-Assembled Soft-Matter Nanostructures at Interface

A biaxial nematic liquid crystal composed of matchbox-symmetric molecules

By means of Monte Carlo simulations in the isothermal-isobaric ensemble, we investigate the structure and phase behaviour of a thermotropic liquid crystal composed of matchbox-symmetric (or board-like) molecules. Besides the isotropic phase the liquid crystal exhibits also uniaxial and biaxial nematic phases. The interaction potential is derived through an expansion in terms of Stone's rotational invariants [A. J. Stone, Mol. Phys. 78, 241–256 (1978).] that can be reexpressed in terms of Cartesian tensors. This latter formulation is particularly well suited for computer simulations. We analyse the orientation distribution function which allows us to distinguish between intrinsic and extrinsic biaxiality. In addition, we study the orientation-dependent correlation functions. In the limit of large intermolecular separations, the value of the orientation correlation function corresponds to the uniaxial and biaxial order parameters which are coupled in a complex fashion

Flow-assisted self-healing of the helical structure in a cholesteric liquid crystal

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 155, 054903 (2021) and may be found at https://doi.org/10.1063/5.0058745.We employ nonequilibrium molecular dynamics simulations to investigate the structure and dynamics of a cholesteric liquid crystal confined between atomically corrugated solid walls. By choosing walls normal to the helical axis, we can study systems with an arbitrary cholesteric pitch without exposing the cholesteric helix to a spurious stress. We investigate the effects of local heating and flow and their joint effects. A steady-state laminar Poiseuille flow is initiated by means of an external body force. Flow alone (i.e., without local heating) in a direction normal to the helical axis does not affect the cholesteric pitch. If the liquid crystal is heated in a small region, the cholesteric helix becomes unstable and melts locally. However, if local heating and flow are combined, a nontrivial synergistic effect is observed in that the helical structure recuperates the better, the higher the speed of the flow is

ORIGAMI: A New Interface for Fuel Assembly Characterization with ORIGEN

<p>ORIGAMI (ORIGen Assembly Isotopics) is a new interface to the well-established ORIGEN code<br>designed to enable simplified, 3-D depletion and decay calculations for used nuclear fuel (UNF) assemblies.</p> <p><br>ORIGAMI calculates assembly characteristics through the use of individual discrete depletion nodes, corresponding to axial zones and pin locations. Using a nominal irradiation history, ORIGAMI individually calculates each node’s<br>isotopic composition using axial and radial relative power distribution maps to create a zone-specific burnup, enabling users to easily specify assembly-wide power distributions for modeling.<br>ORIGAMI includes several useful capabilities for UNF assembly analyses, including visualization of key parameters by generating MeshView-compatible output files, generation of materials cards for MCNP and KENO for further source term analysis (for example, cask loading and dose studies for dry<br>storage and transport and simulation of full assemblies for NDA instrumentation studies).</p> <p>Additionally, ORIGAMI offers the ability to store individual depletion zone concentrations in an ORIGEN-compatible binary file for later re-evaluation (such as decay analysis of previously-depleted assemblies).</p> <p><em>Paper presented at the 2015 International High-Level Radioactive Waste Management Conference in Charleston, SC.</em></p

Flow-assisted self-healing of the helical structure in a cholesteric liquid crystal

We employ nonequilibrium molecular dynamics simulations to investigate the structure and dynamics of a cholesteric liquid crystal confined between atomically corrugated solid walls. By choosing walls normal to the helical axis, we can study systems with an arbitrary cholesteric pitch without exposing the cholesteric helix to a spurious stress. We investigate the effects of local heating and flow and their joint effects. A steady-state laminar Poiseuille flow is initiated by means of an external body force. Flow alone (i.e., without local heating) in a direction normal to the helical axis does not affect the cholesteric pitch. If the liquid crystal is heated in a small region, the cholesteric helix becomes unstable and melts locally. However, if local heating and flow are combined, a nontrivial synergistic effect is observed in that the helical structure recuperates the better, the higher the speed of the flow is

The temperature dependence of the helical pitch in a cholesteric liquid crystal

We investigate the temperature dependence of the helical pitch of a cholesteric liquid crystal by means of Monte Carlo simulations. We carry out both lattice and off-lattice simulations to assess the impact of geometric and modelling constraints on the properties of the cholesteric phase. For the off-lattice simulations we develop boundary conditions commensurate with the cholesteric phase and derive an analytic expression for the helical wavenumber q that works well qualitatively. We find that the common simplification of constraining the orientation of the mesogens to planes normal to the helical axis makes q temperature-independent, as predicted by a mean-field theory of van der Meer et al. [J. Chem. Phys. 65, 3935 (1976)]. However, if mesogens are allowed to rotate in three dimensions, q will increase with temperature, as the isotropic-cholesteric transition is approached from below, in agreement with experiments for a number of substances. Our simulations indicate that the temperature-independent q is merely a consequence of the overly restricted orientational degrees of freedom to points on the unit circle in the model on which the mean-field theory is based

Emergent biaxiality in nematic microflows illuminated by a laser beam

Anisotropic fluids (e.g. liquid crystals) offer a remarkable promise as optofluidic materials owing to the directional, tunable, and coupled interactions between the material, flow, and the optical fields. Here we present a comprehensive in silico treatment of this anisotropic interaction by performing nonequilibrium molecular dynamics simulations. We quantify the response of a nematic liquid crystal (NLC) undergoing a Poiseuille flow in the Stokes regime, while being illuminated by a laser beam incident perpendicular to the flow direction. We adopt a minimalistic model to capture the interactions, accounting for two features: first, the laser heats up the NLC locally; and second, the laser polarises the NLC and exerts an optical torque that tends to reorient molecules of the nematic phase. Because of this reorientation the liquid crystal exhibits small regions of biaxiality, where the nematic director is one symmetry axis and the axis of rotation for the reorientation of the molecules is the other one. We find that the relative strength of the viscous and the optical torques mediates the flow-induced response of the biaxial regions, thereby tuning the emergence, shape and location of the regions of enhanced biaxiality. The mechanistic framework presented here promises experimentally tractable routes toward novel optofluidic applications based on material-flow-light interactions