Micellar shape anisotropy on lyotropic nematic phases

In this work we show, that with the introduction of the micellar shape anisotropy in the order parameter of a lyotropic liquid crystal, it is possible to understand, in a unified way, the features observed in the phase diagram of these materials. Using the de Gennes connection between microscopic and macroscopic order parameters, such an extended order parameter is constructed from a geometrical conception of the micelles. Our theoretical results will be correlated with refractive index experimental data near the uniaxial-biaxial nematic phase transitions of a lyotropic mixture of potassium laurate, decanol and deuterium oxide.     

Role of dislocation loops on the elastic constants of lyotropic lamellar phases

We study the role of dislocation loops defects on the elasticity of lamellar phases by investigating the variation of the lamellar elastic constants, ˉ and K, induced by the proliferation of these defects. We focus our interest on one particular lamellar phase made up of a mixture of C₁₂E₅ and DMPC in water, which is already well-characterised. This lamellar phase undergoes a second-order (or weakly first-order) lamellar-to-nematic phase transition at about 19^°C and dislocation loops are seen to proliferate within the lamellar structure when temperature is decreased below 30^°C. The values of both elastic constants of this given lamellar phase are measured as a function of temperature, approaching the lamellar-to-nematic transition, with the help of Quasi-Elastic Light Scattering (QELS) on oriented lamellar phases. Very surprisingly we observe a strong and rapid increase in both ˉ and K as the lamellar-to-nematic transition temperature is approached. These increases are seen to start as soon as dislocation loops can be observed in the lamellar phase. We interpret our results as being the consequence of the appearance and proliferation of dislocation loops within the lamellar structure. According to a simple model we developped we show that ˉ and K are proportional to the density of dislocation loops in the lamellar phase.     

Rheological behaviour of polyoxometalate-doped lyotropic lamellar phases

We study the influence of nanoparticle doping on the lyotropic liquid crystalline phase of the industrial surfactant Brij30 ( C₁₂E₄ and water, doped with spherical polyoxometalate nanoparticles smaller than the characteristic dimensions of the host lamellar phase. We present viscometry and in situ rheology coupled with small-angle X-ray scattering data that show that, with increasing doping concentration, the nanoparticles act to decrease the shear viscosity of the lamellar phase, and that a shear-induced transition to a multilamellar vesicle “onion” phase is pushed to higher shear rates, and in some cases completely suppressed. X-ray data reveal that the nanoparticles remain encapsulated within the membranes of the vesicles, thus indicating a viable method for the fabrication of nanoparticle incorporating organic vesicles.     

Magnetic stabilization of nematic turbulence

Under an applied magnetic field, turbulence starts off in a nematic liquid at higher critical Reynolds numbers. Also, the internal scale of turbulence is enlarged.     

Birefringence study on lyotropic micellar nematic phase in a binary mixture

We report the results of our studies on optical and thermal properties of two non-mesogenic compounds, namely, didodecyl dimethyl ammonium bromide (DDAB) and glacial acetic acid. The mixture exhibits schlieren texture of lyotropic micellar nematic (N_D), SmA, SmB, and SmG phases, respectively, at different concentrations of DDAB sequentially when the specimen is cooled from isotropic phase. The order parameter (S) of lyotropic micellar nematic (N_D) phase is estimated with the help of temperature dependence of optical anisotropy from the measured values of refractive index and density data. The temperature variation of order parameter of the experimental curve is in agreement with the Mayer–Saupe theoretical curve. The formation of the above phases has been confirmed by optical studies.     

Mapping out electron-electron interactions at surfaces

Using a high resolution coincidence technique, we measured for the first time the angular and energy correlation of an electron pair emitted from the valence band of a single crystal upon the impact of an electron with a specified momentum. We observe a hole in the measured two-particle correlation function when the two excited electrons have comparable momentum vectors, a fact traced back to exchange and repulsion among the electrons. We find the hole is not isotropic, has a finite extension, and is strongly suppressed when decoherence is operating.     

