1,721,139 research outputs found
From idealised to predictive models of liquid crystals
Full text linkA brief review and personal perspective of modelling and simulations of
liquid crystals and of how this has dramatically changed over the years,
from idealised lattice to molecular level and now to predictive
atomistic models.
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From Chiral Islands to Smectic Layers: A Computational Journey Across Sexithiophene Morphologies on C-60
No full textA theoretical investigation of the molecular organization at a sexithiophene (T6)-C-60 fullerene planar heterojunction, based on atomistic molecular dynamics, is presented, in which the effect of two different sample preparation processes on the resulting interface morphology is explored. First, the landing of T6 on C-60(001) substrate is considered, which leads to crystalline layers of standing and tilted molecules, in accordance with experiments. The observation and the quantitative characterization of the nucleation and growth provide detailed insights on this out-of-equilibrium process, including the establishment of an epitaxial relationship between the substrate and the interfacial T6 layer, and the spontaneous formation of defective islands, characterized by chiral edges, during the growth of the second and third layers. It is then shown that molecular orientations can be radically changed upon annealing at 600 K, at which T6 forms a smectic phase with planar alignment, whose layers are perpendicular to the interface. The interfacial T6 morphologies are then analyzed in detail at room temperature and compared to the known bulk polymorphs
Rotational diffusion of shape switching particles in nematic liquid crystals
No full textThe theory of rotational diffusion of particles of various symmetry embedded in a liquid crystal host, essential to interpret a variety of spectroscopic observables, has been available for some time, but only for the case of rigid molecules. Here we generalize the treatment and present a theory to describe the rotational diffusion of shape-changing particles dispersed in nematic liquid crystals. The interaction of the particles with the environment is modeled by an effective field potential, while the particles are allowed to assume an arbitrary discrete number of shapes. The transition between shapes is modeled by a Markovian process which is combined with rotational diffusion. Our model is applied to the simple case of a particle that can exchange between three shapes: a rod, a disk, and a sphere. We consider in detail the effect of shape transitions in some selected correlation functions which are relevant for experiments
Ferroelectric and structured phases from polar tapered mesogens
No full textWe investigate the effect of changing the molecular dipole strength on the liquid crystal phases formed in a system of tapered Gay-Berne particles with an axial dipole. We show that increasing the strength of the dipole has a profound effect on the stability and molecular organisation of the phases and eventually destroys the polar ferroelectric nematic
Submicron object recognition in nematic liquid crystals
No full textWe present large-scale lattice Monte Carlo simulations of nematic ordering in the vicinity of submicron objects â colloids or viruses â of different shapes (spherical, cubical or icosahedral) immersed in a nematic liquid crystal. We simulate polarized light transmission patterns for the various particles and find that they are distinguishable, suggesting that they might be useful for object shape recognition or, in case of viruses, for diagnostic purposes
Molecular organizations of conical mesogenic fullerenes
No full textWe have studied liquid crystal phases formed by fullerenes functionalized with mesogenic groups yielding a cone-shaped molecular structure. We have modelled these shuttlecock-like molecules with a set of Gay-Berne particles grafted with flexible springs to a spherical core and we have studied, using Monte Carlo simulations, their phase organization, also with a view to examining their possible use as candidate organic photovoltaic materials. We have found that, upon cooling from the isotropic phase, the system forms a columnar phase, like in the experimental work of Kato and coworkers [T. Kato et al., Nature, 2002, 419, 702]. However the phase is made of polar stacks extending not more than about ten molecules, which could limit their usefulness in enhancing and directing charge transport for possible photovoltaic applications
On the effects of dispersing polar nanoparticles in liquid crystals
Full text linkWe have investigated the effect of adding polar rodlike nanoparticles (NPs) to a liquid crystal using Monte Carlo simulations. The mesogens (Ms) are represented with Gay–Berne elongated ellipsoids endowed with a central axial electric dipole. A NP is instead modelled by a overall rod-like set of rigidly assembled Lennard–Jones spherical beads (Orlandi et al. Phys. Chem. Chem. Phys. 18, 2428–2441 (2016). doi:10.1039/C5CP05754J) that are either non-polar or endowed with a central axial dipole of different strengths. We consider two cases: one of strong NP-M affinity and weak NPNP interactions (case 1) and the opposite one of weak NP-M affinity and strong NP-NP interactions (case 2). We find that for case 1 adding polar NPs slightly lowers the nematic isotropic transition temperature TNI which instead, for case 2, is essentially unaffected. Having strongly polar, instead of non-polar NPs reduces the TNI difference with the pristine one, while significantly increasing the dielectric anisotropy in the nematic phase, which could be useful in applications
Can elastic constants and surface alignment be obtained from polarized microscopy images of nematic droplets? A Monte Carlo study
No full textWe present a Monte Carlo study of the effects of elastic anisotropy on
the textures of nematic droplets with different boundary conditions.
Simulated polarized optical microscopy (POM) images are analysed for
uniaxial nematics with radial, bipolar and toroidal boundaries and for
numerous combinations of the splay, twist and bend elastic constant
values. An atlas of simulated textures is presented as an aid to
understanding new experimental results. (C) 2017 Elsevier B.V. All
rights reserved
Predicting the Conditions for Homeotropic Anchoring of Liquid Crystals at a Soft Surface. 4-n-Pentyl-4′-cyanobiphenyl on Alkylsilane Self-Assembled Monolayers
No full textWe have studied, using atomistic molecular dynamics simulations, the alignment of the nematic liquid-crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) on self-assembled monolayers (SAMs) formed from octadecyl- and/or hexyltrichlorosilane (OTS and HTS) attached to glassy silica. We find a planar alignment on OTS at full coverage and an intermediate situation at partial OTS coverage because of the penetration of 5CB molecules into the monolayer, which also removes the tilt of the OTS SAM. Binary mixtures of HTS and OTS SAMs instead induce homeotropic (i.e., perpendicular) alignment. A comparison with the existing experimental literature is provided
Can off-centre mesogen dipoles extend the biaxial nematic range?
No full textWe have investigated the possibility of extending the stability range of the biaxial nematic phase by adding an off-centre dipole of various strengths and orientations to elongated biaxial Gay-Berne (GB) mesogens yielding a relatively narrow biaxial nematic (Nbx) phase, and a smectic (Sbx) phase when dipole-less. The effect of dipoles is not easy to predict, and our previous investigations have shown the limited benefits of having a central dipole. Here we show, employing molecular dynamics (MD) simulations, that a not too strong off-centre dipole positioned along the longest axis of the nematogen can extend the temperature range of stability of the biaxial nematic phase, also shifting it towards lower temperatures
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