Orientational Parameter Research Articles

Square ice formation in CrOCl and graphene confinement

The structure and thermodynamic properties of water in nanoscale confinement environment are greatly involved in the research field of material science and nanotechnology. However, a complete picture of the ordered structure formation and thermodynamics behavior of the confined water inside two parallel nanosheets with different surface atomic arrangement is still lacking. In this paper, by using molecular dynamics (MD) simulations, we study the structural variation and thermodynamics behavior for water molecules confined between two parallel CrOCl with a square surface atomic arrangement and two parallel graphene nanosheets with a hexagonal surface atomic arrangement. Square ice, with a lattice constant 2.1 and 2.0 Å, is observed inside the two parallel CrOCl and graphene nanosheets, respectively. By calculating the configuration entropy of the molecular dipoles St , it is found that, in the CrOCl confinement, St reaches a peak value and then is greatly reduced due to the square ice formation. On the other hand, in the graphene confinement, St continues to grow after the square ice formation and is then reduced after reaching its peak value. Interestingly, it is found that the square ice could be stable at a higher entropy state under the external pressure than the bulk water at ambient condition. By calculating the orientational order parameters M, it shows that the conventional tetrahedral geometry of hydrogen bonding between water molecules breaks due to the square ice formation. By analyzing the average number of hydrogen bonds of water molecules Nh , it is found that the hydrogen bond interaction of the square ice relies on the confinement environment, where Nh is reduced in the CrOCl confinement and increased in the graphene confinement. Probability distribution functions of the dipole orientation angles between the x- or z-axis and the projection of the oxygen atoms of the water molecules are also calculated. It is observed that the square ice structure is paralleled with the x-axis (unit cell axis) in the CrOCl confinement and tilted with the x-axis (the zigzag direction of graphene) at an angle 30° in the graphene confinement. Furthermore, the square ice formation is found to be insensitive to temperature. Our result reveals the peculiar ordered structure and thermodynamics behavior of water in different nanoscale confinement environments.

Disordering two-dimensional magnet-particle configurations using bidispersity.

In various types of many-particle systems, bidispersity is frequently used to avoid spontaneous ordering in particle configurations. In this study, the relation between bidispersity and disorder degree of particle configurations is investigated. By using magnetic dipole-dipole interaction, magnet particles are dispersed in a two-dimensional cell without any physical contact between them. In this magnetic system, bidispersity is introduced by mixing large and small magnets. Then, the particle system is compressed to produce a uniform particle configuration. The compressed particle configuration is analyzed by using Voronoi tessellation for evaluating the disorder degree, which strongly depends on bidispersity. Specifically, the standard deviation and skewness of the Voronoi cell area distribution are measured. As a result, we find that the peak of standard deviation is observed when the numbers of large and small particles are almost identical. Although the skewness shows a non-monotonic behavior, a zero skewness state (symmetric distribution) can be achieved when the numbers of large and small particles are identical. In this ideally random (disordered) state, the ratio between pentagonal, hexagonal, and heptagonal Voronoi cells becomes roughly identical, while hexagons are dominant under monodisperse (ordered) conditions. The relation between Voronoi cell analysis and the global bond orientational order parameter is also discussed.

Two-step crystallisation in a 2D active magnetic granular system confined by a parabolic potential

We studied the two-step crystallisation process of a magnetic active 2D-granular system placed on different lens concaveness and under the action of an alternating magnetic field which controls its effective temperature. We have observed that the two-step features of the crystallisation process are more evident as the depth of the parabolic potential increases. At the initial formation of the nucleus, as a first step, in the central region of the lens an amorphous aggregate is formed. In an ulterior second step, this disordered aggregate, due to the effective temperature and the perturbations caused by the impacts of free particles moving in the surrounding region, evolves to an ordered crystalline structure. The nucleus size is larger for deeper concaveness of the parabolic potential. However, if the depth of the parabolic potential exceeds a certain value, the reordering process of the second step does not occur. The crystal growth occurs similarly; small disordered groups of particles join the nucleus, forming an amorphous shell of particles which experiments a rearranging while the aggregate grows. In the explored range of the depths of the parabolic potential, crystallisation generally occurs quicker as the deeper parabolic potential is. Also, aggregates are more clearly round-shaped as parabolic potential depth increases. On the contrary, the structures are more branched for a smaller depth of the parabolic potential. We studied the structural changes and features in the system by using the sixth orientational order parameter and the packing fraction.