Impact of spin-orbit coupling on quantum Hall nematic phases

Anisotropic charge transport is observed in a two-dimensional (2D) hole system in a perpendicular magnetic field at filling factors nu=7/2, nu=11/2, and nu=13/2 at low temperature. In stark contrast, the transport at nu=9/2 is isotropic for all temperatures. Isotropic hole transport at nu=7/2 is restored for sufficiently low 2D densities or an asymmetric confining potential. The density and symmetry dependences of the observed anisotropies suggest that strong spin-orbit coupling in the hole system contributes to the unusual transport behavior.     

Carbon phases on nickel surfaces

The conditions of the formation of different carbon surface phases on nickel substrates by the example of a planar Ni( 110) surface and a stepped Ni(771 ) surface similar in structure were determined. The structure of the phases was investigated by means of scanning tunneling microscopy (STM), and the influence of carbon on the structure of the nickel surface was demonstrated. The process of graphene synthesis by propylene cracking is described. A method for forming graphene islands on nickel is proposed. A variety of phase transitions between the carbon surface phases (e.g., surface carbide, graphene, and graphene islands) and the reasons for their irreversibility are discussed. The relation between the structures of the surface carbide phases and the crystal structures of the initial surfaces for two different substrates is shown.     

Effective interactions of colloids on nematic films

The elastic and capillary interactions between a pair of colloidal particles trapped on top of a nematic film are studied theoretically for large separations d. The elastic interaction is repulsive and of quadrupolar type, varying as d^-5. For macroscopically thick films, the capillary interaction is likewise repulsive and proportional to d^-5 as a consequence of mechanical isolation of the system comprised of the colloids and the interface. A finite film thickness introduces a nonvanishing force on the system (exerted by the substrate supporting the film) leading to logarithmically varying capillary attractions. However, their strength turns out to be too small to be of importance for the recently observed pattern formation of colloidal droplets on nematic films.     

Stability of the uniform alignment of lyotropic nematic liquid crystals doped with magnetic grains

The stability of the uniform alignment of lyotropic nematic liquid crystals doped with magnetic grains is theoretically analysed. The sample is assumed to be prepared in a uniformly magnetized state by means of an external magnetic field, which is switched off subsequently. We show that there is a critical concentration of magnetic grains above which the distorting effect of the magnetic interaction overcomes the stabilizing effect of the steric interactions among grains and micelles.     

Observations of the collapse of dilute lyotropic lamellar phases under shear flow

Experimental evidence of the collapse of dilute lamellar phases due to shear flow is presented. Two systems are used: one composed of brine and an ionic surfactant, and another composed of water, a nonionic surfactant, and cosurfactant. We observe this transition for a range of lamellar spacings and brine salinity. The results are in reasonable agreement with recent theory in which the suppression of fluctuations by shear plays an important role.     

Confined diffusion of hydrophilic probes inserted in lyotropic lamellar phases

The dynamic behaviour of three hydrophilic probes (two dyes and one fluorescently-labelled protein) inserted in the water layers of lyotropic lamellar phases has been studied by confocal fluorescence recovery experiments. Two different, ionic (AOT/NaCl/ H(2)O and non-ionic ( C(12)E(5) /hexanol/ H(2)O host systems were studied. The confinement effect has been carefully monitored using the swelling properties of the lamellar phases. In all cases, we measure the evolution of the probe diffusion coefficient in the layer plane D ( perpendicular) versus the separation between the membranes d(w). Depending on the composition of the lamellar phase, this distance can be continuously adjusted from 500A to about 20A. For all systems, we observe a first regime, called dilute regime, where the diffusion coefficient decreases almost linearly with 1/d (w) . In this regime, the Faxén theory for the friction coefficient of a spherical particle symmetrically dragged between two rigid walls can largely explain our results. More unexpectedly, when the membranes are non-ionic, and also quite flexible ( C(12)E(5) /hexanol in water), we observe the existence of a second, concentrated (or confined) regime, where the diffusion coefficient is nearly constant and different from zero for membrane separations smaller than the particle size. This new regime can be heuristically explained by simple arguments taking into account the membrane fluidity.     

