Short-range Orientational Order Research Articles

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.

Observing short-range orientational order in small-molecule liquids

Local molecular ordering in liquids has attracted a lot of interest from researchers investigating crystallization, but is still poorly understood on the molecular scale. Classical nucleation theory (CNT), a macroscopic thermodynamic description of condensation, has shortcomings when dealing with clusters consisting of tens of molecules. Cluster formation and local order fluctuations in liquid media are difficult to study due to the limited spatial resolution of electron- and photon-imaging methods. We used NMR relaxometry to demonstrate the existence of dynamic clusters with short-range orientational order in nominally isotropic liquids consisting of elongated molecules. We observed clusters in liquids where the local ordering is driven by polar, steric, and hydrogen-bond interactions between the molecules. In the case of a liquid crystal, measuring the local orientational order fluctuations allowed us to observe the size of these clusters diverging when approaching the phase transition from the isotropic to the nematic phase. These fluctuations are described in terms of rotational elasticity as a consequence of the correlated reorientations of the neighbouring molecules. Our quantitative observations of the dynamic clusters in liquids, numbering about ten or fewer molecules, indicate that this is a general phenomenon in various types of liquids.

Rough or crumpled: Phases in kinetic growth with surface relaxation.

We show that generic kinetic growth processes with surface relaxations can exhibit a hitherto unexplored crumpled phase with short-range orientational order at dimensions d<4. A sufficiently strong spatially nonlocal part of the chemical potential associated with the particle current above a threshold in the system can trigger this crumpling. The system can also be in a perturbatively accessible rough phase with long-range orientational order but short-range positional order at d<4 with known scaling exponents. Intriguingly, in d>4 we argue that there is no crumpling transition; instead, there is a roughening transition from a smooth to a rough phase for large enough nonlocal particle chemical potential. Experimental and theoretical implications of these results are discussed.

Nanostructuring Polystyrene in a Melt

The melts of four fractions of polystyrene are investigated by the method of electric birefringence. It is found that the electro-optical properties of the melt greatly depend on the length of the polymer chain. The fractions with the highest molecular weight showed an increase in the Kerr constant K at temperatures above 120°C. The anomalous nature of the temperature dependence of the form K ∝ T is explained by the fact that at high temperatures the flexibility of the polymer chain begins to grow rapidly. This process is called the liquid-liquid transition. As a result, the short-range orientational order increases, which is associated with a change in the mutual orientation of phenyl rings.

On regular and random two-dimensional packing of crosses

Packing problems, even of objects with regular geometries, are in general non-trivial. For few special shapes, the features of crystalline as well as random, irregular two-dimensional (2D) packing structures are known. The packing of 2D crosses does not yet belong to the category of solved problems. We demonstrate in experiments with crosses of different aspect ratios (arm width to length) which packing fractions are actually achieved by random packing, and we compare them to densest regular packing structures. We determine local correlations of the orientations and positions after ensembles of randomly placed crosses were compacted in the plane until they jam. Short-range orientational order is found over 2 to 3 cross lengths. Similarly, correlations in the spatial distributions of neighbors extend over 2 to 3 crosses. There is no simple relation between the geometries of the crosses and the peaks in the spatial correlation functions, but some features of the orientational correlations can be traced to typical local configurations.

Packing structure of semiflexible rings

Unraveling the packing structure of dense assemblies of semiflexible rings is not only fundamental for the dynamical description of polymer rings, but also key to understand biopackaging, such as observed in circular DNA of viruses or genome folding. Here we use X-ray tomography to study the geometrical and topological features of disordered packings of rubber bands in a cylindrical container. Assemblies of short bands assume a liquid-like disordered structure, with short-range orientational order, and reveal only minor influence of the container. In the case of longer bands, the confinement causes folded configurations and the bands interpenetrate and entangle. Most of the systems are found to display a threading network which percolates the system. Surprisingly, for long bands whose diameter is more than twice the diameter of the container, we found that all bands interpenetrate each other, in a complex fully entangled structure.