Structural Quantification of the Surface-Confined Metal-Organic Precursors Simulated with the Lattice Monte Carlo Method.

The diversity of surface-confined metal-organic precursor structures, which recently have been observed experimentally, poses a question of how the individual properties of a molecular building block determine those of the resulting superstructure. To answer this question, we use the Monte Carlo simulation technique to model the self-assembly of metal-organic precursors that precede the covalent polymerization of halogenated PAH isomers. For this purpose, a few representative examples of low-dimensional constructs were studied, and their basic structural features were quantified using such descriptors as the orientational order parameter, radial distribution function, and one- and two-dimensional structure factors. The obtained results demonstrated that the morphology of the precursor (and thus the subsequent polymer) could be effectively tuned by a suitable choice of molecular parameters, including size, shape, and intramolecular distribution of halogen substituents. Moreover, our theoretical investigations showed the effect of the main structural features of the precursors on the related indirect characteristics of these constructs. The results reported herein can be helpful in the custom designing and characterization of low-dimensional polymers with adjustable properties.

Influence of Thiophene Ring on Molecular Order of Structurally Simple π-Conjugated Smectogens:13C NMR Study.

Structurally simple rod-like π-conjugated mesogens with thiophene directly connected to phenyl, biphenyl, and fluorenone rings with terminal chains are synthesized respectively.The occurrence of smectic A/smectic C phases is concurred by a hot-stage optical polarising microscope (HOPM), differential scanning calorimetry (DSC), and X-Ray diffraction (XRD). The static 1D and 2D13C nuclear magnetic resonance (NMR) studies in the liquid crystalline phase are carried out to find the alignment-induced chemical shifts (AIS) and13C-1H dipolar couplings.The orientational order parameters of the mesogens determined from13C-1H dipolar couplings disclose that the long axis is not only collinear to the C3-C4 bond of the thiophene ring but also for the local axes of phenyl and biphenyl rings.For fluorenone-based mesogen, the molecular biaxiality is found to be high owing to the increased breadth of the molecule. The study unveils that the orientation of thiophene and the phenyl rings is similar in the current mesogens in stark contrast to mesogens, where thiophene is connected to phenyl rings through linking groups.

Molecular dynamics simulation of polymorphic transformation between ε-CL-20 and β-CL-20 by short range order and orientational order parameter

Phase transition simulation of molecular crystals is a challenge. In order to clarify the polymorphic transformation mechanism of molecular crystal, a molecular dynamics (MD) simulation of the transformation between ε-CL-20 and β-CL-20 was carried out by the short range order g(r) and local orientational order parameter (OPs) P2(cosθ) as well as the spatial orientation correlation function. The effects of the temperature on them were performed. The results indicate that the g(r), orientational OPs and spatial orientation correlation function, composed of the structural parameters from three CL-20 molecules in three different single crystal, could be used to simulate the process of the polymorphic transformation between ε-CL-20 and β-CL-20. Only at a high temperature, could two kinds of the CL-20 supercells realize the transformation of crystal form. Furthermore, there were local and global minimum energy points, indicating that there could be the metastable or stable crystal forms in the process of the transformation from β-CL-20 to ε-CL-20. The values of P2(cosθ) were well related to results of the surface electrostatic potentials (ESPs), suggesting that the ESPs have an influence on polymorphic transformation.

Interactions of cationic surfactant-fatty alcohol monolayers with natural human hair surface: Insights from dissipative particle dynamics

