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Mesogens, which are typically stiff rodlike or disklike molecules, are able to self-organize into liquid crystal (LC) phases in a certain temperature range. Such mesogens, or LC groups, can be attached to polymer chains in various configurations including within the backbone (main-chain LC polymers) or at the ends of side-chains attached to the backbone in an end-on or side-on configuration (side-chain LC polymers or SCLCPs), which can display synergistic properties arising from both their LC and polymeric character. At lower temperatures, chain conformations may be significantly altered due to the mesoscale LC ordering; thus, when heated from the LC ordered state through the LC to isotropic phase transition, the chains return from a more stretched to a more random coil conformation. This can cause macroscopic shape changes, which depend significantly on the type of LC attachment and other architectural properties of the polymer. Here, to study the structure–property relationships for SCLCPs with a range of different architectures, we develop a coarse-grained model that includes torsional potentials along with LC interactions of a Gay–Berne form. We create systems of different side-chain lengths, chain stiffnesses, and LC attachment types and track their structural properties as a function of temperature. Our modeled systems indeed form a variety of well-organized mesophase structures at low temperatures, and we predict higher LC-to-isotropic transition temperatures for the end-on side-chain systems than for analogous side-on side-chain systems. Understanding these phase transitions and their dependence on polymer architecture can be useful in designing materials with reversible and controllable deformations. [ABSTRACT FROM AUTHOR]

Normally, defects in two-dimensional, circular, confined liquid crystals can be classified into four types based on the position of singularities formed by liquid crystal molecules, i.e., the singularities located inside the circle, at the boundary, outside the circle, and outside the circle at infinity. However, it is considered difficult for small aspect ratio liquid crystals to generate all these four types of defects. In this study, we use molecular dynamics simulation to investigate the defect formed in Gay–Berne, ellipsoidal liquid crystals, with small aspect ratios confined in a circular cavity. As expected, we only find two types of defects (inside the circle and at the boundary) in circular, confined, Gay–Berne ellipsoids under static conditions at various densities, aspect ratios, and interactions between the wall and liquid crystals. However, when introducing an external field to the system, four types of defects can be observed. With increasing the strength of the external field, the singularities in the circular, confined system change from the inside to the boundary and the outside, and the farthest position that the singularities can reach depends on the strength of the external field. We further introduce an alternating, triangular wave, external field to the system to check if we can observe the transformation of different defects within an oscillating period. We find that the position of the singularities greatly depends on the oscillating intensity and oscillating period. By changing the oscillating intensity and oscillating period of the external field, the defect types can be adjusted, and the transformation between different defects can be easily observed. This provides a feasible way to modulate liquid crystal defects and investigate the transformation between different defects. [ABSTRACT FROM AUTHOR]

The interfacial stiffness for nematic–smectic B (nm–smB) interface in a liquid crystalline (LC) material is calculated using Capillary Wave Theory (CWT) and molecular dynamics simulations. The Gay–Berne (GB) pair potential with parameters κ, κ′, μ, and ν equal to 3, 5, 2, and 1 is used to model the LC material. Using a smart three-step recipe, we have obtained an nm–smB phase coexistence in our simulations where the nm and smB directors are nearly parallel to each other and perpendicular to the interface normal. The density profiles are used to compute the nm–smB coexisting density range, the interfacial width, and its position. The smectic phase is differentiated from the nematic phase by using the local bond order parameter (q6q6), which has helped us to demonstrate that the interface is indeed rough. Finally, the interfacial stiffness of the nm–smB interface is computed by following the CWT analysis and is found to be γ ̃ n m − s m B = 0.398 61 k B T / σ e e 2 = 0.044 29 / σ s s 2 , where σee and σss are the length and diameter of the GB LC particles. [ABSTRACT FROM AUTHOR]