Viscosity, undercoolability and short-range order in quasicrystal-forming Al-Cu-Fe melts

Understanding the mechanisms which relate properties of liquid and solid phases is crucial for fabricating new advanced solid materials, such as glasses, quasicrystals and high-entropy alloys. Here we address this issue for Al-Cu-Fe alloys which can serve as a model for studying microscopic mechanisms of quasicrystal formation. We study experimentally two structural-sensitive properties of the liquid – viscosity and undercoolability – and compare results with ab initio investigations of short-range order (SRO). It is shown that the main features of interatomic interaction in Al-Cu-Fe system, extracted from radial distribution function and bong-angle distribution function, are the same for both liquid and solid states. In particular, the system demonstrates pronounced effective repulsion between Fe and Cu as well as strong chemical interaction between Fe and Al. Analysis of orientational short-range order reveals existence in the supercooled liquid Fe-centered icosahedral clusters which are discussed in the literature as structural motifs of the quasicrystal phase. In the concentration region corresponding to the composition of the icosahedral phase, minima on the viscosity and undercoolability isotherms are observed which affect noticeably the initial stage of solidification. Thus, we argue that SRO and structural-sensitive properties of a melt may serve as useful indicators of solid phase formation.

Unveiling the vortex glass phase in the surface and volume of a type-II superconductor

Order-disorder transitions between glassy phases are common in nature and yet a comprehensive survey on the entailed structural changes is challenging since the constituents are in the micro-scale. Vortex matter in type-II superconductors is a model system where some of these experimental challenges can be tackled. Samples with point disorder present a glassy transition on increasing the density of vortices. A glassy yet quasi-crystalline phase, the Bragg glass, nucleates at low densities. The vortex glass stable at high densities is expected to be disordered, however its detailed structural properties remained experimentally elusive. Here we show that the vortex glass has large crystallites with in-plane positional displacements growing algebraically and short-range orientational order. Furthermore, the vortex glass has a finite and almost constant correlation length along the direction of vortices, in sharp contrast with strong entanglement. These results are important for the understanding of disorder-driven phase transitions in glassy condensed matter.

Hyperuniformity of generalized random organization models.

Studies of random organization models of monodisperse (i.e., identical) spherical particles have shown that a hyperuniform state is achievable when the system goes through an absorbing phase transition to a critical state. Here we investigate to what extent hyperuniformity is preserved when the model is generalized to particles with a size distribution and/or nonspherical shapes. We begin by examining binary disks in two dimensions and demonstrate that their critical states are hyperuniform as two-phase media, but not hyperuniform nor multihyperuniform as point patterns formed by the particle centroids. We further confirm the generality of our findings by studying particles with a continuous size distribution. Finally, to study the effect of rotational degrees of freedom, we extend our model to noncircular particles, namely, hard rectangles with various aspect ratios, including the hard-needle limit. Although these systems exhibit only short-range orientational order, hyperuniformity is still preserved. Our analysis reveals that the redistribution of the "mass" of the particles rather than the particle centroids is central to this dynamical process. The consideration of the "active volume fraction" of generalized random organization models may help to resolve which universality class they belong to and hence may lead to a deeper theoretical understanding of absorbing-state models. Our results suggest that general particle systems subject to random organization can be a robust way to fabricate a wide class of hyperuniform states of matter by tuning the structures via different particle-size and -shape distributions. This in turn potentially enables the creation of multifunctional hyperuniform materials with desirable optical, transport, and mechanical properties.

Local order and vibrational coupling of the C=O Stretching Mode of \u03b3-Caprolactone in liquid binary mixtures

The isotropic and anisotropic parts Raman spectra of γ-Caprolactone in the binary mixture at different concentrations have been measured. The non-coincidence effect (NCE) of γ-Caprolactone was determined in carbon tetrachloride solution and DMSO solution. The NCE of the ν11(C=O) stretching mode in the γ-Caprolactone/DMSO mixtures exhibits a linear plot, in contrast to that in the γ-Caprolactone/CCl4 mixtures, which shows an upward (convex) curvature. The reduction and enhancement of the dimer structure (short-range orientational order) of γ-Caprolactone in the γ-Caprolactone/DMSO and γ-Caprolactone/CCl4 mixtures respectively may play a major role in shifting of peak frequencies, thus the geometries of monomer and dimer of γ-Caprolactone were calculated at the B3LYP-D3/6–311 G (d,p) level of theory. We proposed aggregated model to explain the γ-Caprolactone C=O vibration NCE phenomenon and its concentration effect and found it largely consistent with our experimental findings. Solvent dependent experiment show the value of NCE declined with the increase of the solvent dielectric constant under the same condition which is consistent with the Logan’s theory.