Fatty alcohols (CnFAs) combined with cationic surfactants are common ingredients of lamellar-phase personal care liquids. We employ mesoscopic modelling to study.how surfactant-CnFA monolayers originating from the corresponding bilayers of the personal care liquids interact with the human hair surface above and below the fluid-gel transition temperature as well as in- and out-of-equilibrium. For the monolayer model, we consider the single-tail cationic surfactant cetyltrimethylammonium chloride (CTAC) and an excess of CnFAs with their alkyl tail length equal to or longer than the CTAC alkyl tail length. The hair surface mimics keratin surface proteins covered by a film of lipid chains covalently bonded to the proteins. Our modelling shows the formation of a dense adsorbed layer due to the interactions of the CTAC and CnFA alkyl tails with the hydrophobic hair surface. The adsorption and the behaviour of the adsorbed layer is different under fluid and gel conditions. The differences are related to the structure of the adsorbed layer as characterised by density profiles across the adsorbed layer and the orientational order parameters of the chains within the adsorbed layer. Under steady-state shearing (an approximation of real, non-equilibrium conditions), increasing the shear rate above a threshold leads to continuous or abrupt desorption of the CTAC and CnFA chains under fluid or gel conditions, respectively; the desorbed chains can then form various self-assembled structures in the bulk solution. The underlying mechanism of CTAC and CnFA desorption from the adsorbed layers is closely related to the corresponding adsorption mechanism.

Local and Global Order in Dense Packings of Semi-Flexible Polymers of Hard Spheres.

The local and global order in dense packings of linear, semi-flexible polymers of tangent hard spheres are studied by employing extensive Monte Carlo simulations at increasing volume fractions. The chain stiffness is controlled by a tunable harmonic potential for the bending angle, whose intensity dictates the rigidity of the polymer backbone as a function of the bending constant and equilibrium angle. The studied angles range between acute and obtuse ones, reaching the limit of rod-like polymers. We analyze how the packing density and chain stiffness affect the chains' ability to self-organize at the local and global levels. The former corresponds to crystallinity, as quantified by the Characteristic Crystallographic Element (CCE) norm descriptor, while the latter is computed through the scalar orientational order parameter. In all cases, we identify the critical volume fraction for the phase transition and gauge the established crystal morphologies, developing a complete phase diagram as a function of packing density and equilibrium bending angle. A plethora of structures are obtained, ranging between random hexagonal closed packed morphologies of mixed character and almost perfect face centered cubic (FCC) and hexagonal close-packed (HCP) crystals at the level of monomers, and nematic mesophases, with prolate and oblate mesogens at the level of chains. For rod-like chains, a delay is observed between the establishment of the long-range nematic order and crystallization as a function of the packing density, while for right-angle chains, both transitions are synchronized. A comparison is also provided against the analogous packings of monomeric and fully flexible chains of hard spheres.

Comparison of compression versus shearing near jamming, for a simple model of athermal frictionless disks in suspension.

Using a simplified model for a non-Brownian suspension, we numerically study the response of athermal, overdamped, frictionless disks in two dimensions to isotropic and uniaxial compression, as well as to pure and simple shearing, all at finite constant strain rates ε[over ̇]. We show that isotropic and uniaxial compression result in the same jamming packing fraction ϕ_{J}, while pure-shear- and simple-shear-induced jamming occurs at a slightly higher ϕ_{J}^{*}, consistent with that found previously for simple shearing. A critical scaling analysis of pure shearing gives critical exponents consistent with those previously found for both isotropic compression and simple shearing. Using orientational order parameters for contact bond directions, we compare the anisotropy of the force and contact networks at both lowest nematic order, as well as higher 2n-fold order.

The orientational order parameter of some ferroelectric liquid crystals by refractivity and image analysis methods

This manuscript describes the determination of the orientational order parameter of the ferroelectric liquid crystals having chemical formulas C36H47NO5, C37H49NO5 and C39H53NO5. The ordinary and extraordinary refractive indices of these compounds in the nematic phase are measured by using a small angle prism and modified spectrometer. By well-known Vuk’s method the orientational order parameter is estimated. The optical textures of these compounds are apprehended using an optical polarizing microscope. The microscopic textures are analysed using MATLAB software.and the orientational order parameter is estimated. The limitations and merits of the image analysis technique is discussed.

Phase transition and birefringence studies on some ferroelectric liquid crystals

In the present studies the homologous series of hydroxy phenylamino methyl phenoxy benzoate liquid crystals (n = 8,9 and m = 6) are used. The temperature variation of density is measured. Using density data the phase variants in the compounds are identified. All the compounds exhibit first-order phase transitions. By Newton rings method the birefringence is measured directly in smectic and nematic phases. The advantages of the newton ring technique are presented over the conventional methods for the determination of orientational order parameter. The orientational order parameter attained by the newton rings scheme is equated through the literature data, the results are discussed.

