arXiv subjects

Demetri J. Photinos

Publications and source records attributed to Demetri J. Photinos.

17 recordsLinked to original sources

arXiv2025

Entropy stabilized form chirality in curved rod nematics: structure and symmetries

Monte Carlo molecular simulations of curve-shaped rods show the propensity of such shapes to polymorphism revealing both smectic and polar nematic phases. The nematic exhibits a nanoscale modulated local structure characterized by a unique, polar, $\rm{C}_2$-symmetry axis that tightly spirals generating a mirror-symmetry-breaking orgnization of the achiral rods=form chirality. A comprehensive characterization of the polarity and its symmetries in the nematic phase confirms that the nanoscale modulation is distinct from the elastic deformations of a uniaxial nematic director in the twist-bend nematic phase. Instead it is shown that, analogous to the isotropic-to-nematic transition, entropy stabilizes the roto-translating polar director in the polar-twisted nematic phase. The conflation of macroscale form chirality in ferroelectric nematics with that in the twist-bend nematic stems from the misattribution of the nanoscale modulation in the lower temperature nematic ''$\rm{N_X}$ phase'' found in CB7CB dimers.

cond-mat.soft↗

arXiv2020

Light scattering by a metallic nanoparticle coated with a nematic liquid crystal

We study the optical properties of gold nanoparticles coated with a nematic liquid crystal whose director field is distributed around the nanoparticle according to the anchoring conditions at the surface of the nanoparticle. The distribution of the nematic liquid crystal is obtained by minimization of the corresponding Frank free-energy functional whilst the optical response is calculated by the discrete-dipole approximation. We find, in particular, that the anisotropy of the nematic liquid-crystal coating does not affect much the (isotropic) optical response of the nanoparticle. However, for strong anchoring of the nematic liquid-crystal molecules on the surface of nanoparticle, the inhomogeneity of the coating which is manifested by a ring-type singularity (disclination or Saturn ring), produces an enhancement of the extinction cross spectrum over the entire visible spectrum.

cond-mat.soft↗

arXiv2020

'Setting things straight' by twisting and bending?

We explain the origins of the controversy about the classification of the $N_X$ phase observed in cyanobiphenyl dimers and why it is a polar twisted phase, an entirely new kind of nematic.

cond-mat.soft↗

arXiv2020

The Twist Bend Nematic: A Case of Mistaken Identity

We review the physics underlying Meyer's conjecture of how macroscopic-scale twist and bend conspire within the Frank-Oseen elasticity theory of nematics to create a heliconical arrangement of the uniaxial, apolar nematic director, the so-called "twist bend nematic" $\rm N_{TB}$. We show that since 2011 a second, lower-temperature nematic phase observed in odd methylene-linked cyanobiphenyl dimers discovered by Toriumi and called $\rm N_X$, has been incorrectly identified as $\rm N_{TB}$. Moreover, as more quantitative data on the $\rm N_X$ emerged, Meyer's simple prediction has been distorted to accommodate those findings. In fact, the molecular organization in the $\rm N_X$ conforms to the $\rm N_{PT}$ phase, a polar, twisted arrangement of nonlinear mesogens advanced in 2016. The attributes of the $\rm N_{PT}$ are summarized and differentiated from those of the $\rm N_{TB}$ in an effort to contribute to a better understanding of the $\rm N_X$ phase and, equally important, to encourage researchers to continue to search for a liquid crystal that exhibits Meyer's pioneering theoretical suggestion, namely that form-chirality can exist in simple nematics composed of achiral molecules.

cond-mat.soft↗

arXiv2019

Probing Molecular Ordering in the Nematic Phases of para-Linked Bimesogen Dimers through NMR Studies of Flexible Prochiral Solutes

The quadrupolar splittings of perdeuteriated n-decane dissolved in nematic phases formed by mesogenic dimers of the CBnCB series, for n=7,9,10,11 are measured throughout the entire temperature range of these phases. The results of the measurements, are reported together with related measurements using the common nematic phase of 5CB as a solvent for n-decane. The data obtained from the $^{13}$C spectra of the cyanobiphenyl mesogenic units of the monomeric and dimeric solvent molecules yield the order parameter of those units. The information obtained from this set of experiments is used to elucidate the structure of the low temperature (N$_X$) and the high temperature (N) nematic phases of CBnCB dimers with n=7,9,11. The polar twisted nematic (N$_{PT}$) model is found to provide a consistent description not only of these measurements, but also of NMR measurements previously reported in the literature for these phases. These findings suggest that the high temperature nematic (N) is not a common, locally uniaxial and apolar nematic, but rather a nematic phase consisting of N$_{PT}$ clusters. The twist-bend (N$_{TB}$) model, often identified with the N$_X$ phase, is shown to be inadequate to account even qualitatively for crucial features of the experimental findings.