About the spatial organization of double-stranded DNA molecules in the cholesteric liquid-crystalline phase and dispersion particles of this phase

The answer to a question on the organization of molecules in a cholesteric phase is well enough proved in case of low molecular mass compounds. However, in case of double-stranded nucleic acids molecules the unequivocal answer to such question is a subject of discussions. In this work an attempt to generalize the well known literary data on the structure of the cholesteric phase formed by double-stranded DNA molecules was undertaken. Besides the experimental results of authors describing the packing of these molecules in the cholesteric liquid-crystalline dispersion particles are added to these data. Comparison of the results obtained offers the possibility to come out with an assumption of high probability of the existence of both the short-range positional and long-range orientational order in arrangement of double-stranded DNA molecules in a liquid-crystalline phase, and in the particles of dispersions of this phase generated under certain conditions. The occurrence of the orientational order, i.e. rotation of 'quasinematic' layers of double-stranded DNA molecules by a small angle, defines the formation of spatially twisted (cholesteric) structure with characteristic for it physical and chemical properties.

On the behavior of indicators of melting: Lennard-Jones system in the vicinity of the phase transition

Various indicators of melting for a system of particles whose pair interaction is described by the Lennard-Jones potential have been considered. The behavior of the radial distribution function g(r) and the associated criteria of melting, modified Lindemann criterion, and criteria based on the properties of short-range orientational order (rotational invariants ql and wl of various orders l) has been analyzed in detail in the vicinity of the melting phase transition. A parameter based on the loss of the nearest neighbors of an atom/particle has been proposed to characterize the melting transition. All considered indicators of melting for the Lennard-Jones system have been compared. It has been shown that the indicators of melting derived from the properties of the short-range orientational order are much more sensitive to the melting phase transition and can be used to construct new phenomenological criteria of melting similar to the Lindemann criterion. An additional important advantage of such indicators is the relatively small number of configurations of the system necessary for their calculation.

Structural features of a Lennard-Jones system at melting and crystallization

The structural features of the crystallization and melting of a system of particles whose pair interaction is described by the Lennard-Jones potential have been considered. The bond order parameter method is used to quantitatively describe the orientational short-range order. The rotational invariants of the second (ql) and third (wl) orders are calculated for each particle of the system. These calculations require only information on the snapshot of atom positions, which is quite easily obtained in experiments, and provide the distribution functions of particles in ql and wl (where l is the rank of an invariant; the results for l = 4, 6 are presented), which are important characteristics of the phase state of the system. It has been shown that the cumulant of the distribution of particles in w6 is very sensitive to the destruction/formation of the short-range orientational order in the Lennard-Jones system and, correspondingly, can be used as a criterion of the melting and crystallization of this system.

Application of analytical and numerical methods to simulation of systems with orientation interactions

The effect of temperature and intermolecular interaction constants on the short-range orientational order in two-dimensional films with a dipole-dipole potential has been investigated using the variational method and the Monte Carlo method. The parameters of the long-range order in three-dimensional systems with dipole-dipole and quadrupole potentials have been computed.

The plastic and liquid phases of CCl3Br studied by molecular dynamics simulations

We present a molecular dynamics study of the liquid and plastic crystalline phases of CCl(3)Br. We investigated the short-range orientational order using a recently developed classification method and we found that both phases behave in a very similar way. The only differences occur at very short molecular separations, which are shown to be very rare. The rotational dynamics was explored using time correlation functions of the molecular bonds. We found that the relaxation dynamics corresponds to an isotropic diffusive mode for the liquid phase but departs from this behavior as the temperature is decreased and the system transitions into the plastic phase.

