Short-range Orientational Correlations Research Articles

Mutant-Dependent Local Orientational Correlation in Biofilms of Vibrio campbellii Revealed through Digital Processing of Light Microscopy Images.

Biofilms are key bacterial communities in genetic and adaptive resistance to antibiotics as well as disease control strategies. The mature high-coverage biofilm formations of the Vibrio campbellii strains (wild type BB120 and isogenic derivatives JAF633, KM387, and JMH603) are studied here through the unstraightforward digital processing of morphologically complex images without segmentation or the unrealistic simplifications used to artificially simulate low-density formations. The main results concern the specific mutant- and coverage-dependent short-range orientational correlation as well as the coherent development of biofilm growth pathways over the subdomains of the image. These findings are demonstrated to be unthinkable based only on a visual inspection of the samples or on methods such as Voronoi tessellation or correlation analyses. The presented approach is general, relies on measured rather than simulated low-density formations, and could be employed in the development of a highly efficient screening method for drugs or innovative materials.

Shape-induced chiral ordering in two-dimensional packing of snowmanlike dimeric particles

Understanding the distinctive phase behaviors in random packing due to particle shapes is an important issue in condensed matter physics. In this paper, we investigate the random packing structure of two-dimensional (2D) snowmen via wax-snowman packing experiments and Brownian dynamics simulations. Both experiments and simulations reveal that neighboring snowmen have a strong short-range orientational correlation and consequently locally form particular conformations. A chiral conformation is dominant for high area fractions near the jamming condition (φ>0.8), and the proportion of the chiral conformation increases with γ. We also found that the attractive interaction does not have a significant impact on the results. The geometry of chirally ordered snowmen causes a mismatch with 2D crystalline symmetries and thus inhibits the development of long-range spatial order, despite the strong orientational correlation between neighbors.

Mechanics of Random Discontinuous Long-Fiber Thermoplastics—Part I: Generation and Characterization of Initial Geometry

The deformation mechanics of dry networks of large-aspect-ratio fibers with random orientation controls the processing of long-fiber thermoplastics (LFTs) and greatly affects the mechanical properties of the final composites. Here, we generate initial geometries of fiber networks in a cubic unit cell with a fiber aspect ratio of l/d = 100 and fully periodic boundary conditions for later numerical simulation. The irreversible random sequential adsorption (RSA) process is first used to generate a quasi-random structure due to the excluded-volume requirements. In order to investigate the nonequilibrium character of the RSA, a second method, which is similar to the mechanical contraction method (MCM) (Williams and Philipse, 2003, “Random Packings of Spheres and Spherocylinders Simulated by Mechanical Contraction,” Phys. Rev. E, 67, pp. 1–9) and based on a simplified Metropolis Monte Carlo (MC) simulation is then developed to produce quasi-equilibrium fiber geometries. The RSA packing results (ϕ ≈ 4.423% when using a fiber aspect ratio of 100) are in good agreement with the maximum unforced random packing limits (Evans and Gibson, 1986, “Prediction of the Maximum Packing Fraction Achievable in Randomly Oriented Short-Fibre Composites,” Compos. Sci. Technol., 25, pp. 149–162). The fiber structures were characterized by several distribution functions, including pair-spatial and pair-orientation distributions, based on either the center-to-center distance or the shortest distance between the particles. The results show that the structures generated by the RSA have an easily-detectable long-range spatial correlation but very little orientational correlation. In contrast, the quasi-equilibrium structures have reduced spatial correlation but increased short-range orientational correlation.

Properties of the discotic-nematic to isotropic transition: influence of short range orientational order

The present investigation concerns the analysis of the influence of short range orientational correlation on the thermodynamic properties of discotic-nematic liquid crystals. Two-site cluster approximation is applied to the orientational molecular coordinates to include the short range orientational correlation. The role of short range orientational order, dispersion interaction, molecular length-to-width ratio and pressure on the thermodynamic and orientational behaviour of discotic nematogens close to the discotic-nematic to isotropic transition are analysed. It is observed that the short range orientational order has a strong influence on the thermodynamic properties and that the transition properties of both the calamitic and discotic mesogens exhibit quite similar behaviour.

Density functional approach to the theory of interfacial properties of nematic liquid crystals

General expressions for the free energy of the nematic liquid crystal in the interfacial region are derived using the density functional approach. The anisotropic part of the surface tension and the coefficients of the Landau–de Gennes theory for the nematic free surface are expressed in terms of the direct correlation function and analytical results are obtained for a reference system composed of molecules with ellipsoidal equipotential surfaces. The assumption of perfect local orientational order is used to account for strong short-range orientational correlations in the nematic liquid. It is shown that in such an idealized system the molecules can be aligned parallel or perpendicular to the free surface of the nematic, depending on the balance between attractive and repulsive forces. However, the condition of the stability of the nematic phase requires this alignment to be parallel. The possible role of specific intermolecular interactions, which can be very important in real liquid crystals, is also discussed.

