Pawel Pieranski

Shear-flow-induced transition in nematics

Abstract The distortion, induced by a shear flow in a nematic (MBBA) layer having its director initially normal to the velocity and to the velocity gradient, has been observed by a conoscopic technique. At low velocity, the sample retains an ideal planar texture. Above a certain velocity threshold vc, a distortion is observed. The threshold is inversely proportional to the sample thickness, and can also be modified by a magnetic field. These results are in agreement with a simple calculation based on the Leslie equations.

Heat Convection in a Nematic Liquid Crystal

Abstract We present a theoretical and experimental study of the thermal properties of a well aligned nematic Liquid Crystal in a vertical temperature gradient. Due to the long time constant of the relaxation of the director and to the anisotropy of the heat conductivity, the value of the temperature gradient threshold for convective instabilities is strongly modified as compared to the isotropic case. Experiments on planar MBBA films show a drastic reduction of the threshold. A detailed description of the shape and temperature distribution of the convective rolls is given.

Optical Study of a Chiral Smectic C Under Shear

Abstract We study the distortion of a chiral smectic C, under a shear applied parallel to the plane of the layers, using an original technique based on the modification of the conoscopic image. Both the amplitude of the distortion and the phase lag between the alternating applied shear and the deformation are found to vary like the flow induced transverse polarization (described in Ref. 7). The behaviour is understood in terms of a viscoelastic-nematic-like model and of a simplified optical description which gives account of the displacement without deformation of the conoscopic image.

Deformation of nematic layers in crossed electric and magnetic fields

Abstract A parallel nematic layer will show a Freedericksz-transition in an electric field when the voltage exceeds a threshold value U c . If a magnetic field is applied parallel to the layer but perpendicular to the molecular orientation the threshold voltage U c increases with increasing magnetic field field H above a threshold field H c2 . From a plot of U c vs H all three elastic constants can be obtained. Theoretical results are compared with data of HBAB.

Solid-solid phase transitions in a low-dimensionality system

Abstract The properties of a bilayer of hard spheres are determined in the uncorrelated-cell approximation. The experimentally observed solid-solid phase transitions between a triangular and a square structure are identified, and interpreted in terms of accessibility of the free volume.

Concave and convex shapes of the Pn3m/L1 interface

Abstract.Shapes of the interface between the L1 and cubic Pn3m phases in the mixture C12EO2/water are studied. The concave and convex variants of the interface are realised using Pn3m crystals surrounded by the L1 phase and L1 inclusions on surfaces and in the bulk of the Pn3m phase. It is shown that both variants of the Pn3m/L1 interface contain the (111)-type facets in coexistence with everywhere else rough surfaces. The matching between facets and curved parts of the interface is angular. In the vicinity of the upper limit of the L1 + Pn3m coexistence domain, additional (200)-type facets appear on the interface. The influence of the contact angle at glass walls on shapes of crystals and of inclusions is discussed.

Symmetry, topology and faceting in bicontinuous lyotropic crystals

Abstract.Phase diagrams of phytantriol/ethanol/water and phytantriol/DSPG/ethanol/water systems are explored and experiments on facetings of Pn3m-in-L1 and Im3m-in-L1 crystals are performed. Observed crystal habits do not agree with the Friedel-Donnay-Harker rules. We argue that this paradox can be explained in terms of constraints imposed on Pn3m/L1 and Im3m/L1 interfaces by the bicontinuous topology of the cubic phases. We point out that when free edges of the surfactant bilayer are prohibited at these interfaces, the two labyrinthes separated by the bilayer cannot anymore be equivalent. The corresponding

Topological defects in bicontinuous phases

{\rm Pn3m}\rightarrow {\rm Fd3m}

New Faceting Phenomena in Lyotropic Liquid Crystals

and

“Magic Spiral” Submitted to a Torque: Topological Flows Driven by Ericksen Stresses in Sm C Films

{\rm Im3m}\rightarrow {\rm Pm3m}