Orientational Order Research Articles

Symbiotic dynamics in living liquid crystals.

An amalgam of nematic liquid crystals and active matter, referred to as living liquid crystals, is a promising self-healing material with futuristic applications for targeted delivery of information and microcargo. We provide a phenomenological model to study the symbiotic pattern dynamics in this contemporary system using the Toner-Tu model for active matter (AM), the Landau-de Gennes free energy for liquid crystals (LCs), and an experimentally motivated coupling term that favours coalignment of the active and nematic components. Our extensive theoretical studies unfold two novel steady states, chimeras and solitons, with sharp regions of distinct orientational order that sweep through the coupled system in synchrony. The induced dynamics in the passive nematic is unprecedented. We show that the symbiotic dynamics of the AM and LC components can be exploited to induce and manipulate order in an otherwise disordered system.

Dynamical theory of topological defects I: the multivalued solution of the diffusion equation

Point-like topological defects are singular configurations that manifest in and out of various equilibrium systems with two-dimensional orientational order. Because they are associated with a nonzero circuitation condition, the presence of defects induces a long-range perturbation of the orientation landscape around them. The effective dynamics of defects is thus generally described in terms of quasi-particles interacting via the orientation field they produce, whose evolution in the simplest setting is governed by the diffusion equation. Because of the multivalued nature of the orientation field, its expression for a defect moving with an arbitrary trajectory cannot be determined straightforwardly and is often evaluated in the quasi-static approximation. Here, we instead derive the exact expression for the orientation created by multiple moving defects, which we find to depend on their past trajectories and thus to be nonlocal in time. Performing various expansions in relevant regimes, we demonstrate how improved approximations with respect to the quasi-static defect solution can be obtained. Moreover, our results lead to so far unnoticed structures in the orientation field of moving defects, which we discuss in light of existing experimental results.

Partial proton ordering during phase transition in Friedel's salt

Friedel's salt, also known as Cl-Hydrocalumite, (Ca2[Al(OH)6]· Cl·2H2O), is one of the calcium aluminates phases (AFm phases). It can accommodate radioactive chlorine isotopes, serving as a potential material in radioactive waste applications. Earlier X-Ray diffraction and nuclear magnetic resonance (NMR) experiments demonstrated the existence of the first-order phase transition between 303 K and 313 K. The phase transition involves an orientational ordering of water molecules while lowering the temperature. However, such behavior was not properly reproduced in computer simulations using classical molecular dynamics. Here we apply density functional theory and ab initio molecular dynamics to study structural properties and rotational dynamics of water molecules in the interlayer space of Cl-AFm. The transition is characterized by the partial proton ordering in the low-temperature phase, which was not previously observed in experiments and computer simulations.

Tensor renormalization group study of orientational ordering in simple models of adsorption monolayers.

A simple lattice model of the orientational ordering in organic adsorption layers that considers the directionality of intermolecular interactions is proposed. The symmetry and the number of rotational states of the adsorbed molecule are the main parameters of the model. The model takes into account both the isotropic and directional contributions to the molecule-molecule interaction potential. Using several special cases of this model, we have shown that the tensor renormalization group (TRG) approach can be successfully used for the analysis of orientational ordering in organic adsorption layers with directed intermolecular interactions. Adsorption isotherms, potential energy, and entropy have been calculated for the model adsorption layers differing in the molecule symmetry and the number of rotational states. The calculated thermodynamic characteristics show that entropy effects play a significant role in the self-assembly of dense phases of the molecular layers. All the results obtained with the TRG have been verified by the standard Monte Carlo method. The proposed model reproduces the main features of the phase behavior of the real adsorption layers of benzoic, terephthalic, and trimesic acids on a homogeneous surface of metal single crystals and graphite.

On the Effects of Different trans and cis Populations in Azobenzene Liquid Crystal Elastomers: A Monte Carlo Investigation.