The Effect of Short-Range Orientational Order on the Nematic-Isotropic Phase Transition

Abstract A statistical mechanical perturbation theory, that is derived from our earlier work, is applied to investigate the influence of short-range orientational correlation on the thermodynamic properties of nematic liquid crystals. Two-particle orientational distribution function is used in order to account for the short-range orientational order. Numerical calculations are reported for a model system interacting via a pair potential having both repulsive and attractive parts. The repulsive interaction is represented by that between hard ellipsoids of revolution and is short-range rapidly varying potential. Dispersion forces give rise to the attractive interaction. The effect of pressure on the stability, ordering and phase transition properties is analysed and found to be in accordance with experimental observations. It is found that the short-range orientational order has strong influence on the phase transition properties and the numerical results are in better agreement with experiments as compared to mean-field results.

The frequency and wavelength dependent dielectric permittivity of water

The static and dynamic dielectric behaviour of a model of liquid water is studied in the supercooled region, as a function of wavelength. The data used for this analysis were obtained in a previous molecular dynamics simulation of a sample of 343 water molecules, modelled by the TIP4P potential, at 245 K. A substantial wavelength dependence of the static dielectric permittivity is observed, while the value found for ϵ(0) seems to indicate that the TIP4P model is able to account qualitatively for the increase of dielectric constant upon temperature decrease, as found in real water. The time correlation function of the longitudinal and transverse components of the dipole density as well as that of the individual and total dipole moment is also calculated, to relate collective dielectric properties to the single molecule relaxation. The collective and single molecule dielectric relaxation times are obtained and their ratio seems to be close to a static property, namely the short-range orientational correlation factor, g s (k). The frequency dependence of the dielectric constant is compared with previous simulation and experimental values. Furthermore, the longitudinal and transverse components of the hydrogen current are discussed. The molecular symmetry relates the latter properties to the corresponding component of the dipole density in the frequency range 80–200 THz, typical of the librational motions of water. From these results, a simple and unifying picture of the dynamics underlying the three dielectric bands of water in the frequency range 1–200 THz emerges.

Nonhydrodynamic molecular motions in a complex liquid: Temperature dependent dynamics in pentylcyanobiphenyl

The isotropic phase of nematogenic substances above the nematic–isotropic phase transition temperature Tni is characterized by short range orientational correlation of the molecules and the existence of pseudonematic domains. This microscopic structure has a strong influence on reorientational dynamics in the liquid. We present time resolved transient grating optical Kerr effect measurements of pentylcyanobiphenyl between 35 and 120 °C, which allow us to monitor dynamics on timescales from subpicoseconds to tens of nanoseconds. The well-known collective component characterizing the reorientational dynamics of the pseudonematic domains follows Landau–deGennes behavior up to 30–35 degrees above Tni where the theoretical correlation length ξ≊three molecular lengths. Additionally fast relaxation channels (with time constants between 1 and 300 ps) are observed which are associated with local reorientational and intermolecular dynamics within the pseudonematic domains. The local dynamics are temperature independent in the range where the pseudonematic domains dominate the liquid structure, demonstrating a decoupling of the molecular motions from the bulk viscosity. Only at high temperatures, T≳70 °C, where the pseudonematic domains cease to exist, is a viscosity dependence observed. At these elevated temperatures, the slowest contribution to the fast molecular dynamics approximately follows hydrodynamic theory.

The effect of water on dielectric relaxations in the glassy states of poly(propylene oxide) and propylene glycol

Abstract The dielectric permittivity and loss of poly(propylene oxide), M w =425 and propylene glycol (PG) and their mixtures with water were measured in the temperature range 77–320 K and frequency range 12−5×10 5 Hz. PPO-425 has two relaxation processes which appear as β-and α-peaks, but the β-peak in PG is masked by a large contribution to the loss from the α-process. Addition of water up to 1.25 wt% changes the temperature or the rate of the α-process in both PPO-425 and PG, and increases the half-width of the α-relaxation in PG. Water decreases both the rate and the strength of the β-process in PPO-425. The strength of the β-process increases with increase in the molecular weight of PPO. Therefore, the β-process is related to the increasingly less efficient packing of a high molecular weight polymer chain. The increase in permittivity on cooling the liquids is more than that anticipated from the temperature effect alone. This increase is due to the enhanced short-range orientational correlation of the dipoles which may also involve H-bonding through the water molecules.

Structural transition on cooling of plastic adamantane: A molecular-dynamics study.

The new molecular-dynamics methods of Andersen and others have been used to study the structural phase transition that occurs in solid adamantane. While the transition between ordered and plastic-crystal phases is spontaneous on heating, cooling leads to metastable structures. The transition to the ordered phase can, however, be induced by extension to the complete molecular-dynamics sample of the short-range orientational correlations characteristic of the high-temperature phase.

