Spontaneous Orientational Order Research Articles

Phase behavior and orientational ordering in block copolymers doped with anisotropic nanoparticles.

A molecular field theory and coarse-grained computer simulations with dissipative particle dynamics have been used to study the spontaneous orientational ordering of anisotropic nanoparticles in the lamellar and hexagonal phases of diblock copolymers and the effect of nanoparticles on the phase behavior of these systems. Both the molecular theory and computer simulations indicate that strongly anisotropic nanoparticles are ordered orientationally mainly in the boundary region between the domains and the nematic order parameter possesses opposite signs in adjacent domains. The orientational order is induced by the boundary and by the interaction between nanoparticles and the monomer units in different domains. In simulations, sufficiently long and strongly selective nanoparticles are ordered also inside the domains. The nematic order parameter and local concentration profiles of nanoparticles have been calculated numerically using the model of a nanoparticle with two interaction centers and also determined using the results of computer simulations. A number of phase diagrams have been obtained which illustrate the effect of nanoparticle selectivity and molar fraction of the stability ranges of various phases. Different morphologies have been identified by analyzing the static structure factor and a phase diagram has been constructed in coordinates' nanoparticle concentration-copolymer composition. Orientational ordering of even a small fraction of nanoparticles may result in a significant increase of the dielectric anisotropy of a polymer nanocomposite, which is important for various applications.

Origin of the spontaneous orientational ordering of islands during thin film growth

Origin of the spontaneous orientational ordering of islands during thin film growth

Patterning of Lyotropic Chromonic Liquid Crystals by Photoalignment with Photonic Metamasks.

Controlling supramolecular self-assembly in water-based solutions is an important problem of interdisciplinary character that impacts the development of many functional materials and systems. Significant progress in aqueous self-assembly and templating has been demonstrated by using lyotropic chromonic liquid crystals (LCLCs) as these materials show spontaneous orientational order caused by unidirectional stacking of plank-like molecules into elongated aggregates. In this work, it is demonstrated that the alignment direction of chromonic assemblies can be patterned into complex spatially-varying structures with very high micrometer-scale precision. The approach uses photoalignment with light beams that exhibit a spatially-varying direction of light polarization. The state of polarization is imprinted into a layer of photosensitive dye that is protected against dissolution into the LCLC by a liquid crystalline polymer layer. The demonstrated level of control over the spatial orientation of LCLC opens opportunities for engineering materials and devices for optical and biological applications.

Hybrid Nanocomposites with Tunable Alignment of the Magnetic Nanorod Filler

For many important applications, the performance of polymer–anisotropic particle nanocomposite materials strongly depends on the orientation of the nanoparticles. Using the very peculiar magnetic properties of goethite (α-FeOOH) nanorods, we produced goethite–poly(hydroxyethyl methacrylate) nanocomposites in which the alignment direction and the level of orientation of the nanorods could easily be tuned by simply adjusting the intensity of a magnetic field applied during polymerization. Because the particle volume fraction was kept low (1–5.5 vol %), we used the orientational order induced by the field in the isotropic phase rather than the spontaneous orientational order of the nematic phase. At the strongest field values (up to 1.5 T), the particles exhibit almost perfect antinematic alignment, as measured by optical birefringence and small-angle X-ray scattering. The results of these two techniques are in remarkably good agreement, validating the use of birefringence measurements for quantifying the degree of orientational order. We also demonstrate that the ordering induced by the field in the isotropic suspension is preserved in the final material after field removal. This work illustrates the interest, for such problems, of considering the field-induced alignment of anisotropic nanoparticles in the isotropic phase, an approach that is effective at low filler content, that avoids the need of controlling the nematic texture, and that allows tuning of the orientation level of the particles at will simply by adjusting the field intensity.

Spontaneous Orientational Ordering of Liquid Crystal Layer During Evaporation onto Silica

The ordering process of nematic liquid crystal (LC) during evaporation onto silica substrate was investigated. The optical second-harmonic generation and polarized absorption measurements were employed to determine the orientational order parameters S 1 and S 2. A sudden increase of S 1 was observed during the first layer deposition while S 2 gradually increased. Interestingly, S 1 was nearly constant after the completion of the first layer deposition, while S 2 was similar to that in the first layer deposition. The results suggest that spontaneous orientational ordering process is induced during the first monolayer formation.

The structure and spontaneous orientational order in surface layers of water-soluble methyl- and hydroxypropylmethyl cellulose films

The structure of the surface layers of thin films cast from water-soluble derivatives of cellulose—methyl cellulose and hydroxypropyl methylcellulose—has been studied by the methods of an oblique polarized beam and molecular hydrodynamics. It has been demonstrated that the molecular chains of the polymers under study are characterized by a high degree of order in surface film layers. The orientational order of molecular chains demonstrates a strong molecular mass dependence. This effect is associated with the concentration of terminal segments whose orientational order parameter relative to the film surface differs from the corresponding value of internal segments composing the molecular chain. The quantitative estimates of the orientational order parameter of terminal and internal segments have been performed.

Long-Lived Giant Number Fluctuations in a Swarming Granular Nematic

Coherently moving flocks of birds, beasts, or bacteria are examples of living matter with spontaneous orientational order. How do these systems differ from thermal equilibrium systems with such liquid crystalline order? Working with a fluidized monolayer of macroscopic rods in the nematic liquid crystalline phase, we find giant number fluctuations consistent with a standard deviation growing linearly with the mean, in contrast to any situation where the central limit theorem applies. These fluctuations are long-lived, decaying only as a logarithmic function of time. This shows that flocking, coherent motion, and large-scale inhomogeneity can appear in a system in which particles do not communicate except by contact.