cond-mat.mtrl-sci↗

arXiv2018

Atomistic simulations of nematic phases formed by cyano-biphenyl dimers

Molecular dynamics simulations of selected members of the cyano-biphenyl series of dimers (CBnCB) have been set up using atomistic detail interactions among intermolecular pairs of united atoms and allowing fully for the flexibility of the spacer chain. Detailed results are presented for the CB7CB dimers, showing an isotropic fluid phase and two nematic phases. The positional and orientational correlation functions extracted from the simulations are used to elucidate the structure of the low-temperature nematic phase. Polar molecular ordering is clearly identified along a direction undergoing helical twisting at right angles to the helical axis, with a constant pitch of about of 8nm. The local ordering of the various molecular segments is calculated and found to be in excellent agreement with experimental NMR measurements. Key findings of the simulation are shown to be correctly predicted by the theoretical model of the polar-twisted nematic (NPT) phase [A.G. Vanakaras, D.J. Photinos, Soft Matter. 12 (2016) 2208-2220]. The complete failure of the usual twist bend model (NTB) to account for these findings is demonstrated.

cond-mat.soft↗

arXiv2017

Polar molecular ordering in the Nx phase of bimesogens and enantiotopic discrimination in the NMR spectra of rigid prochiral solutes

The potential of mean torque governing the orientational ordering of prochiral solutes in the two nematic phases (N and Nx) formed by certain classes of symmetric achiral bimesogens is formulated and used for the analysis of existing NMR measurements on solutes of various symmetries dissolved in the two phases. Three distinct attributes of the solvent phase, namely polarity of the orientational ordering, chirality of the constituent molecules and spatial modulation of the local director, are identified as underlying three possible mechanisms for the generation of chiral asymmetry in the low temperature nematic phase (Nx). The role and quantitative contribution of each mechanism to enantiotopic discrimination in the NX phase are presented and compared with the case of the conventional chiral nematic phase (N*). It is found that polar ordering is essential for the appearance of enantiotopic discrimination in small rigid solutes dissolved in the Nx phase and that such discrimination is restricted to solutes belonging to the point group symmetries $C_s$ and $C_{2v}$.

cond-mat.soft↗

arXiv2016

Molecular Interactions in Chiral Nematic Liquid Crystals and Enantiotopic Discrimination through the NMR Spectra of Achiral Molecules I: Rigid Solutes

We have developed a molecular theory for the enantiotopic discrimination in prochiral solutes dissolved in chiral nematic solvents by means of NMR spectroscopy. The leading rank tensor contributions to the proposed potential of mean torque include symmetric as well as antisymmetric terms with respect to spatial inversion, these lead to a consistent determination of all the prochiral solute symmetries for which enantiotopes are distinguishable by NMR and also to excellent quantitative agreement when tested against the available experimental data for the rigid solute acenaphthene and for the moderately flexible ethanol.

cond-mat.soft↗

arXiv2015

A Molecular Theory of the Nematic-Nematic Phase Transitions in Mesogenic Dimers

We present a simplified molecular model of mesogenic dimers consisting of two identical uniaxial mesogenic cores separated by a fixed-length spacer and allowed to assume only two, statistically equivalent, conformations which are non-planar and of opposite handedness. In the mean-field approximation, with additive interactions among the mesogenic cores, the model yields up to three positionally disordered phases, one isotropic and two nematic. The low temperature nematic phase ($\mathrm{N_X}$) has a local two-fold symmetry axis which is also a direction of molecular polar ordering and is tightly twisted about a macroscopic phase axis. The onset of polar ordering generates spontaneous chiral symmetry breaking, manifested primarily by the twisting of the polar director and the formation of chiral domains of opposite handedness. Within these domains the statistical balance between the two enantiomer conformations is shifted and the principal axes of the ordering tensors of the molecular segments twist at constant tilt angles with the helix axis. Key experimental results on the $\mathrm{N_X}$ phase of liquid crystalline dimers are discussed in the light of the theoretical predictions of the model, which are also contrasted with the predictions of the twist-bend nematic model.

cond-mat.soft↗

arXiv2015

Molecular simulation study of polar order in orthogonal bent core smectic liquid crystals

We explore the phase behavior and structure of orthogonal smectic liquid crystals consisting of bent-core molecules (BCMs) by means of Monte Carlo molecular simulations. A simple athermal molecular model is introduced that describes the basic features of the BCMs. Phase transitions between uniaxial and biaxial (antiferroelectric) orthogonal smectics are obtained. The results indicate the presence of local in-plane polar correlations in the uniaxial smectic phase. The macroscopic uniaxial-biaxial transformation is rationalized in terms of local polar correlations giving rise to polar domains. The size of these polar domains grows larger under the action of an external vector field and their internal ordering is enhanced, leading to field-induced biaxial order-disorder transitions.

