Macroscopic Orientational Order Research Articles

Mesostructured Materials with Controllable Long-Range Orientational Ordering and Anisotropic Properties.

Inorganic-organic mesophase materials provide a wide range of tunable properties, which are often highly dependent on their nano-, micro-, or meso-scale compositions and structures. Among these are macroscopic orientational order and corresponding anisotropic material properties, the adjustability of which are difficult to achieve. This is due to the complicated transient and coupled transport, chemical reaction, and surface processes that occur during material syntheses. By understanding such processes, general criteria are established and used to prepare diverse mesostructured materials with highly aligned channels with uniform nanometer dimensions and controllable directionalities over macroscopic dimensions and thicknesses. This is achieved by using a micropatterned semipermeable poly(dimethylsiloxane) stamp to manage the rates, directions, and surfaces at which self-assembling phases nucleate and the directions that they grow. This enables mesostructured surfactant-directed silica and titania composites, including with functional guest species, and mesoporous carbons to be prepared with high degrees of hexagonal order, as well as controllable orthogonal macroscopic orientational order. The resulting materials exhibit novel anisotropic properties, as demonstrated by the example of direction-dependent photocurrent generation, and are promising for enhancing the functionality of inorganic-organic nanocomposite materials in separations, catalysis, and energy conversion applications.

Snowdrift game induces pattern formation in systems of self-propelled particles.

Evolutionary games between species are known to lead to intriguing spatiotemporal patterns in systems of diffusing agents. However, the role of interspecies interactions is hardly studied when agents are (self-)propelled, as is the case in many biological systems. Here, we combine aspects from active matter and evolutionary game theory and study a system of two species whose individuals are (self-)propelled and interact through a snowdrift game. We derive hydrodynamic equations for the density and velocity fields of both species from which we identify parameter regimes in which one or both species form macroscopic orientational order as well as regimes of propagating wave patterns. Interestingly, we find simultaneous wave patterns in both species that result from the interplay between alignment and snowdrift interactions-a feedback mechanism that we call game-induced pattern formation. We test these results in agent-based simulations and confirm the different regimes of order and spatiotemporal patterns as well as game-induced pattern formation.

Phase-dependent shear-induced order of nanorods in isotropic and nematic wormlike micelle solutions.

Small angle X-ray scattering with in situ shear was employed to study the assembly and ordering of dispersions of gold nanorods within wormlike micelle solutions formed by the surfactant cetylpyridinium chloride (CPyCl) and counter-ion sodium salicylate (NaSal). Above a threshold CPyCl concentration but below the isotropic-to-nematic transition of the micelles, the nanorods self-assembled under quiescent conditions into isotropically oriented domains with hexagonal order. Under steady shear at rates between 0.5 and 7.5 s-1, the nanorod assemblies acquired macroscopic orientational order in which the hexagonal planes were coincident with the flow-vorticity plane. The nanorods could be re-dispersed by strong shear but re-assembled following cessation of the shear. In the nematic phase of the micelles at higher surfactant concentration, the nanorods did not acquire hexagonal order but instead formed smectic-like layers in the gradient-vorticity plane under shear. Finally, at still higher surfactant concentration, where the micelles form a hexagonal phase, the nanorods showed no translational ordering but did acquire nematic-like order under shear due to alignment in the flow. Depletion forces mediated by the wormlike micelles are identified as the driving mechanism for this sequence of nanorod ordering behaviors, suggesting a novel mechanism for controlled, reconfigurable assembly of nanoparticles in solution.

The inherent dynamics of isotropic- and nematic-phase liquid crystals.