Phase transition behavior of pentylcyanobiphenyl liquid crystal containing n-dodecyl-2-hydroxy-1-naphthaldimine

The present work focuses on the phase transition behavior of a thermotropic liquid crystal incorporating a Schiff base having a terminal alkyl chain of 12 carbons, such as, n-dodecyl-2-hydroxy-1-naphthaldimine (SB). Given its interesting molecular structure, the SB was chosen as doping agent for a liquid crystal of cyano-biphenyl type, such as, 4-cyano-4′-n-pentyl-biphenyl (5CB). The study of the guest–host systems, at different concentrations of the solute, was performed using differential scanning calorimetry and UV–visible spectroscopy, equipped with the hot compartment. The fusion and the nematic – isotropic transition temperatures as well as the enthalpies ΔH 0 and entropies ΔS 0 variations of the 5CB-based systems were determined from their spectral data. The thermal properties and orientational order parameters were also determined for the pure and SB doped 5CB, as well as the activation energy for the guest system. The filamentary texture was observed by polarized light optical microscopy for both the pure and Schiff base-doped 5CB.

On the intensity of light scattered by molecular liquids-Comparison of experiment and quantum chemical calculations.

The intensity of light scattered by liquids has been studied for over a century since the valuable microscopic information about the molecules can be obtained, such as the anisotropy of the molecular polarizability tensor or preferred orientations of neighboring molecules. However, in modern dynamic light scattering experiments, the scattering intensity is usually disregarded, unlike in dielectric spectroscopy, which can be considered as a complementary experimental method, where the dielectric strength is routinely evaluated. The reason lies partly on the fact that the exact form of the equations relating the macroscopically measured light scattering intensity to the microscopic properties of the molecules is debated in the literature. Therefore, as a first step, we compare anisotropy parameters from the literature, calculated from light scattering intensities using different equations, with quantum chemical calculations for over 150 medium-sized molecules. This allows us to identify a consistent form of equations. In a second part, we turn to the depolarized light scattering spectra of 13 van der Waals liquids and some mixtures thereof, recorded with a combination of Tandem-Fabry-Perót and Raman spectroscopies, giving direct access to the reorientational dynamics of the molecules. We discuss how the strength of the structural α-relaxation is connected to the anisotropy parameter, what implication this has for the shape of the α-relaxation, how the components of a mixture-also for the case of ionic liquids-can be identified in this way, and how orientational correlation parameters can be extracted. Additionally, we point out for the example of n-alkanes that for highly flexible molecules, the reorientational motion might not be the decisive source of the depolarized scattered light. We also show that light scattering might serve as a sensitive tool to check the accuracy of a conformer ensemble obtained by quantum chemical calculations.

Novel Orientation-Sensitive Spin Probes for Graphene Oxide Membranes Study.

Spin probe EPR spectroscopy is currently the only method to quantitatively report on the orientational ordering of graphene oxide membranes. This technique is based on the analysis of EPR spectra of a membrane containing stable radicals sorbed on oxidized graphene planes. The efficiency of the method depends on the spin probe structure; therefore, it is important to find stable paramagnetic substances that are most sensitive to the alignment of graphene oxide membranes. In the present work, three novel stable nitroxide radicals containing aromatic fragments with two nitrogen atoms were tested as spin probes to study graphene oxide membranes. The spin-Hamiltonian parameters of the radicals in graphite oxide powder and orientational order parameters of the probes inside graphene oxide membrane were determined. The sensitivity of one of these radicals to membrane orientational ordering was found to be higher than for any of spin probes used previously. A likely reason for this higher sensitivity is the presence of heteroatoms which can facilitate interaction between paramagnetic molecules and oxygen-containing groups on the inner surface of the membrane. The new high-sensitivity spin probe may significantly increase the potential of EPR spectroscopy for studying the internal structure of graphene oxide membranes.

