arXiv subjects

R. L. B. Selinger

Publications and source records attributed to R. L. B. Selinger.

5 recordsLinked to original sources

arXiv2001

Monte Carlo Simulation of Liquid-Crystal Alignment and of Chiral Symmetry-Breaking

We carry out Monte Carlo simulations to investigate the effect of molecular shape on liquid-crystal order. In our approach, each model mesogen consists of several soft spheres bonded rigidly together. The arrangement of the spheres may be straight (to represent uniaxial molecules), Z-shaped (for biaxial molecules), or banana-shaped (for bent-core molecules). Using this approach, we investigate the alignment of the nematic phase by substrates decorated with parallel ridges. We compare results for wide and narrow ridge spacing and examine local order near the substrates, and show that our results are consistent with the predictions of Landau theory. We also investigate chiral symmetry-breaking in systems of bent-core molecules. We find a chiral crystalline phase as well as a nonchiral smectic-A phase, but not a chiral smectic-C phase.

cond-mat.soft↗

arXiv2000

Theory of Chiral Modulations and Fluctuations in Smectic-A Liquid Crystals Under an Electric Field

Chiral liquid crystals often exhibit periodic modulations in the molecular director; in particular, thin films of the smectic-C* phase show a chiral striped texture. Here, we investigate whether similar chiral modulations can occur in the induced molecular tilt of the smectic-A phase under an applied electric field. Using both continuum elastic theory and lattice simulations, we find that the state of uniform induced tilt can become unstable when the system approaches the smectic-A--smectic-C* transition, or when a high electric field is applied. Beyond that instability point, the system develops chiral stripes in the tilt, which induce corresponding ripples in the smectic layers. The modulation persists up to an upper critical electric field and then disappears. Furthermore, even in the uniform state, the system shows chiral fluctuations, including both incipient chiral stripes and localized chiral vortices. We compare these predictions with observed chiral modulations and fluctuations in smectic-A liquid crystals.

cond-mat.soft↗

arXiv1999

Monte Carlo Simulation of Smectic Liquid Crystals and the Electroclinic Effect: the Role of the Molecular Shape

Using Monte Carlo simulation methods, we explore the role of molecular shape in the phase behavior of liquid crystals and the electroclinic effect. We study a "bent-rod" mesogen shaped like the letter Z, composed of seven soft spheres bonded rigidly together with no intra-molecular degrees of freedom. For strongly angled molecules, we find that steric repulsion alone provides the driving force for a smectic-C phase, even without intermolecular dipole-dipole interactions. For weakly angled (nearly rod-like) molecules, we find a stable smectic-A (SmA) phase and a strong electroclinic effect with a saturation tilt angle of about 19 degrees. In the SmA phase we find evidence of vortex-like point defects. We also observe a field-induced nematic-smectic phase transition.

cond-mat.soft↗

arXiv1996

Cooperative Chiral Order in Copolymers of Chiral and Achiral Units

Polyisocyanates can be synthesized with chiral and achiral pendant groups distributed randomly along the chains. The overall chiral order, measured by optical activity, is strongly cooperative and depends sensitively on the concentration of chiral pendant groups. To explain this cooperative chiral order theoretically, we map the random copolymer onto the one-dimensional random-field Ising model. We show that the optical activity as a function of composition is well-described by the predictions of this theory.

cond-mat↗

arXiv1995

Theory of Chiral Order in Random Copolymers

Recent experiments have found that polyisocyanates composed of a mixture of opposite enantiomers follow a chiral ``majority rule:'' the chiral order of the copolymer, measured by optical activity, is dominated by whichever enantiomer is in the majority. We explain this majority rule theoretically by mapping the random copolymer onto the random-field Ising model. Using this model, we predict the chiral order as a function of enantiomer concentration, in quantitative agreement with the experiments, and show how the sharpness of the majority-rule curve can be controlled.

cond-mat↗