cond-mat.soft↗

arXiv2014

On the structure of the Nx phase of symmetric dimers

NMR measurements on a selectively deuteriated liquid crystal dimer CB-C9-CB exhibiting two nematic phases show that the molecules in the lower temperature nematic phase, Nx, experience a chiral environment and are ordered about a unique direction. The results are contrasted with previous reports that proposed a twist-bend spatial variation of the director. A structural model is proposed wherein the molecules show organization into highly correlated assemblies of opposite chirality.

cond-mat.soft↗

arXiv2011

Molecular simulation of hierarchical structures in bent-core nematics

The structure of nematic liquid crystals formed by bent-core mesogens (BCMs) is studied in the context of Monte Carlo simulations of a simple molecular model that captures the symmetry, shape and flexibility of achiral BCMs. The results indicate the formation of (i) clusters exhibiting local smectic order, orthogonal or tilted, with strong in-layer polar correlations and anti-ferroelectric juxtaposition of successive layers and (ii) large homochiral domains through the helical arrangement of the tilted smectic clusters, whilst the orthogonal clusters produce achiral (untwisted) nematic states.

cond-mat.soft↗

arXiv2010

Liquid crystalline phases and demixing in binary mixtures of shape-anisometric colloids

A theoretical model of shape-anisometric particles embedded in a cubic lattice is formulated for binary mixtures combining rod-like, plate-like and spherical particles. The model aims at providing a tool for the prediction and interpretation of complex phase behavior in a variety of liquid crystalline colloids, biological and macromolecular systems. Introducing just repulsive interactions among the particles, a rich variety of phase structures and multiphasic equilibria is obtained, including isotropic, nematic, lamellar and columnar phases, demixing into phases of the same or different symmetries and structural microsegregation of the different species of the mixture within the same phase.

cond-mat.soft↗

arXiv2010

Extending the Maier-Saupe theory to cybotactic nematics

A theory of thermotropic nematic liquid crystals in which molecules form internally ordered clusters is presented. The formulation is based on the same mean field approximation and form of the anisotropic potential used in the Maier-Saupe theory. A uniaxial nematic and two macroscopically isotropic phases are predicted. One of the isotropic phases consists of thermodynamically stable clusters with internal orientational order. The nematic phase shows cybotactic order throughout its range of stability. This order persists above the transition temperature, either as a stable feature or as a pretransitional effect. The values of the order parameter and of the entropy change at the nematic to isotropic phase transition depend on the size of the clusters. The Maier-Saupe theory is obtained in the limit of extremely large or extremely small clusters.

cond-mat.soft↗

arXiv2009

Symmetries and alignment of biaxial nematic liquid crystals

The possible symmetries of the biaxial nematic phase are examined against the implications of the presently available experimental results. Contrary to the widespread notion that biaxial nematics have orthorhombic symmetry, our study shows that a monoclinic ($C_{2h}$) symmetry is more likely to be the case for the recently observed phase biaxiality in thermotropic bent-core and calamitc tetrapode nematic systems. The methodology for differentiating between the possible symmetries of the biaxial nematic phase by NMR and by IR spectroscopy measurements is presented in detail. The manifestations of the different symmetries on the alignment of the biaxial phase are identified and their implications on the measurement and quantification of biaxiality as well as on the potential use of biaxial nematic liquid crystals in electro-optic applications are discussed.

cond-mat.soft↗

arXiv2007

Thermotropic Biaxial Nematics: Spontaneous or Field-Stabilized?

An intermediate nematic phase is proposed for the interpretation of recent experimental results on phase biaxiality in bent-core nematics. The phase is macroscopically uniaxial but consists of microscopic biaxial, and possibly polar, domains. On applying an electric field the phase exhibits substantial macroscopic biaxial ordering resulting from the collective alignment of the domains. A phenomenological theory is developed for the molecular order in this phase and for its transitions to purely uniaxial and to spontaneously biaxial nematic phases.

cond-mat.soft↗

arXiv2005

Molecular Theory of Dendritic Liquid Crystals: Self Organization and Phase Transitions

We formulate the configurational partition function for dendrimers, taking explicit account of their conformations and segmental interactions. Two approximate schemes are presented, one based on the effective dendrimer-dendrimer interactions and the other based on the interactions among the mesogenic units comprising the dendrimers. In the latter scheme, the description of the dendromesogenic system reduces to that of an ensemble of mesogenic dimers. Results of lattice calculations for phase transitions are presented showing that the minimal inclusion of shape anisotropy and of sub-molecular partitioning into chemically distinct parts reproduces the variety of phases and phase sequences observed experimentally and provides insights into the conformational aspect of these transitions.

cond-mat.mtrl-sci↗