Deterministic topological defects and quantum dot assembly in a nematic liquid crystalline medium

Liquid crystal (LC) materials exhibit interesting electro-optic switching and molecular ordering properties. Furthermore, the addition of chemically compatible active emitters such as core-shell quantum dots (QD) in a LC medium allows optical as well as dielectric tuning in an anisotropic, reconfigurable ordered medium. Order in a nematic LC phase is characterized by an orientational order parameter. In this work, we demonstrate the use of patterned substrates to generate arrays of integer topological defects in a nematic LC medium doped with cadmium selenide (core) cadmium sulfide (shell) core–shell QDs. We demonstrate the formation of metastable air-pillar-induced integer topological defects (TDs) in relatively thinner 9 μm LC sandwich cells, and the formation of field-induced TDs in thicker (25 μm) cells. Simultaneously, the self-assembly of core-shell QDs into square arrays on the patterned substrates is discussed, highlighting potential electro-optic device applications. The surfactant hexadecyltrimethylammonium bromide (CTAB) is found to play a significant role in LC TD formation as well as QD spatial organization at the optimized concentration. Self-assembly and ordering of single- and multi-component LCs within structured devices is a highly relevant problem for modern optoelectronic devices. This work opens new possibilities for classical as well as quantum light sources which require spatially ordered optical emitters in a reconfigurable dielectric medium at a micron-scale.

Binary salt structure classification with convolutional neural networks: Application to crystal nucleation and melting point calculations

Convolutional neural networks are constructed and validated for the crystal structure classification of simple binary salts such as the alkali halides. The inputs of the neural network classifiers are the local bond orientational order parameters of Steinhardt, Nelson, and Ronchetti [Phys. Rev. B 28, 784 (1983)], which are derived solely from the relative positions of atoms surrounding a central reference atom. This choice of input gives classifiers that are invariant to density, increasing their transferability. The neural networks are trained and validated on millions of data points generated from a large set of molecular dynamics (MD) simulations of model alkali halides in nine bulk phases (liquid, rock salt, wurtzite, CsCl, 5-5, sphalerite, NiAs, AntiNiAs, and β-BeO) across a range of temperatures. One-dimensional time convolution is employed to filter out short-lived structural fluctuations. The trained neural networks perform extremely well, with accuracy up to 99.99% on a balanced validation dataset constructed from millions of labeled bulk phase structures. A typical analysis using the neural networks, including neighbor list generation, order parameter calculation, and class inference, is computationally inexpensive compared to MD simulations. As a demonstration of their accuracy and utility, the neural network classifiers are employed to follow the nucleation and crystal growth of two model alkali halide systems, crystallizing into distinct structures from the melt. We further demonstrate the classifiers by implementing them in automated MD melting point calculations. Melting points for model alkali halides using the most commonly employed rigid-ion interaction potentials are reported and discussed.

Configuration design and dimensional synthesis of an asymmetry 2R1T parallel mechanism

AbstractThis paper focuses on configuration design, dimensional synthesis, and engineering application of a novel asymmetric 2R1T parallel mechanism (PM) with zero-coupling degree. The analytical forward and inverse displacement solutions are deduced by the means of vector method. The mathematical models between Euler angles and the orientational parameters (i.e., azimuth and tilt angles) of the offset output axis are established. Using screw theory as mathematical tool, this paper worked out evaluation indices of motion/force transmissibility and presented the definitions and calculation methods of good transmission orientation workspace and good transmission orientation capacity (GTOC). Furthermore, a comparative example with respect to kinematic performance of asymmetric UPS-RPU-PU PM and planar symmetric 2UPS-PU PM is carried out, and the result demonstrates that UPS-RPU-PU significantly outperforms 2UPS-PU in terms of GTOC. The constrained optimization model is constructed to formulate optimal problem of dimensional parameters on the maximizing GTOC, which is then solved by differential evolution algorithm. Finally, an engineering case demonstrates that the optimized mechanism has a good application prospect in hydraulic support test bed.