We investigate main-chain liquid crystal elastomers (LCEs) formed by photoresponsive azobenzene units with different populations of trans and cis conformers (from fully trans to fully cis). We study their macroscopic properties as well as their molecular organization using extensive Monte Carlo simulations of a simple coarse-grained model where the trans and cis conformers are represented by soft-core biaxial Gay-Berne particles with size and interaction energy parameters obtained by fitting a bare bone azobenzene moiety represented at atomistic level. We find that increasing the fraction of cis conformers, as could be obtained by near-UV irradiation, shifts the nematic-isotropic transition to a lower temperature, consistently with experiment, while generating internal stress in a clamped sample. An analysis of pair distributions shows that the immediate surroundings of a bent cis molecule are slightly less dense and more orientationally disordered in comparison with that of a trans conformer. Comparing nematic and smectic LCEs, actuation in the smectic phase proved less effective, disrupting the smectic layers to some extent but preserving orientational order of the azobenzene moieties.

Extracellular matrix sensing via modulation of orientational order of integrins and F-actin in focal adhesions.

Specificity of cellular responses to distinct cues from the ECM requires precise and sensitive decoding of physical information. However, how known mechanisms of mechanosensing like force-dependent catch bonds and conformational changes in FA proteins can confer that this sensitivity is not known. Using polarization microscopy and computational modeling, we identify dynamic changes in an orientational order of FA proteins as a molecular organizational mechanism that can fine-tune cell sensitivity to the ECM. We find that αV integrins and F-actin show precise changes in the orientational order in an ECM-mediated integrin activation-dependent manner. These changes are sensitive to ECM density and are regulated independent of myosin-II activity though contractility can enhance this sensitivity. A molecular-clutch model demonstrates that the orientational order of integrin-ECM binding coupled to directional catch bonds can capture cellular responses to changes in ECM density. This mechanism also captures decoupling of ECM density sensing from stiffness sensing thus elucidating specificity. Taken together, our results suggest relative geometric organization of FA molecules as an important molecular architectural feature and regulator of mechanotransduction.

Effect of quantum dots on the phase behavior and order of 8CB liquid crystal

We experimentally investigate the impact of surface functionalized hydrophobic core-shell CdSe/ZnS quantum dots on the phase behavior and order of the host liquid crystal compound 4-cyano-4'-octylbiphenyl (8CB) by means of polarized optical microscopy, polarized micro-Raman spectroscopy and X-ray diffraction. The orientational order of the nanocomposite system is probed by means of birefringence measurements as a function of temperature and mass fraction of the quantum dots. The dispersion quality of the dopant in the samples, partial-aggregation phenomena and/or nanoparticle organization give rise to non-monotonic variation of the probed thermodynamic quantities. A critical dopant mass fraction above which the nematic phase becomes paranematic is evidenced. A phenomenological model accounts for phase separation effects and a qualitative interpretation of the experimental results. The impact of nanoparticles on the macroscopic nematic order is discussed in the frame of a random anisotropy Lebwohl-Lasher model.

Liquid Crystal Materials for Biomedical Applications.

Liquid crystal is a state of matter being intermediate between solid and liquid. Liquid crystal materials exhibit both orientational order and fluidity. While liquid crystals have long been highly recognized in the display industry, in recent decades, liquid crystals provide new opportunities into the cross-field of material science and biomedicine due to their biocompatibility, multifunctionality, and responsiveness. In this review, the latest achievements of liquid crystal materials applied in biomedical fields are summarized. The start is made by introducing the basic concepts of liquid crystals, and then shifting to the components of liquid crystals as well as functional materials derived therefrom. After that, the ongoing and foreseeable applications of liquid crystal materials in the biomedical field with emphasis put on several cutting-edge aspects, including drug delivery, bioimaging, tissue engineering, implantable devices, biosensing, and wearable devices are discussed. It is hoped that this review will stimulate ingenious ideas for the future generation of liquid crystal-based drug development, artificial implants, disease diagnosis, health status monitoring, and beyond.

Coupling of nematic in-plane orientational ordering and equilibrium shapes of closed flexible nematic shells

The impact of the intrinsic curvature of in-plane orientationally ordered curved flexible nematic molecules attached to closed 3D flexible shells was studied numerically. A Helfrich-Landau-de Gennes-type mesoscopic approach was adopted where the flexible shell’s curvature field and in-plane nematic field are coupled and concomitantly determined in the process of free energy minimisation. We demonstrate that this coupling has the potential to generate a rich diversity of qualitatively new shapes of closed 3D nematic shells and the corresponding specific in-plane orientational ordering textures, which strongly depend on the shell’s volume-to-surface area ratio, so far not predicted in mesoscopic-type numerical studies of 3D shapes of closed flexible nematic shells.