The geodesic (shortest) pathways through the potential energy landscape of a liquid can be thought of as defining what its dynamics would be if thermal noise were removed, revealing what we have called the "inherent dynamics" of the liquid. We show how these inherent paths can be located for a model liquid crystal former, showing, in the process, how the molecular mechanisms of translation and reorientation compare in the isotropic and nematic phases of these systems. These mechanisms turn out to favor the preservation of local orientational order even under macroscopically isotropic conditions (a finding consistent with the experimental observation of pseudonematic domains in these cases), but disfavor the maintenance of macroscopic orientational order, even in the nematic phase. While the most efficient nematic pathways that maintain nematic order are indeed shorter than those that do not, it is apparently difficult for the system to locate these paths, suggesting that molecular motion in liquid-crystal formers is dynamically frustrated, and reinforcing the sense that there are strong analogies between liquid crystals and supercooled liquids.

Cluster and constraint analysis in tetrahedron packings.

The disordered packings of tetrahedra often show no obvious macroscopic orientational or positional order for a wide range of packing densities, and it has been found that the local order in particle clusters is the main order form of tetrahedron packings. Therefore, a cluster analysis is carried out to investigate the local structures and properties of tetrahedron packings in this work. We obtain a cluster distribution of differently sized clusters, and peaks are observed at two special clusters, i.e., dimer and wagon wheel. We then calculate the amounts of dimers and wagon wheels, which are observed to have linear or approximate linear correlations with packing density. Following our previous work, the amount of particles participating in dimers is used as an order metric to evaluate the order degree of the hierarchical packing structure of tetrahedra, and an order map is consequently depicted. Furthermore, a constraint analysis is performed to determine the isostatic or hyperstatic region in the order map. We employ a Monte Carlo algorithm to test jamming and then suggest a new maximally random jammed packing of hard tetrahedra from the order map with a packing density of 0.6337.

Synchronization and liquid crystalline order in soft active fluids.

We introduce a phenomenological theory for a new class of soft active fluids with the ability to synchronize. Our theoretical framework describes the macroscopic behavior of a collection of interacting anisotropic elements with cyclic internal dynamics and a periodic phase variable. This system can (i)spontaneously undergo a transition to a state with macroscopic orientational order, with the elements aligned, a liquid crystal, (ii)attain another broken symmetry state characterized by synchronization of their phase variables, or (iii)a combination of both types of order. We derive the equations describing a spatially homogeneous system and also study the hydrodynamic fluctuations of the soft modes in some of the ordered states. We find that synchronization can promote or inhibit the transition to a state with orientational order, and vice versa. We provide an explicit microscopic realization: a suspension of microswimmers driven by cyclic strokes.

Defect accommodation in nanostructured soft crystals

A detailed analysis of the structure of lyotropic micellar FCC soft crystals was performed by scanning small-angle neutron scattering. Soft crystals have a large number of structural defects, leading to characteristic features in the scattering patterns such as secondary Bragg peaks, diffuse scattering lines, and paracrystalline distortions. We find that the presence of a large number of defects locally breaks the three-dimensional symmetry of the crystal, leading to weakly correlated assemblies of stacked {111} layers. Positional correlations of micelles in different layers are very short ranged, with correlation lengths corresponding to only a few layers. Within the layers, in-plane positional correlations are somewhat longer ranged, but still corresponding to only a few unit cells. Depending on the polydispersity, soft crystals accommodate defects to form mesocrystals of iso-oriented mosaic domains, or paracrystals. The soft layer structures already show characteristic features of two-dimensional systems, exhibiting short-range positional order and longer-ranged orientational order, with similarities to hexatic and recently observed soft quasicrystalline structures. The study shows that defects can be differently accommodated in soft crystals, thereby strongly affecting local and macroscopic positional and orientational order.

