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

Adhesion of surfaces mediated by adsorbed particles: Monte Carlo simulations and a general relationship between adsorption isotherms and effective adhesion energies

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.In colloidal and biological systems, interactions between surfaces are often mediated by adsorbed particles or molecules that interconnect the surfaces. In this article, we present a general relationship between the adsorption isotherms of the particles and the effective, particle-mediated adhesion energies of the surfaces. Our relationship is based on the analysis and modeling of detailed data from Monte Carlo simulations. As general properties that should hold for a wide class of adsorption scenarios, we find (i) that the particle-mediated adhesion energies of surfaces are maximal at intermediate bulk concentrations of the particles, and (ii) that the particle coverage in the bound state of the surfaces is twice the coverage in the unbound state at these bulk concentrations.DFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface

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

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. 145, 164903 (2016) and may be found at https://doi.org/10.1063/1.4965711.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 K1, K2, and K3 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.DFG, 65143814, GRK 1524: Self-Assembled Soft-Matter Nanostructures at Interface

Coarse-grained treatment of the self-assembly of colloids suspended in a nematic host phase

The complex interplay of molecular scale effects, nonlinearities in the orientational field and long-range elastic forces makes liquid-crystal physics very challenging. A consistent way to extract information from the microscopic, molecular scale up to the meso- and macroscopic scale is still missing. Here, we develop a hybrid procedure that bridges this gap by combining extensive Monte Carlo (MC) simulations, a local Landau-de Gennes theory, classical density functional theory, and finite-size scaling theory. As a test case to demonstrate the power and validity of our novel approach we study the effective interaction among colloids with Boojum defect topology immersed in a nematic liquid crystal. In particular, at sufficiently small separations colloids attract each other if the angle between their center-of-mass distance vector and the far-field nematic director is about 30 degrees. Using the effective potential in coarse-grained two-dimensional MC simulations we show that self-assembled structures formed by the colloids are in excellent agreement with experimental data.DFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface

Nichtlineare Nichtgleichgewichtsdynamik in nematischen Flüssigkristallen

Liquid crystals are elongated molecules with a rich and surprising phase behavior. Nonequilibrium conditions open a myriad possibilities of manipulating matter, and reach collective states not accessible under equilibrium conditions. We perform nonequilibrium molecular dynamics simulations of a nematic liquid crystal flowing around a colloidal particle. Because of a mismatch between the nematic far field alignment and the local orientation of the liquid-crystal molecules at the surface of the colloid, defect topologies arise if the host is in thermodynamic equilibrium. We study the flow-induced modifications of these topological defects. We find that Saturn ring defects are convected downstream along the flow direction, which is in agreement with experimental observations [1]. As Poiseuille flow is initiated, the Saturn ring is deformed. The degree of deformation is analyzed quantitatively in terms of characteristic geometric parameters fitted to suitable projections of the Saturn ring. Our results suggest that smaller Saturn rings are shifted downstream while approximately maintaining their circular shape, whereas larger ones exhibit an elastic deformation in addition. Additionally, we show that flow distorts Boojum defects into an asymmetrically larger downstream lobe. For a Janus colloid, exhibiting a Boojum defect and a Saturn ring defect, we find that the Boojum defect facing the upstream direction is destroyed and the Saturn ring is convected downstream. Furthermore, we study a similar system of a nematic liquid crystal flowing around a cylindrical pillar. We report flow-induced cavitation in an anisotropic fluid. Cavitation domains nucleate due to a sudden drop in pressure upon flow past the cylindrical obstacle. The inception and growth of cavitation domains ensue in the laminar flow regime. We study the physical principles governing the cavitation phenomena in nematic liquid crystals, and identify a critical value of the Reynolds number for cavitation inception that scales inversely with the characteristic order parameter of the nematic liquid crystal. Strikingly, the critical Reynolds number can be as low as about 50% of the cavitation threshold in the isotropic liquid crystal. These findings suggest that long range ordering, and its tunability, can be potentially applied as a novel control parameter to modulate cavitation inception in anisotropic fluids. Additionally, we find very good agreement with earlier microfluidic experiments [2] at smaller flow speeds before cavitation initiates. Our simulations are able to reproduce the structural changes within the microfluidic channel at different flow speeds.Mit Hilfe von Molekulardynamiksimulationen im Nichtgleichgewicht untersuchen wir einen nematischen Flüssigkristall, welcher um ein kolloidales Teilchen fließt. Die Diskrepanz zwischen der globalen nematischen Vorzugsrichtung und der lokalen Orientierung der Flüssigkristallmoleküle an der Kolloidoberfläche führt zu Defekttopologien im thermodynamischen Gleichgewicht. Wir untersuchen die Modifikation der topologischen Defekte durch Fluss. Unsere Untersuchungen ergeben, in Übereinstimmung mit experimentellen Befunden [1], dass ein Saturnringdefekt in Flussrichtung verschoben wird. Zusätzlich wird der Saturnring durch den Poiseuillefluss verformt. Die Verformung lässt sich anhand von geometrischen Parametern aus Anpassungen geeigneter Projektionen des Saturnrings quantitativ analysieren. Unsere Simulationen zeigen, dass kleinere Saturnringe stromabwärts verschoben werden und ihre Kreisform nahezu behalten. Größere Saturnringdefekte hingegen weisen eine elastische Verformung auf, welche von der Kreisform abweicht. Weiterhin zeigen wir, dass der Fluss einen Boojumdefekt in einen asymmetrischen größeren Defekt flussabwärts verformt. Januskolloide weisen eine Kombination aus Boojum- und Saturnringdefekt auf. Unsere Untersuchung ergibt, dass der Boojumdefekt flussaufwärts zerstört wird und der Saturnring flussabwärts verschoben wird. Außerdem untersuchen wir ein vergleichbares System, in welchem der nematische Flüssigkristall um eine zylindrische Säule fließt. Wir beobachten flussinduzierte Kavitation in einer anisotropen Flüssigkeit. Kavitationsdomänen bilden sich auf Grund des Druckverlusts hinter der zylindrischen Säule. Sowohl die Entstehung als auch das Wachstum der Kavität findet unter laminaren Flussbedingungen statt. Wir untersuchen im Detail die physikalischen Ursachen, welche zum Phänomen der Kavitation in einem nematischen Flüssigkristall führen. Weiterhin bestimmen wir den kritischen Wert der Reynoldszahl für den Beginn der Kavität. Dieser kritische Wert skaliert invers mit dem charakteristischen Ordnungsparameter für einen nematischen Flüssigkristall. Markanterweise kann die kritische Reynoldszahl bis zu 50% niedriger sein als der Kavitationsbeginn in einem isotropen Flüssigkristall. Diese Befunde weisen darauf hin, dass die langreichweitige Ordnung potentiell als Kontrollparameter verwendet werden kann um den Beginn von Kavitation in anisotropen Flüssigkeiten zu beeinflussen. Zusätzlich erreichen wir sehr gute Übereinstimmung mit mikrofluidischen Experimenten [2] für geringere Flussgeschwindigkeiten, bevor die Kavitation einsetzt. Unsere Simulationen sind in der Lage die strukturellen Veränderungen innerhalb des Mikrokanals für verschiedene Flussgeschwindigkeiten zu reproduzieren