Spontaneous orientational ordering in magnetic fluids

The problem of spontaneous orientational order in dipolar hard sphere fluid is studied on the basis of the BBGKY equation self-consistent solution. This self-consistent approach is used in the density functional method and proves to be equivalent to the mean field theory. In the framework of dipole–dipole interaction second order perturbation model the following effect is discovered: the self-consistent density functional approach leads to the spontaneous “ferrimagnetic’’ state of the magnetic fluid induced by the dipole–dipole interaction.

Spontaneous ferromagnetic ordering in magnetic fluids.

This paper is devoted to the theoretical justification of spontaneous orientational order in magnetic fluids. We study the self-consistent solutions of the Bogoliubov-Born-Green-Kirkwood-Yvon equation connecting the one-particle distribution function with the pair correlation function. This self-consistent approach is used in the specific density functional method and proves to be equivalent to the mean field theory. On the basis of the second-order perturbation method over the intensity of dipole-dipole interparticle interaction the following effect is discovered: the self-consistent density functional approach leads to the spontaneous "ferrimagnetic" state of the magnetic fluid induced by the dipole-dipole interaction. This strange result seems to be physically meaningless and prejudices the validity of the density functional methods and mean field theories applied to orientational microstructure in ferrofluids.

Spontaneous orientational order in confined dipolar fluid films

We report Monte Carlo simulation results for a strongly coupled dipolar soft-sphere (DSS) fluid confined to a nanoscopic slit pore with structureless, nonconducting walls. The central topic of our investigation are the conditions under which the pore fluid can spontaneously order into a globally polarized (i.e., ferroelectric) state. Polarized states are observed in bulk DSS fluids at sufficiently low temperatures and high densities/pressures. The confined system is simulated in the (N,Lz,P∥,T) ensemble, where N is the particle number, Lz the wall separation, P∥ the pressure parallel to the walls, and T the temperature. Fixing T and P∥ such that the corresponding bulk system is ferroelectric, and considering confined films with various thicknesses proportional to Lz, we first demonstrate that the long-range orientational order persists down to Lz≈6σ. We then specialize to the case Lz=7σ, for which we investigate in detail the spatial and orientational structure as functions of P∥. It turns out that the transition from the globally isotropic to the globally polarized phase occurs at significantly lower pressures/densities than in the bulk, indicating that spatial confinement can support the onset of ferroelectric order. We explain this phenomenon within the framework of a simple mean-field theory based on the assumption that confinement effectively restricts orientational fluctuations, as suggested by the Monte Carlo results.

Mean field theories and ferromagnetic ordering in ferrofluids

This paper addresses basic questions concerning spontaneous “ferromagnetic” order in ferrofluids. We obtain the exact equation connecting the one-particle distribution function with the pair correlation function. We study four expressions for the pair correlation function in terms of the one-particle distribution function, and only one of them results in the spontaneous orientational order. This self-consistent uncoupling is used in the density functional approach and proves to be equivalent to the mean field model. In this case an influence of dipole–dipole interaction is overestimated and so the theoretical prediction of ferromagnetic order in ferrofluids has to be considered as questionable.

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 <N>, diverges as <N >(2/3 ) in polar-ordered SPP suspensions.

Structure of polydisperse dipolar hard-sphere fluids

We report Monte Carlo simulations of dipolar hard-sphere fluids with a nonuniform distribution of the dipole strengths or different hard-sphere diameters. Dipolar polydispersity significantly reduces the spontaneous orientational order observed in the ferroelectric phase of monodisperse dipolar hard-sphere fluids at high densities and low temperatures. Equimolar binary mixtures of dipolar hard spheres of different diameters σa and σb also exhibit ferroelectric order. However, the order parameters are very dependent on the ratio σa/σb. Chains of dipolar hard spheres, typical of strongly interacting dipolar hard spheres at low densities and low temperatures, were observed in binary fluids of dipolar and neutral hard spheres of different sizes.

Theory and computer simulation of bent-core molecules

Fluids of hard bent-core molecules have been studied using theory and computer simulation. The molecules are composed of two hard spherocylinders, with length-to-breadth ratio L/D, joined by their ends at an angle 180°−γ. For L/D=2 and γ=0,10,20°, the simulations show isotropic, nematic, smectic, and solid phases. For L/D=2 and γ=30°, only isotropic, nematic, and solid phases are in evidence, which suggests that there is a nematic-smectic-solid triple point at an angle in the range 20°&amp;lt;γ&amp;lt;30°. In all of the orientationally ordered fluid phases the order is purely uniaxial. For γ=10° and 20°, at the studied densities, the solid is also uniaxially ordered, whilst for γ=30° the solid layers are biaxially ordered. For L/D=2 and γ=60° and 90° we find no spontaneous orientational ordering. This is shown to be due to the interlocking of dimer pairs which precludes alignment. We find similar results for L/D=9.5 and γ=72°, where an isotropic-biaxial nematic transition is predicted by Onsager theory. Simulations in the biaxial nematic phase show it to be at least mechanically stable with respect to the isotropic phase, however. We have compared the quasi-exact simulation results in the isotropic phase with the predicted equations of state from three theories: the virial expansion containing the second and third virial coefficients; the Parsons–Lee equation of state; an application of Wertheim’s theory of associating fluids in the limit of infinite attractive association energy. For all of the molecule elongations and geometries we have simulated, the Wertheim theory proved to be the most accurate. Interestingly, the isotropic equation of state is virtually independent of the dimer bond angle—a feature that is also reflected in the lack of variation with angle of the calculated second and third virial coefficients.