Insight into the Viscoelasticity of Self-Assembling Smectic Liquid Crystals of Colloidal Rods from Active Microrheology Simulations.

The rheology of colloidal suspensions is of utmost importance in a wide variety of interdisciplinary applications in formulation technology, determining equally interesting questions in fundamental science. This is especially intriguing when colloids exhibit a degree of long-range positional or orientational ordering, as in liquid crystals (LCs) of elongated particles. Along with standard methods, microrheology (MR) has emerged in recent years as a tool to assess the mechanical properties of materials at the microscopic level. In particular, by active MR one can infer the viscoelastic response of a soft material from the dynamics of a tracer particle being dragged through it by external forces. Although considerable efforts have been made to study the diffusion of guest particles in LCs, little is known about the combined effect of tracer size and directionality of the dragging force on the system's viscoelastic response. By dynamic Monte Carlo simulations, we apply active MR to investigate the viscoelasticity of self-assembling smectic (Sm) LCs consisting of rodlike particles. In particular, we track the motion of a spherical tracer whose size is varied within a range of values matching the system's characteristic length scales and being dragged by constant forces that are parallel, perpendicular, or at 45° to the nematic director. Our results reveal a uniform value of the effective friction coefficient as probed by the tracer at small and large forces, whereas a nonlinear, force-thinning regime is observed at intermediate forces. However, at relatively weak forces the effective friction is strongly determined by correlations between the tracer size and the structure of the host fluid. Moreover, we also show that external forces forming an angle with the nematic director provide additional details that cannot be simply inferred from the mere analysis of parallel and perpendicular forces. Our results highlight the fundamental interplay between tracer size and force direction in assessing the MR of Sm LC fluids.

Molecular organization and molecular order of two rod-like smectogens in mesophases

ABSTRACT Structurally simple two rod-like mesogens with a three-ring core consisting of biphenyl and phenyl rings are synthesised to investigate the mesophase properties, molecular organisation and the orientational order parameter. In mesogen-1, a dodecyloxy terminal chain is present on the biphenyl ring while the phenyl ring is free from substituent. For mesogen-2, in addition to the dodecyloxy chain, the butyl chain is placed on the phenyl ring. The Hot-stage optical polarising microscopy (HOPM) and differential scanning calorimetry (DSC) measurements show smectic A (SmA), smectic B (SmB), and crystal E (CryE) phases for mesogen-1 and SmA, smectic C (SmC) and smectic F (SmF) phases for mesogen-2. The presence of smectic mesophases is established by X-ray Diffraction (XRD). 1D and 2D 13C solid-state NMR experiments are performed in smectic mesophases to find the orientational order parameters of the phenyl rings of the core. The main order parameter (Szz) of the mesogens in the SmB and SmF phase is in the range of 0.87–0.93. The excellent spectral resolution noticed in 13C NMR spectra in the SmB mesophase (mesogen-1) indicates that the internal rotations of the molecules along the long axis are primarily unhindered.

Evaluating the effects of oxidation and the deagglomerating potential of selected additives on asphaltene aggregation via computational modeling

Economic and environmental sustainability has been a significant stimulus to the use of recycled asphalt materials in the pavement industry. In this practice, rejuvenators are usually required to treat the oxidized asphalts with the aim to restore the physical properties and engineering performance. A critical function required for rejuvenators is that they can deagglomerate the asphaltene aggregates by effectively interacting with them. This study employed the techniques of molecular dynamics simulations and quantum chemical calculations, and investigated the impacts of oxidation on asphaltenes and the deagglomerating potential of three selected additives with different molecular structures and polarities, i.e., tributyl citrate (TBC), limonene, and myristamide. Analyses based on the aggregation number, orientational order parameter, and decomposition of the interaction energy indicated that oxidation strengthened the binding between the asphaltenes and thus promoted their aggregation behaviors. Among the three additives, TBC exhibited the highest rejuvenating effectiveness as it restrained the formation of large asphaltene aggregates and improved the orientational randomness of the aromatic cores. This capability was attributed to the highest number of polar sites in TBC and its three-dimensional spatial structure, which enhanced the compatibility and interactions with the aged asphaltenes. Based on the independent gradient model (IGM) method, the asphaltene aggregation is driven by the π-π interactions dominated by dispersion forces between the fused aromatic cores. The additives weakened the asphaltene interactions to different degrees, but the reversing effects were limited compared to the unaged state.