Photoacoustic Monitoring of the Macroscopic Orientational Order in Disperse Red 1 Azo-Dye-Based Dissolutions

The polarized pulsed-laser photoacoustic (PLPA) methodology was implemented to characterize the macroscopic molecular orientational order as a function of the applied electric field in an azo-dye-based dissolution. It is shown in this work that photoacoustic signals are highly sensitive to the material anisotropy when PLPA analyses are performed. In particular, the macroscopic molecular symmetries and structural organization of rod-like azo-dyes (Disperse Red 1: DR1) prepared in heavily loaded toluene dissolutions, have been studied. Experiments evidence unambiguous information concerning the molecular organization detected via simultaneous optical transmission and PLPA measurements. Indeed, in the electrically poled stage, it is shown that the Fourier, correlation, and root-mean-square analyses performed on the PLPA signals, as functions of the incident linear polarization of the laser source (orthogonal states $$P_\mathrm{in}$$ and $$S_\mathrm{in}$$ , respectively), reveal the main optical axis, symmetry, and structure of the aligned compounds in the liquid phase. An important advantage of the PLPA method is that it can be implemented with poor or null optical transmission anisotropic materials.

편광 흡수성 광기전성 고분자 박막 연구

We present the results of a study of the polarizing photovoltaic (PV) effects in an aligned polymer bulk heterojuction PV layer. The fairly uniform in-plane uniaxial alignment of the PV layer with a macroscopic axial orientational order parameter of 0.40 was achieved by means of a simple rubbing technique. Moreover, reflective polarizing PSCs having the aligned PV layers were applied to power-generating reflective type liquid crystal displays (LCDs), which exhibited a maximum contrast ratio of 1.7. These results form a promising foundation for various energy harvesting polarization dependent opto-electrical LCD device applications.

Elucidation of the Orientational Order and the Phase Diagram of p-Quinquephenyl

The orientational order in the nematic phase of p-quinquephenyl, the pentamer of p-phenylene, has been determined by means of birefringence measurements and by wide-angle X-ray scattering (WAXS). The experimentally determined order parameters <P(2)> are compared with the temperature-dependent order parameter predicted by the Maier-Saupe theory. The order parameters derived from the birefringence at different temperatures in the nematic phase of p-quinquephenyl were in excellent agreement with the Maier-Saupe predictions. The <P(2)> values calculated from the azimuthal profiles derived from WAXS measurements were significantly lower than those determined by the birefringence measurements, especially at higher temperatures. For the birefringence measurements, alignment of the director is achieved by using polyimide alignment layers, whereas director alignment for the WAXS experiments was achieved by a magnetic field. We assign the low overall order parameters that were measured by WAXS to a lower macroscopic orientational order. In addition, upon reinvestigating the mesophase behavior of p-quinquephenyl, a monotropic smectic A phase has been observed upon cooling at 390 °C, just before the crystallization temperature is reached.

Dynamics of the Kerr effect in the isotropic phase of comb-shaped nematic polymers

The equilibrium Kerr effect and its dynamics are investigated in isotropic melts of the comb-shaped LC polymer poly(4-[(4′-cyano-1,1′-biphenyl-4-yl)oxy]butyl-2-acrylate) and its copolymers with acrylic acid or methyl methacrylate and isotropic melts of the LC ionomer containing rubidium ions in a wide temperature range. A sharp change in the temperature dependence of relaxation times for the induced macroscopic orientational order is found for the LC copolymers and the LC ionomer in the temperature range 10–15 K higher than the temperature of the nematic-isotropic phase transition. In contrast, the phenomena caused by the small-scale dynamics, such as electric conductivity and dipole relaxation, are approximated by the conventional Arrhenius type dependence, and the equilibrium electrooptical properties of the copolymers are well characterized via the Landau-De Gennes model. The nonclassical behavior of macroscopic relaxation times may be associated with temperature variations in the contribution of main polymer chains to the orientation macroscopic dynamics of the melt, in which the decisive role is played by side mesogenic groups.

Reentrant isotropic – discotic nematic phase transition: a critical exponent study

In this work the critical exponents associated with the behavior of the macroscopic orientational order parameter are determined near the reentrant isotropic (IRE)–discotic nematic (ND) and ND–isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and D2O by measurement of optical birefringence. The critical exponents found indicate that the low temperature ND–IRE phase transition is quasi‐continuous, having a critical exponent, β1∼0.17, which is markedly different from that found at the classical ND–I phase transition, β2∼0.38. The results are discussed and compared with others obtained with different lyotropic mixtures.

