Colloidal interactions in suspensions of rods

We report direct measurements of entropic interactions of colloidal spheres in suspensions of rodlike fd bacteriophage. We investigate the influence of sphere size, rod concentration, and ionic strength on these interactions. Although the results compare favorably with a recent calculation, small discrepancies reveal entropic effects due to rod flexibility. At high salt concentrations, the potential turns repulsive as a result of viral adsorption on the spheres and viral bridging between the spheres.     

Hydrodynamic fluctuations and instabilities in ordered suspensions of self-propelled particles

We construct the hydrodynamic equations for suspensions of self-propelled particles (SPPs) with spontaneous orientational order, and make a number of striking, testable predictions: (i) Nematic SPP suspensions are always absolutely unstable at long wavelengths. (ii) SPP suspensions support novel propagating modes at long wavelengths, coupling orientation, flow, and concentration. (iii) In a wave number regime accessible only in low Reynolds number systems such as bacteria, polar-ordered suspensions are invariably convectively unstable. (iv) The variance in the number N of particles, divided by the mean , diverges as (2/3 ) in polar-ordered SPP suspensions.     

Orientational order and topological defects in two-dimensional Yukawa systems

New numerical results concerned with formation of orientational order and topological defects in non-ideal systems of particles, interacted via screened Coulomb potential, are presented. Calculations have been performed in a wide range of parameters, corresponding to the experimental conditions in the laboratory dusty plasmas. Relations between a number of topological defects and shape of a bond-angular correlation function are obtained for the first time.     

Orientational correlations and order in A3C60

A tight-binding method which has been previously applied to study the effect of uncorrelated orientational disorder on conduction-band properties is extended here to the case of systems with long-range order and/or short-range correlations. The density of states and conductivity are not highly sensitive to the specific short-range correlations, so long as the system is not too close to being fully ordered. Hence the strong effects of disorder found previously appear to be robust and should play an important role in the interpretation of normal-state properties of A₃C₆₀.     

Orientational order and formation of topological defects in two-dimensional systems

The formation dynamics of topological defects and the orientational order in nonideal systems of particles interacting with a screened Coulomb potential is numerically simulated. Calculations are performed in a broad range of parameters corresponding to experimental conditions in a laboratory dusty plasma. A relation is obtained between the number of defects, the shape of the orientational correlation function, and the coupling parameter of the system. New approximations are proposed for the orientational correlation function in the liquid and hexatic phases of structures analyzed.     

Orientational order of molecular assemblies on inorganic crystals

Surfactant micelles form oriented arrays on crystalline substrates although registration is unexpected since the template unit cell is small compared to the size of a rodlike micelle. Interaction energy calculations based on molecular simulations reveal that orientational energy differences on a molecular scale are too small to explain matters. With atomic force microscopy, we show that orientational ordering is a dynamic, multimolecule process. Treating the cooperative processes as a balance between van der Waals torque on a large, rodlike micellar assembly and Brownian motion shows that orientation is favored.     

Orientational order and polarizability of molecules in a nematic liquid crystal

The influence of the orientational order of molecules in a nematic liquid crystal on the mean value $\bar \gamma$ and anisotropy Δγ of the molecular polarizability has been studied in the framework of the molecular statistical approach with allowance for the perturbation of the electronic structure of molecules due to the change in the conformation of their aromatic core and intermolecular interactions. Experimental dependences of $\bar \gamma$ and Δγ on the molecular orientational order parameter S have been derived, and their specific features for the known objects have been explained. The possibility of separating the contributions of opposite signs to the dependence Δγ(S) due to the change in the conformation of molecules and intermolecular interactions has been shown using nematic MBBA as an example.     

Orientational order in the vicinity of a second order smectic-A to nematic phase transition

Measurements of the quadrupolar splitting of the central N¹⁴ are reported for the liquid crystal NBOA. The splitting is a measure of the orientational order parameter. At the nematic to smectic-A transition, the orientational order parameter varies continuously as a function of temperature, but with a discontinuous slope, as expected of a second order phase transition. The results are interpreted in terms of the Kobayashi-McMillan theory.     

Optical bistability,instabilities and higher order bifurcations

We discuss the transient and steady state behaviour of the self-pulsing instability of a bistable system using the dressed mode approach of Benza and Lugiato. This formalism, in a suitable limit, is shown to afford an exact adiabatic elimination of the atomic modes, and to lead to explicit and manageable equations of motion describing the evolution of the transmitted radiation. In steady state, the self-pulsing solutions display first and second order phase transition-type behavior. A higher bifurcation of the Hopf type, where the self-pulsing becomes unstable, is also identified, and the accompanying unstable limit cycle is displayed by integrating the equations of motion backward in time.     

Orientational order and evolution of the properties of biomolecules in anisotropic media

Experimental values of the Lorentz tensor components and the local field of a light wave have been obtained for anisotropic films of polypeptide and DNA. These data indicate an increase in the mean value and anisotropy of the polarizability of biomolecules with respect to an isotropic solution. These changes in the polarizability as an indicator of the response of biomolecules to external actions, as well as their chemical and biological activity, specify the direction of the evolution of the properties of biomolecules in orientationally ordered media.     

Orientational order in three nO.m liquid crystalline compounds

Refractive index and density measurements have been carried out on three nO.m liquid crystalline compounds, namely, 4O.2, 4O.3 and 1O.10 belonging to the N-(p-n-alkoxybenzylidene)-p-n-alkylaniline series. From the data, the orientational order parameter has been estimated using Vuks and Neugebauer local field models. Furthermore, the orientational order parameter has been calculated directly from refractive index data employing the Vuks scaling factor method, Neugebauer f(B) parameter, effective geometry parameter and a method proposed by Kuczyński et al. It is observed that order parameter values estimated from different methods agree well near the nematic–isotropic transition and diverge as the nematic phase attains equilibrium. The temperature gradient of refractive indices and the nematic crossover temperatures have also been estimated for these compounds. The results obtained are compared and discussed.     

Hexagonal order in crystalline and columnar phases of hard rods

We report a study of colloidal suspensions of highly monodisperse semiflexible chiral rodlike viruses, denoted fd, in the range of high concentrations. Small angle x-ray scattering experiments reveal the existence of two hexagonal phases: the first one is crystalline and the second one is hexatic columnar, as shown by its short-range positional order. The suspension of rodlike viruses is the first experimental system showing the whole phase sequence with increasing particle concentration theoretically predicted for systems of hard rods, ranging from the chiral nematic via the smectic to columnar and crystalline phases.     

On coexistence of crystalline and liquid order in colloidal suspensions

Possible coexistence of crystalline and liquid order (phases) with $\text{widely different}$ particle concentrations in colloidal suspensions proposed recently by Udo and de Souza is investigated. In contrast to their proposed model the crystalline and liquid orders are found to coexist with $\text{nearly same}$ particle concentration. The particle concentration obtained from the structure factor (light scattering) is found to be typically 1.5 times higher than the actual concentration. Various possible sources which could lead to this disagreement are examined both theoretically and experimentally and their implications are discussed. The reason for the disagreement is not well understood at present and requires further attention.