Collective movement and organization of cell monolayers are important for wound healing and tissue development. Recent experiments highlighted the importance of liquid crystal order within these layers, suggesting that +1 topological defects have a role in organizing tissue morphogenesis. We study fibroblast organization, motion, and proliferation on a substrate with micron-sized ridges that induce +1 and -1 topological defects using simulation and experiment. We model cells as self-propelled deformable ellipses that interact via a Gay-Berne potential. Unlike earlier work on other cell types, we see that density variation near defects is not explained by collective migration. We propose instead that fibroblasts have different division rates depending on their area and aspect ratio. This model captures key features of our previous experiments: the alignment quality worsens at high cell density and, at the center of the +1 defects, cells can adopt either highly anisotropic or primarily isotropic morphologies. Experiments performed with different ridge heights confirm a prediction of this model: Suppressing migration across ridges promotes higher cell density at the +1 defect. Our work enables a mechanism for tissue patterning using topological defects without relying on cell migration. [ABSTRACT FROM AUTHOR]

Liquid crystalline (LC) materials are ubiquitous in our modern life. Most of the applications where LC materials find their use depend on maintaining the orientation of the constituent LC particles. In this article, we report a recipe to control the molecular orientation. We have done a computer simulation study of a two-dimensional (2D) liquid crystalline material consisting of soft ellipses. The pair interaction is modelled by using the Gay–Berne potential. The 2D LC material is set to interact with an underlying substrate which varies periodically in one-dimension (1D) only. By controlling the substrate parameters like periodicity and strength, we showed that a nematic phase LC material transitions into different novel phases. We finally show that by applying a substrate with large periodicity, the substrate strength can be used as a control parameter to steer the LC director. [ABSTRACT FROM AUTHOR]

We examined the conformation of polymer chains with varying stiffness, immersed in a nematic liquid crystal solvent of varying qualities due to the known ability of nematogenic solvents to elongate and align polymer chains. Using a coarse-grained model and molecular dynamics simulations, we considered single- and multi-chain polymer systems in both isotropic and nematic solvent mesophases. In the isotropic regime, chains exhibited a typical range of conformations from globular to random coil, while in the nematic regime, chains displayed a globular conformation with poorer solvent quality or anisotropic alignment with better solvent quality. Higher polymer stiffness and the application of an alignment field to enhance the long-range orientation order of solvent particles further improved chain anisotropic alignment. We also observed that polymer chains locally disrupted the nematic ordering of nearby solvent particles. Both stiffness-added single- and multi-chain systems in a nematic solvent spent a fraction of time in configurations with hairpin defects, similar to phenomena observed in biopolymers. We hope that molecular-level insights into the interplay between nematogenic solvent quality, polymer stiffness, and concentration will be useful in designing processes to create anisotropic chain conformations of polymers of inherently isotropic chemistries. [Display omitted] • Gay–Berne ellipsoidal solvent can align simple Kremer–Grest type of polymer chain. • Without alignment field, chain elongation diminishes even in nematic solvent. • Good solvent quality increases alignment effect. • Stiffer chains in nematic solvents exhibit hairpin defects. [ABSTRACT FROM AUTHOR]

Due to the importance of liquid crystals in modern technology and food industry, understanding their structural and dynamical properties are scientifically important. In this study, we applied extensive molecular dynamics simulations to infer the effect of the molecular elongation on the formation, stability and dynamics of liquid crystal mesophases in a system of Gay-Berne model mesogens with parameters GB (k , k ′ = 20 , μ = 1 , ν = 1). Four aspect ratio values, k = 2 , 3 , 4.4 and 5 , were studied at pressure P * = 4. Thermodynamics properties, the order parameter and pair distribution functions (both translational and positional), helped to identify phases, hysteresis, and consequently phase diagrams. By increasing aspect ratio from 2 to 5 , nematic and smectic phases were induced and all transition temperatures were shifted to higher values, hence, the range of stability of the nematic and smectic mesophases became wider. The analysis of the coordination numbers and angular distribution functions showed that the structure of the solid phase of system with k = 2 at lowest temperatures was fcc which by increasing the aspect ratio changed to a complete hexagonal in-layer structure. Moreover, by increasing the axial ratio of particles, decoupling between the principal components of diffusion coefficients increased, where it reflects an increase in the molecular orientational order in the system with higher k. The effect of increasing aspect ratio was more pronounced on inter-layers diffusion rather than the in-layers. In all mesophases parallel components of diffusion coefficients were greater than the perpendicular components. We analyzed the results based on the existing theoretical descriptions and experimental observations. • For elongations k = 3 , 4.4 a n d 5 , in all mesophases, D ∥ * > D ⊥ *. • The effect of increasing k was more pronounced on parallel diffusion than on perpendicular one. • By increasing the aspect ratio and pressure, the range of stability of each mesophase expanded over a wider temperature interval. • The results of this work are commonly observed in experiments on real thermotropic liquid. • The Chu and Moroi model better described the diffusion coefficients than the Hess-Frenkel-Allen model. [ABSTRACT FROM AUTHOR]