Polar order and relaxation of second order nonlinear optical susceptibility in an electric field polarized amorphous polymer

The dependence of the second harmonic generation (SHG) signal of an amorphous polymer system containing nonlinear optical (NLO) chromophores on the polarizing electric field is investigated. It is shown that under a weak poling field condition, the SHG intensity is proportional to Ep2, provided that the polarizing electric field Ep is greater than a threshold field Es. Above Es, the macroscopic polar orientational order is established. The lengthening of the relaxation time for polar orientational order with increasing Ep is also observed. Theoretical analysis shows that the relaxation time is proportional to the polar order initially induced by the polarizing field. Experiments have been carried out to confirm the theoretical prediction.

Molecular and macroscopic orientational order in aramid solutions: a model to explain the influence of some spinning parameters on the modulus of aramid yarns

A semi-empirical model is presented for the calculation of the overall orientation in nematic aramid solutions during the fibre spinning process. The model describes the effect of coagulation-bath temperature, polymer concentration, and draw ratio on the modulus of poly( p-phenylene terephthalamide) yarns. The results of the model are compared with experimental results for the E modulus of as-spun aramid fibres.

Changes in the conformation and electronic structure of molecules upon phase transitions in a uniaxial liquid crystal

Changes in the electronic structure (the displacement of levels and changes in the oscillator strength of the long-wavelength transition) and the conformation of the aromatic skeleton of the molecules in a liquid crystal of 4-n-octyloxy-4'-cyanobiphenyl have been discovered experimentally upon the isotropic-liquid-nematic-smectic A phase transitions. A functional dependence of the oscillator strengths of the electronic transitions of the molecules on the macroscopic orientational and transitional order parameters of the liquid crystal has been established and confirmed experimentally.

The Search for Quantum Liquid Crystals

Abstract We report our progress in the search fro quantum liquid crystals. By “quantum liquid crystals,” we mean quantum liquids which exhibit macroscopic orientational order but no (or incomoplete) spatial order. Such a system is necessarily composed of light molecules, so that even at very low temperatures the zero-point motion of the molecules keeps the system in the liquid state. The molecules must be sufficiently anisotropic and theinteractions sufficiently orientation-dependent to bring about macroscopic orientational order under appropriate conditions. Our goal is to occur. The most natural candidate is ortho-hydrogen. Despite the light mass of the molecules, the strong intermolecular attraction forces the system at low temperature into a crystal in free space. To prevent this from happening, we introduce an external field which makes it energetically advantageous to keep the molecules well apart, by means of an adsorbing substrate such as a graphite surface. At zero temperature and varying areal d...

Quantum liquid crystals: Orientationally ordered quantum liquids

Abstract We summarize here a theory of quantum liquid crystals: quantum liquids of anisotropic molecules which exhibit macroscopic orientational order. As an exercise, the theory is applied to ortho-hydrogen adsorbed on graphite surface. Parameters in the H2-graphite potential are varied to determine conditions favorable to the existence of orientationally ordered phases.

Monodomain samples of dipalmitoyl phosphatidylcholine with varying concentrations of water and other ingredients

Methods are presented for the preparation of large monodomain phospholipid bilayer arrays containing variable amounts of water approaching the two-phase limit. The optical birefringence of these lamellar phases of dipalmitoyl phosphatidylcholine (DDPC) is measured over a range of temperature and water content, and phase transitions are observed. The techniques employed for pure DPPC and water are extended in order to produce macroscopically aligned samples containing varying concentrations of cholesterol, inorganic salts, antibiotics, and chlorophyll a. Polarization studies of the 670-nm band of chlorophyll a indicate macroscopic orientational order in the chromophore under the same conditions.