Order-disorder transitions in solutions of discoid micelles

X-ray scattering, electrical conductivity and NMR measurements are presented for a sample of caesium pentadecafluoroctanoate (CsPFO) (55 per cent by weight)/water (2H2O). A first order lamellar to nematic phase transition occurs at 325.13 (1) to 325.50 (1) K and a first order nematic to isotropic micellar solution transition occurs at 330.23 (1) to 330.83 (1) K. The fundamental mesogenic particle is shown to be a discrete discoid micelle in all three phases, that is, the lamellar phase consists of planes of discoid micelles. The thickness of the micelle is of the order 2.2 nm. In the isotropic micellar solution, the average diameter of the micelle grows slowly on cooling. At the isotropic to nematic transition, it increases discontinuously from 5.26 nm (NA = 88 ) at the upper boundary to 5.82 nm ( N A = 113) at the lower boundary, but there is no discontinuity in size at the nematic to lamellar transition where the diameter is 6.38 nm (N A =140). The growth of the micelle in the nematic phase appears to be driven mainly by the variation with temperature of the orientational order parameter (this is 0.35 at the nematic to isotropic transition and 0.72 at the nematic to lamellar transition). This behaviour is as predicted by the theories of Gelbart et al., yet the transition to the lamellar phase is not driven by the predicted explosive growth of the micelles into bilayers. The X-ray scattering patterns are characteristically different from those generally observed in the analogous thermotropic nematic and smectic A phases. It is argued that this is because the micelles (volume fraction 0.35) are relatively loosely packed: in fact, in all of the three phases they may well undergo large amplitude uncorrelated reorientational fluctuations. Short range orientational correlations are consequently either absent or very limited in their extent. This is manifest in the unexpectedly large values (0.92 to 0.71 in the nematic phase) of the orientational order parameters obtained from the X-ray measurements ; it is suggested that these order parameters are mainly determined by the long-range collective director fluctuations. Thus, the model which emerges for the nematic phase is one in which the individual orientational fluctuations of the micelles are essentially decoupled from the collective director fluctuations.

Molecular morphology in amorphous and glass polymers

Theory advanced many years ago predicted that the randomness characteristics of a long polymer chain when isolated from other macromolecules, as in a dilute solution for example, should be preserved in the amorphous state. This prediction is now supported by a large body of evidence, most notably by the results of small angle neutron scattering, which show the correspondence to be quantitative within experimental error in every instance examined to date. The absence of significant perturbations of the molecular configurations in amorphous and glassy polymers seems to suggest total intermolecular disorder. Nevertheless, small angle x-ray scattering demonstrates the prevalence of short-range radial order comparable to that in liquids reveals an enhancement indicative of short-range orientational correlations as well. The degree of correlation is very small, however, and its effect on the molecular configuration may be presumed to be negligible.

Equation of state for nematic liquid crystals

Taking the point of view that a nematic is an ordinary simple liquid, whose anisotropic properties can be treated in terms of perturbation theory, an equation of state for nematic liquid crystals is derived, based on both anisotropic repulsion between (slightly) nonspherical molecular hard cores and anisotropic attractive forces. The spatial order in the liquid is, in a first approximation, thought to be determined by a hardsphere repulsion. The orientational ordering is considered to result from the effect of the eccentricity of the molecular hard core and the superimposed attractive interaction, both of which are taken into account by means of perturbation theory. A molecular field approximation and, in order to account for short-range orientational correlations, a two-site cluster approximation, are both applied to the orientational molecular coordinates. Expressions for the equilibrium thermodynamics quantities are derived. Extensive numerical model calculations are reported and compared with experimental data for $p$-azoxyanisole (PAA) and the results of Cotter. As a special case an equation of state for the Maier-Saupe model (spherical molecules) is presented. If short-range orientational order is taken into account, quantitative agreement with the experimental data for PAA can be obtained.

Contribution to the theory of dielectric relaxation in polar media

A discussion of the connection between the frequency dependent dielectric constant ε and the short range orientational correlation function d is given for a simple model of polar media. The introduction of separate spatial regions with the attendent difficulties in determining the connection between their (time dependent) correlation functions is avoided by noting that ε must satisfy two relations. The use of these relations allows the long range orientational correlations to be eliminated from the expression for ε, the resulting ε being given as a function of d. The result is a generalization of that given by Nee and Zwanzig [J. Chem. Phys. 52, 6353 (1970)]. A short discussion of the results is given, including a physical picture of the connection between the long and short range orientational correlations.

Calculations of neutron diffraction patterns obtained with a nematic liquid crystal

Recently, neutron diffraction data obtained with a fully deuterated sample of the nematic liquid crystal para-azoxyanisole (PAA) were reported. In the present paper a number of simple ideas which contribute to an understanding of these data are presented. It is shown that part of the structure of the observed diffraction patterns can be attributed to the wavelength dependence of a molecular form-factor. Further, it is found that diffuse scattering observed in PAA just below its melting point may be explained in terms of oscillations of molecules about their long axes. The temperature dependence of this diffuse scattering suggests that the melting of PAA may be driven by a soft torsional mode or modes. The explanation of the diffuse scattering from the solid and the similarities between the diffraction patterns of solid and nematic phases suggest that scattering from the nematic phase might be explicable in terms of hindered rotations of molecules about their long axes. A simple model based on this hypothesis and on the neglect of short-range orientational correlations of the molecular long axes is proposed. A comparison of results obtained from this model with the experimental data demonstrates that there is considerable short-range ordering of the long molecular axes in both the nematic and isotropic-liquid phases of PAA.