• A MD study of self aggregation of active particles, endowed with a propelling force and suspended in a liquid crystal. • Active and inactive particles are modelled with Gay-Berne mesogens known to form isotropic, nematic and smectic phases. • Active particles streams flowing in opposite directions form in a range of temperature, concentration and propelling force. • A nematic or smectic phase is needed for laning, while lanes do not form for active particles in isotropic fluids. • This is one of the first molecular level investigations of active liquid crystals. We studied a binary mixture of self propelling (active) and standard (passive), Gay-Berne mesogens using a molecular dynamics technique. We found that in a certain range ofactive particle concentration, propelling force and temperature, here identified, a liquid crystalline sample where active particles are initially randomly distributed can form self-assembling parallel streams (lanes) of active particles flowing in opposite directions. As far as we are aware this is the first microscopic study showing the dynamic self organization of active particles in thermotropic liquid crystals. [ABSTRACT FROM AUTHOR]

• Phase behavior of mixtures of Gay-Berne/Lennard-Jones spheres has been determined. • The de-mixing temperature is coincident with the iso -to-nem transition for GB rich systems. • The de-mixing temperature is coincident with the nem-to-Col h transition for LJ-rich systems. • A universal behaviour is found for the iso -to-nem transition of GB-rich systems. Liquid crystals (LC) mixtures are ubiquitous in all real-life applications of LCs. This is due to the need of modulating the macroscopic properties of the LC phase used in specific devices. However, mixing particles of different shapes, not to mention different strength of interaction, often results in de-mixing (phase separation) with the formation of one phase rich in one type of particles separated from the other phase. In this work we investigate the phase behaviour of mixtures of discotic Gay-Berne (GB) particles with spherical Lennard-Jones (LJ) particles using molecular dynamics (MD) simulations. We have considered compositions ranging from the pure GB system up to a GB mole fraction of 0.1 and we have studied the systems at constant volume as a function of the temperature keeping all systems at the same packing fraction. We observe an interesting behaviour where GB-rich systems undergo a nematic-to-isotropic transition, with decreasing transition temperature as the fraction of LJ solutes is increased, keeping a single phase with a relatively large amount of LJ particles dissolved. Lowering the temperature eventually results in a phase separated hexagonal columnar in coexistence with an isotropic phase. In contrast, for LJ-rich systems the de-mixing occurs together with the transition to the nematic phase and the transition point raises again close to the values observed for the pure GB. A clear correlation is also observed between the orientational order parameter of the LC phase and the solubility of LJ particles within the LC phase itself. [ABSTRACT FROM AUTHOR]

We studied the equilibrium self-assembly of an equimolar mixture of uniaxial liquid crystals (LCs) and magnetic nanoparticles (MNPs) using molecular dynamics simulations. The LCs are modeled by ellipsoids interacting via Gay–Berne potential, and MNPs are represented by dipolar soft spheres (DSS). We found that the LCs show isotropic, nematic, and smectic phases when the mixture is compressed at a fixed temperature. The DSS form chain-like structures, which remain randomly oriented at low densities where the LCs are in the isotropic phase. At intermediate and high densities, the DSS chains align along the nematic and smectic directors of LCs. We found that the DSS inside a chain follow a ferromagnetic ordering. However, the mixture does not show a significant macroscopic magnetization. The extent of nematic order in the DSS remains very similar to the LCs in intermediate densities. At high densities, the DSS have a lower extent of nematic order than the LCs. The structure of the LC–DSS mixture was further analyzed via projected pair correlation functions for distances parallel and perpendicular to directors in the nematic and smectic phases. [ABSTRACT FROM AUTHOR]