Development of thin film quarter-waveplate for polarization-independent liquid crystal on silicon phase modulators

Liquid crystal on silicon (LCoS) phase modulators spatially modulate the phase of light across the panel. The orientation order and elongated molecules of nematic liquid crystals (LCs) means that traditional LCoS phase modulators are inherently polarization-dependent, resulting in either complicated polarization manipulation systems or high optical power loss for unpolarized incident light. We propose a polarization-independent LCoS (PI-LCoS) device that combines existing technologies, which allows for cost-effective fabrication. Our PI-LCoS phase modulator is suitable for a wide variety of applications in telecommunication, adaptive optics, and display technologies. We have demonstrated the feasibility of the proposed PI-LCoS device by fabricating polarization-independent LC cells using a thin-film quarter-wave plate composed of a photoalignment layer and a layer of LC polymer. We show experimentally that the proposed design can efficiently modulate the phase of light with arbitrary input polarization.

Analysis of Structural Change across the Liquid-Liquid Transition and the Widom Line in Supercooled Stillinger-Weber Silicon.

We consider the changes in the structure of supercooled Stillinger-Weber silicon at pressures at which the studied range of temperatures traverses the liquid-liquid transition or the "Widom line" (at which the isothermal compressibility or the specific heat exhibits a maximum). In addition to the conventional characterizations in terms of the pair-correlation function and bond orientational order, we analyze the statistics of rings in the bond network as well as the statistics of clusters of low density liquid (LDL)- and high density liquid (HDL)-like atoms. We investigate the nature of the change in these structural characterizations when the liquid-liquid transition line or the Widom line is crossed. We find that the isobaric temperature variation of these structural features reveals clear indications of maximal structural heterogeneity or frustration upon crossing the liquid-liquid transition or the Widom line, as in the case of water, but with some differences in detail, which we discuss.

Pristine and Fullerene between Hasselmann and Van Hasselt

It was about K3C60 organic superconductors related to a region found in Belgie at least funding & cash case denoted in mathematics dealt with economical realms, instead of econophysics. We also dealt with fusion also discussed in pure mathematics of tensor & Connes fusion as well as µ-catalyzed fusion. Accompany the ‘the most economical covering of space by sphere’ [the so-called “thinnest covering”] and thus they mainly arrange in a superlattice, - due to the thick organic shell, the orientational ordering of nanocrystals within this type of superlattice is low, therefore this type of crystal is not a monocrystal.

Molecular Orientations of Myristic Acid Derivatives with Different Perfluoroalkyl Chain Lengths at the Air/Water Interface Evaluated by Heterodyne-Detected Sum Frequency Generation Spectroscopy

Surface-related properties of perfluoroalkyl compounds such as hydrophobicity and adhesion force can have a close relationship with the orientational order of the molecules at the surface. Myristic acid (MA) derivatives with perfluoroalkyl (Rf) groups have been known to form a well-ordered monolayer at the air/gold substrate interface. In this study, molecular orientations of the MA derivative monolayers at the air/water interface were investigated by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. We estimated the molecular orientation of MA derivative monolayers with a marker band observed at around 1370 cm–1. The Rf chain length and the surface pressure dependence of the molecular orientation of four MA derivatives were examined. The marker bands were observed except for the derivative with the shortest Rf chain at the higher surface pressure, whereas the marker bands were observed only for the two derivatives with relatively longer Rf chain lengths at the lower surface pressure. The observation indicates that the longer the Rf chain is and the higher the surface pressure is, MA derivatives tend to adopt a standing orientation closer to perpendicular to the interface.

