Shear-induced collapse in a lyotropic lamellar phase

An entropically stabilized cetylpyridinium chloride, hexanol, and heavy brine lyotropic lamellar phase subjected to shear flow has been observed here by small angle neutron scattering to undergo collapse of smectic order above a threshold shear rate. The results are compared with theories predicting that such a lamellar phase sheared above a critical rate should lose its stability by a loss of resistance to compression due to the suppression of membrane fluctuations.     

Diffusion coefficients in a lamellar lyotropic phase: evidence for defects connecting the surfactant structure

We measure diffusion coefficients in the lamellar phase of the nonionic binary system C(12)EO(6)/H(2)O using fluorescence recovery after photobleaching. The diffusion coefficient across the lamellae shows an abrupt increase upon approaching the lamellar-isotropic phase transition. We interpret this feature in terms of defects connecting the surfactant structure. An estimation of the defect density and of the variation in defect energy close to the transition is given in terms of a simple model.     

Oscillating viscosity in a lyotropic lamellar phase under shear flow

We report some time-dependent behavior of lyotropic lamellar phase under shear flow. At fixed stress, near a layering instability, the system presents an oscillating shear rate. We build up a new stress versus shear rate diagram that includes temporal behavior. This diagram is made of two distinct branches of stationary states which correspond, respectively, to disordered and ordered multilamellar vesicle phases. When increasing the shear stress, prior to the transition to the ordered structural state, sustained oscillations of the viscosity are recorded. They correspond to periodic structural change of the entire sample between a disordered and a ordered state of multilamellar vesicles.     

Curvature walls and focal conic domains in a lyotropic lamellar phase

In the quasi-ternary CPCl/brine/hexanol lyotropic system, the interface of the lamellar and L₃ sponge phases displays a phenomenon of epitaxy: the layers of the lamellar phase tend to make a constant non-trivial angle with the interface. Thin samples of lamellar phase embedded in the sponge phase are thus submitted to oblique anchoring conditions and defects are created in the lamellar phase in order to satisfy the bulk lamellar ordering and the boundary conditions. We have studied small droplets of lamellar phase in the sponge phase. They do not exhibit the classic defects (focal conic domains) but wall defects, which appear in order to satisfy the smectic elasticity and the boundary conditions. Moreover we show through experiments in controlled geometry that, even in the presence of focal conic domains, wall defects control the size and periodicity of the textures which are observed at the interface. Received 4 November 1998     

Rheological properties of a highly confined film of a lyotropic lamellar phase

We investigated the rheological properties of a thin film of a lyotropic lamellar phase with a Dynamic Surface Forces Apparatus. The minimum thickness of the film is varied between one to several tens of layers by confining the materials between solid surfaces. The rheometric properties are measured with the application of a small harmonic compression. These properties depend clearly on the smectic order of the material. Whole mechanical properties may be easily described by taking into account interactions between membranes and motion of the dislocation line loops. In particular, it is shown that at the dynamical frequencies investigated in this study, the solvent flows between membranes which remain undeformed. Consequences and perspectives of this study will be discussed.Received: 22 April 2004, Published online: 11 August 2004PACS: 61.30.St Lyotropic phases - 61.30.Jf Defects in liquid crystals - 83.80.Qr Surfactant and micellar systems, associated polymers     

Motionally induced asymmetry parameter in NMR spectral patterns from a lyotropic lamellar phase

A large asymmetry parameter in the deuterium spectral pattern is idetified for the first time near the gel position of the of the potassium palmitate-water system. This asymmetry, measured at the deuterated terminal methyl position of the hydrocarbon chain, implies biaxial order suggesting cooperative tilt of the chains in the bilayer.     

Aging and yielding in a sheared AOT/iso-octane/water lyotropic lamellar phase

We define a creep-flow-based measurement procedure to allow reliable and reproducible results on aging and yielding materials to be obtained. Investigation of the effects of different parameter such as the pre-shear time, the recovery time and the applied stress magnitude on the viscoelastic properties of a lyotropic liquid crystal phase is reported. Cryo-TEM observations indicate the formation of multiconnected bilayers at rest. Shearing the investigated material shows a propensity to acquire all the macroscopic properties of “soft jammed systems”. These properties are then interpreted in terms of shear-induced structural rearrangement on the basis of cryofracture observation obtained at different times after the preshear imposed.     

Polymer-mediated interactions of fluid membranes in a lyotropic lamellar phase: a small angle X-ray and neutron scattering study

Small angle X-ray and neutron scattering data on an effective three-component lamellar phase composed of water, a non adsorbing water-soluble polymer (polyvynilpyrolidone), fluid membranes, made from a mixture of a cationic surfactant (cetylpiridiumchloride) and a cosurfactant (hexanol), are presented for various membrane as well as polymer concentrations. The data are fitted with a recently proposed model which takes into account the geometry and the fluctuations of these periodic structures. This allows a quantitative study of the polymer contribution to the smectic compression modulus of the lamellar phase. Four different regimes of polymer confinement are expected. The associated variations in are compared to a recent theoretical model, which predicts the polymer-mediated contribution to the smectic compression modulus. Received 20 January 1998     

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.     

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.     

Optical properties of lyotropic mesophases

Summary A brief review of early studies of optical properties of lyotropic mesophases is presented. New results about refractive-index measurements in various phases of a ternary system are shown. Emphasis is given to the optical properties of the different lyotropic nematic mesophases. Finally a short review is then devoted to the optical effects of external applied fields such as magnetic, electric and mechanical ones. Associated to the GNCL (National Discussion Group on Liquid Crystals) and supported by the GNSM of the National Research Council.     

Optical nonlinearities in lyotropic liquid crystals

Summary The existence of an optical nonlinearity of the Kerr-like type in a lyotropic liquid crystal is described in c.w. operation. The nonlinearity is found using the liquid crystal as a cladding material in a waveguide, or studying the interface properties between the liquid crystal and an ordinary glass. The used material is potassium caprillate and the nonlinearity is ascribed to thermal effects. Work performed with partial support from MPI-40% (Italian Education Ministry) and GNEQP-CNR (Italian National Research Council-Quantum Electronics Group).     

Thermal diffusivity measurements in a lyotropic discotic nematic phase

The present work is focused on thermal diffusivity (D)$(D)$ measurements, via Z  -scan experiment, in a discotic nematic phase of the lyotropic mixture comprised of potassium laurate, decanol and D₂O${\text{D}}_2\text{O}$. In this experiment, the nematic sample is translated through the focal region of a focused Gaussian laser beam. The experimental data are analyzed according to the thermal lens model and the ratio between the thermal diffusivities parallel (D_∥)$(D_{\parallel })$ and perpendicular (D_⊥)$(D_{\perp })$ to the director of the nematic sample has been found to be smaller than one. The results are compared to others obtained with different liquid crystals and explained by using a simple model where this ratio is correlated to the shape anisotropy of the micelles.     

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.     

Cracks and topological defects in lyotropic nematic gels

We report on the effects of the coupling of nematic order and elasticity in anisotropic lyotropic gels consisting of large nematic domains of surfactant coated single wall carbon nanotubes embedded in a cross-linked N-isopropyl acrylamide polymer matrix. We observe the following striking features: (i) undulations and then cusping of the gel sidewalls, (ii) a nematic director field that evolves as the gel sidewalls deform, (iii) networks of surface cracks that are orthogonal to the nematic director field, and (iv) fissures at the sidewall cusps and associated topological defects that would not form in liquid nematics.