Spontaneous Polarization in Smectic A Phase of Carbon Nanotubes Doped Deformed Helix Ferroelectric Liquid Crystal

Observation of non-zero spontaneous polarization in paraelectric phase in multiwalled carbon nanotubes doped deformed helix ferroelectric LC is reported. Electro-optical response, spontaneous polarization, rotational viscosity, dielectric permittivity, dielectric loss factor, and electrical resistance of pure and multiwalled carbon nanotubes doped deformed helix ferroelectric LC have been measured. Non-zero spontaneous polarization in paraelectric phase has been attributed to the possible short range orientational order of deformed helix ferroelectric LC surrounding the multiwalled carbon nanotubes through surface anchoring and ionic impurities. The results presented will help to understand the basic mechanism of interaction of multiwalled carbon nanotubes with deformed helix ferroelectric LC.

Fluid-Fluid Phase Separation in Cholesterol-Containing Membranes: Rafts and Non-Rafts

Fluid-fluid phase separation is the functionally important mode of in-plane domain formation in lipid-bilayer membranes. The most common, if not sole, well-established case of fluid phase separation is that between liquid-ordered (Lo) and liquid-disordered (Lα) phases in cholesterol-containing membranes. The liquid-ordered phase is characterised by short-range orientational order, but long-range translational disorder. It is well documented for gel-fluid (Lα-Lo) phase coexistence in binary lipid mixtures with cholesterol but, surprisingly, direct evidence for the functionally significant fluid-fluid (Lα-Lo) phase coexistence is rather sparse. Interestingly, evidence for Lα-Lo coexistence is more abundant in ternary mixtures of a high-chain-melting lipid and a low-chain-melting lipid with cholesterol than in binary mixtures. Here, I shall review the current status of the field: it seems that no aspect is without some controversy. Published phase diagrams for the canonical “raft” lipid mixtures, N-palmitoyl sphingomyelin/palmitoyl-oleoyl phosphatidylcholine/cholesterol will be compared, and also new spin-label EPR results on this system will be presented.Marsh, D., 2009. Cholesterol-induced fluid membrane domains: a compendium of lipid-raft ternary phase diagrams, Biochim. Biophys. Acta 1788, 2114-2123.Marsh, D., 2010. Liquid-ordered phases induced by cholesterol: a compendium of binary phase diagrams, Biochim. Biophys. Acta 1798, 688-699.

Structure and Phase Boundaries of Compressed Liquid Hydrogen

We have mapped the molecular-atomic transition in liquid hydrogen using first principles molecular dynamics. We predict that a molecular phase with short-range orientational order exists at pressures above 100 GPa. The presence of this ordering and the structure emerging near the dissociation transition provide an explanation for the sharpness of the molecular-atomic crossover and the concurrent pressure drop at high pressures. Our findings have nontrivial implications for simulations of hydrogen; previous equation of state data for the molecular liquid may require revision. Arguments for the possibility of a first order liquid-liquid transition are discussed.

Liquid-ordered phases induced by cholesterol: A compendium of binary phase diagrams

Mixtures of phospholipids with cholesterol are able to form liquid-ordered phases that are characterised by short-range orientational order and long-range translational disorder. These L o-phases are distinct from the liquid-disordered, fluid L α-phases and the solid-ordered, gel L β-phases that are assumed by the phospholipids alone. The liquid-ordered phase can produce spatially separated in-plane fluid domains, which, in the form of lipid rafts, are thought to act as platforms for signalling and membrane sorting in cells. The areas of domain formation are defined by the regions of phase coexistence in the phase diagrams for the binary mixtures of lipid with cholesterol. In this paper, the available binary phase diagrams of lipid–cholesterol mixtures are all collected together. It is found that there is not complete agreement between different determinations of the phase diagrams for the same binary mixture. This can be attributed to the indirect methods largely used to establish the phase boundaries. Intercomparison of the various data sets allows critical assessment of which phase boundaries are rigorously established from direct evidence for phase coexistence.

Chiral Isotropic Liquids from Achiral Molecules

A variety of simple bent-core molecules exhibit smectic liquid crystal phases of planar fluid layers that are spontaneously both polar and chiral in the absence of crystalline order. We found that because of intralayer structural mismatch, such layers are also only marginally stable against spontaneous saddle splay deformation, which is incompatible with long-range order. This results in macroscopically isotropic fluids that possess only short-range orientational and positional order, in which the only macroscopically broken symmetry is chirality--even though the phases are formed from achiral molecules. Their conglomerate domains exhibit optical rotatory powers comparable to the highest ever found for isotropic fluids of chiral molecules.