Molecular orientation of lyotropic nematic phase in a mixture of two non-mesogenic compounds

We report the results of our studies on the optical and thermal properties of the mixture of two non-mesogenic compounds, namely, sodium dodecyl sulfate (SDS) and glacial acetic acid (GAA). The mixture exhibits very interesting schlieren texture of lyotropic micellar nematic (N_D) phase, SmA and SmB phases, respectively, at different concentrations of SDS in GAA sequentially when the specimen is cooled from its isotropic phase. The order parameter (S) of the lyotropic micellar nematic (N_D) phase is estimated with the help of temperature dependence of optical anisotropy from the measured values of refractive index and density data. The experimental curve showing the temperature variation of order parameter is very well fitted with the Mayer–Saupe theoretical curve. X-ray studies have also been discussed. The formation of the above phases has been confirmed by optical and differential scanning calorimetry studies.     

Influence of the lengths and conformation states of hydrocarbon chains on the stability of lyotropic phases

The changes in the lyotropic phases after mixing amphiphilics with different chain lengths or after changing the environmental temperature are investigated theoretically. Three cases are considered: (a) chains with the same number of carbon atoms deformed because of the thermal motion; (b) a mixture of amphiphilics with hard hydrocarbon chains of different lengths; (c) in a more general phenomenological approach, an expression is proposed for the elastic energy, needed for the packing of the hydrocarbon chain in the building block of every lyotropic phase.     

Charged particles on surfaces: coexistence of dilute phases and periodic structures at interfaces

We consider a mixture of two immiscible oppositely charged molecules strongly adsorbed to an interface, with a neutral nonselective molecular background. We determine the coexistence between a high density ionic periodic phase and a dilute isotropic ionic phase. We use a strong segregation approach for the periodic phase and determine the one-loop free energy for the dilute phase. Lamellar and hexagonal patterns are calculated for different charge stoichiometries of the mixture. Molecular dynamics simulations exhibit the predicted phase behavior. The periodic length scale of the solid phase is found to scale as epsilon/(lB psi3/2), where psi is the effective charge density, lB is the Bjerrum length, and epsilon is the cohesive energy.     

Structure and rheology of the defect-gel states of pure and particle-dispersed lyotropic lamellar phases

We present important new results from light-microscopy and rheometry on a moderately concentrated lyotropic smectic, with and without particulate additives. Shear-treatment aligns the phase rapidly, except for a striking network of oily-streak defects, which anneals out much more slowly. If spherical particles several microns in diameter are dispersed in the lamellar medium, part of the defect network persists under shear-treatment, its nodes anchored on the particles. The sample as prepared has substantial storage and loss moduli, both of which decrease steadily under shear-treatment. Adding particles enhances the moduli and retards their decay under shear. The data for the frequency-dependent storage modulus after various durations of shear-treatment can be scaled to collapse onto a single curve. The elasticity and dissipation in these samples thus arises mainly from the defect network, not directly from the smectic elasticity and hydrodynamics. Received 19 April 1999 and Received in final form 20 May 1999     

Quantitative analysis of lyotropic lamellar phases SANS patterns in powder oriented samples

We have developed a detailed numerical method based on the Caillé model to fit Small Angle Neutron Scattering profiles of powder-oriented lyotropic lamellar phases. We thus obtain quantitative values for the Caillé parameter and the smectic penetration length from which we can derive the smectic compression modulus and the membrane mean bending modulus. Our method, applied to a surfactant lamellar phase system decorated by amphiphilic copolymers, provides excellent fits for any intermembrane spacing or membrane concentration over the entire q-range of the SANS experiments. We compare our fits with those obtained from the model of Nallet et al. (J. Phys. II 3, 487 (1993)), which is reviewed. Good fits are obtained with both methods for samples exhibiting “hard” smectic order (sharp Bragg peak, moderate small angle scattering). Only our procedure, however, gives good fits in the case of “soft” smectic order (smooth Bragg peak, strong small angle scattering). A quantitative criterion to discriminate between these “soft” and “hard” samples is also proposed, based on a simple analogy with smectic-A liquid crystal in contact with an undulating solid surface. This allows us to anticipate the type of thermodynamic information that can be derived from the fits.