Principal Polarizability and Orientational Order Parameter Study of some Pure Cholesteric Liquid Crystals and their Homogeneous Mixtures: Phase Transition Behaviour

Objectives: To study the behavior of the molecular polarizabilities (ae, ao) and order parameter (S) in various phases of Cholesteryl Propionate, Cholesteryl Benzoate and their homogeneous mixtures of weight fractions 0.25, 0.50 and 0.75. Methods: The measurement of extraordinary refractive index (ne) and ordinary refractive index (no) was carried out with the help of Abbe’s refractometer and modified wedge method, in the temperature range of 850C– 1800C. The anisotropic internal field model is used to calculate the internal field factor (a) and order parameter (S). Findings: The decrease in the ratio of the principal polarizabilities (ao =a e)with increase in temperature has been observed. At the transition points of solid to cholesteric (S-Ch transition) and cholesteric to isotropic (Ch-I transition), the decrease in the ratio of principal polarizabilities is much sharper for both Cholesteryl Benzoate and Cholesteryl Propionate and also for their mixtures. The value of the order parameter (S) remains constant in solid phase and sharp decrease occurs in its values for pure samples as well as their mixtures, when materials go from solid to cholesteric phase (S-Ch transition). After the S-Ch transition, the order parameter (S) goes on decreasing with further increase in temperature until the cholesteric to isotropic transition (Ch-I transition) occurs. Novelty: Some researchers have calculated the principal polarizabilities and order parameter of optically positive nematic liquid crystals and aligned cholesteric liquid crystals by using isotropic internal field model such as Vuk’s approach. This itself raises the question of how can an anisotropic environment give rise to isotropic field. The present study is based on the anisotropic field model suggested by Maier and Saupe in which the internal field acting on a molecule is considered to be anisotropic. This approach used to calculate principal polarizabilities and order parameter is more realistic and gives more accurate results. Keywords: Cholesteric; Isotropic; Birefringence; Polarizability; Order Parameter; Anisotropic Internal Field Factor

Silica-based fibers with axially aligned mesopores from chitin self-assembly and sol-gel chemistry

Polysaccharide nanorods (chitin, cellulose) can be used as colloidal templates to form silica-based mesoporous materials by combining self-assembly and sol-gel chemistry. Chitin nanorods are chosen here for their self-assembly properties, facile preparation from natural resources, and because of the complexing properties of the free amino groups at their surface in view of further functionalization. Electrospinning of ethanolic co-suspensions, containing the chitin nanorods as pore templates, siloxane oligomers as silica precursors and polyvinylpyrrolidone (PVP) as spinning polymer, allows for forming mats of hybrid organic-inorganic fibers with diameters in the 200–300 nm range. The relative proportions of each component can be adjusted to meet specific characteristics (e.g. micro-/mesoporosity ratio). After calcination, the fibers present an open porosity studied by TEM and N2 volumetry. Elongated mesopores (100–200 nm long, 2–4 nm wide) are created by individual chitin monocrystals forming the nanorods. They are highly oriented along the fibers' axes with orientational order parameters P2 > 0.95 for volume chitin contents φchitin* ≥ 0.2. Additional microporosity is provided by the removal of the spinning polymer, PVP. Complementary viscosity measurements on the initial co-suspensions allow discussing the interactions between colloids and their possible impact on the materials’ textures observed. The introduction of a monomeric Ti4+ precursor in the co-suspensions leads to isolated and tetrahedral Ti units, distributed in the calcined porous silica fibers and possibly close to the pores surface. In addition, preliminary investigations on processing the fibers with a Zn xanthate as ZnS precursor leads to carbonized fibers with nanoparticles and a Zn/S molar ratio close to 1.

Boundaries control active channel flows

Boundary conditions dictate how fluids, including liquid crystals, flow when pumped through a channel. Can boundary conditions also be used to control internally driven active fluids that generate flows spontaneously? By using numerical simulations and stability analysis we explore how parallel surface anchoring of active agents at the boundaries and substrate drag can be used to rectify coherent flow of an active polar fluid in a 2D channel. Upon increasing activity, a succession of dynamical states is obtained, from laminar flow to vortex arrays to eventual turbulence, that are controlled by the interplay between the hydrodynamic screening length and the extrapolation length quantifying the anchoring strength of the orientational order parameter. We highlight the key role of symmetry in both flow and order and show that coherent laminar flow with net throughput is only possible for weak anchoring and intermediate activity. Our work demonstrates the possibility of controlling the nature and properties of active flows in a channel simply by patterning the confining boundaries.