Vertically aligned polyimide films prepared from lyotropic liquid-crystalline smectic precursors

Vertically aligned (VA), fully aromatic polyimide (PI) thin films were successfully prepared from a lyotropic smectic liquid-crystalline precursor of poly(amic ester) (PAE) containing perfluorooctyl (Rf) terminal groups using a “push-coating” method. Because the repeating units of PAE form the smectic layer, the main chains of PAE are oriented perpendicular to the layer. A homeotropically aligned smectic structure is induced by the step-by-step stacking growth of the layer from the monolayer of the Rf groups self-organized on the hydrophobic interfaces (interfaces between the PAE solution and air or a surface-modified substrate), leading to a vertically aligned PAE. By optimizing the preparation conditions, the values of the orientational order parameter (S) of the push-coated PAE film exceeded 0.7. The homeotropically aligned PAE films were imidized by heating to 300 °C, and the high S values were maintained in the subsequent imidization to give VA PI films. Despite the large standard deviation in the temperature variation of the optical path length of the VA PI films, the small and reversible variations in S confirmed that the vertical orientation of the PI films was thermodynamically stable.

Hierarchy of anomalies in the simple rose model of water

The density, diffusion, and structural anomalies of the simple two-dimensional model of water were determined by Monte Carlo simulations. The rose model was used which is a very simple model for explaining the origin of water properties. Rose water molecules are modelled as two-dimensional Lennard-Jones disks with rose potentials for orientation dependent pairwise interactions mimicking formations of hydrogen bonds. The model can be seen also as a variance of silica-like models. Two parameters of potential in this work were selected in a way that (1) the model exhibits similar properties to Mercedes-Benz (MB) water model; and (2) that the model has real-like properties of water. Beside the known thermodynamic anomaly for the model we also found diffusion and structural anomalies. The orientational order parameters were calculated and maximum encountered for three and six-fold symmetry. For the MB parametrization, the anomalies occur in hierarchy order, which is a slight variation of the hierarchy order in real water. The diffusion anomaly region is the innermost in the hierarchy while for water it is the density anomaly region. In case of real water parametrization the most inner is the structural anomaly.

Orientational Ordering in Nano-confined Polar Liquids.

Water and other polar liquids exhibit nanoscale structuring near charged interfaces. When a polar liquid is confined between two charged surfaces, the interfacial solvent layers begin to overlap, resulting in solvation forces. Here, we perform molecular dynamics simulations of polar liquids with different dielectric constants and molecular shapes and sizes confined between charged surfaces, demonstrating strong orientational ordering in the nanoconfined liquids. To rationalize the observed structures, we apply a coarse-grained continuum theory that captures the orientational ordering and solvation forces of those liquids. Our findings reveal the subtle behavior of different nanoconfined polar liquids and establish a simple law for the decay length of the interfacial orientations of the solvents, which depends on their molecular size and polarity. These insights shed light on the nature of solvation forces, which are important in colloid and membrane science, scanning probe microscopy, and nano-electrochemistry.

Liquid Crystal Assembly for Ultra-dissymmetric Circularly Polarized Luminescence and Beyond.

Circularly polarized luminescence (CPL) is attracting much interest because it can carry extensive optical information. CPL shows left- or right-handedness and can be regarded as part of high-level visual perception to supply an extra dimension of information with regard to regular light. A key to meeting the needs for practical applications is to develop the emerging field of ultra-dissymmetric CPL. Chiral liquid crystal (LC) assemblies─otherwise referred to as cholesteric liquid crystals (CLCs)─are essentially organized helical superstructures with a highly ordered one-dimensional orientation, and distinctly superior to regular helical supramolecules. CLCs can achieve a perfect equilibrium of molecular short-range interaction and long-range orientational order, enabling molecule-scale chirality on a helical pitch and observable scale. LC assembly could be an ideal strategy for amplifying chirality, making it accessible to ultra-dissymmetric CPL. Herein, we focused on some basic but important issues regarding CPL: (i) How can CPL be created from chiral dyes? (ii) Is the chirality of luminescent dyes an essential factor for the generation of CPL? That is, can all chiral dyes emit CPL and vice versa? (iii) How can CPL be transferred within intermolecular systems, and what principles of CPL transmission should be followed? Given these queries and our work, in this Perspective we discuss the generation, transmission, and modulation of CPL with chiral LC assembly, aiming to design and build up novel chiroptical materials. Recent applications of CPL-active LC microstructures in three-dimensional displays, circularly polarized lasers, and asymmetric catalysis are also discussed.