En vue d’études ultérieures sur de nouvelles couches d’alignement, nous nous intéressons àcaractériser les phénomènes de dynamique de réorientation locale d’un nématique sur unsubstrat bien connu : SiO évaporé à 60° sur lames ITO/verre donnant un ancrage planaire.Les défauts topologiques ±1/2 apparaissant dans la phase nématique du 5CB lors de sanucléation dans la phase isotrope relaxent spontanément en l’absence de tension appliquée[1].L’application d’un champ électrique valant environ 2 fois le champ seuil de Freederickszprévient cette relaxation. Les lignes de défauts, soumis à une force résultante très faible, sontalors très sensibles à l’état de surface du substrat et permettent de sonder localement ladynamique de réorientation locale du cristal liquide à la surface du substrat. L’étude envideo-microscopie de cette dynamique nous a permis de mettre en évidence des phénomènescomplexes s’apparentant aux phénomènes de piégeage-dépiégeage d’un objet élastique dansun paysage énergétique aléatoire.A travers l’étude en champ nul de la relaxation de « boucles de désorientation » inscrites surles substrats par le déplacement forcé des défauts ±1/2 sous champ, nous avons montré queces phénomènes se traduisaient par l’existence d’une force de friction solide macroscopiqueF0. Nous discuterons des origines microscopiques d’une telle force.[1] A. Bogi, P. Martinot-Lagarde, I. Dozov, and M. Nobili, Phys. Rev. Lett. 89, 225501-1 (2002).

The effects of a strong confinement on the textures of liquid crystals have beenthoroughly studied for various mesophases, such as nematics or cholesterics. But theconfinement of smectics has been less studied, because the presence of parallel layersyields complex textures and macroscopic defects, commonly identified as focal conicdomains1.Although it has been shown that any interface can be relaxed by focal conic domains2,the choice of a given texture by the material has not yet been clearly formalized. Forexample, direct smectic droplets confined in a sphere with a planar anchoring displaynumerous domains whereas inverse smectic droplets submitted to the same anchoringare much simpler and usually show only two singular lines arising from the surface (theequivalent of the boojums of inverse nematic droplets3).We have studied the topological and geometrical consequences of a strong anchoringat any tilt angleθ0 on a complex surface. We have introduced the notion of “STS”(smectic texture on a surface), in contrast with “STB” (smectic texture in the bulk), andwe have classified the possible point defects on any surface . The bipolar STS is thesimplest possible texture, but can be found on surfaces of revolution4 only.This analysis has been used to explain several generic textures of confined smectics,such as the typical size of the domains, the evolution of the textures with the zenithalanchoring angle θ0, the discrepancies in the textures of direct and inverse droplets…

A number of investigations, led over many years1-6, all indicate that defects, viz. focalconic domains, screw and edge dislocations, play an important role in the plasticbehavior of smectics under shear. Although some of the mechanisms in play have beendescribed2,6, the situation is far from being satisfactory, because of a lack of in-situobservations. In this work, we bring some new elements to the question of theinteractions between dislocations at a microscopic level.We have developed an experimental setup designed to observe the motion ofmicroscopic defects under polarized light microscope in a homeotropic slab of a SmAmaterial submitted to a strain7. The deformation of the cell and its temperature arecontrolled and the motion is followed by video. The following results have been reachedand discussed in a theoretical framework.(a) The existence of several regimes for the motion of dislocations and their transitions(b) The existence of a yield stress due to the depinning of the mobile edge dislocationsdislocations from the fixed screw dislocations; the associated forces, energies, andshapes of the moving lines are calculated;(c) The role of the heterogeneous/ homogeneous nucleation of dislocations understrainWe conclude that the interactions between defects actually rule the plastic deformationof the confined smectic and we discuss the origin of observed interactions.References :1 Bartolino and G. Durand, Phys. Rev. Lett., 1977, 39, 1346.2 R.G. Horn and M. Kléman,. Ann. de Phys. 1978, 3, 229.3 L. Bourdon, M. Kléman, L. Lejcek and D. Taupin, J. de Phys. 1981, 42, 261.4 P. Oswald and M. Kléman, J. de Phys. Lettres 1984, 45, L319.5 R. A. Herke, N. A. Clark, and M. Handshy, Science 1995, 267, 651. R. A. Herke, N. A. Clark, and M. Handshy, Phys.Rev. E 1997, 56, 3028.6 C. Meyer, S. Asnacios, and M. Kleman, Eur. Phys. J. E, 2001, 6, 245.7 I.Lelidis, M.Kleman, and J.L.Martin, Mol. Cryst. Liq. Cryst., 2000 , 351,187.

In lyotropic systems, the sponge and the lamellar phases possess the same local structure: a membrane made of a bilayer of surfactants. In the quasiternary lyotropic system CPCl/brine/hexanol, the bilayer is continuous through the interface between the lamellar and the sponge phases. A model based on this phenomenon has predicted a very low value of the interfacial free energy. The study of hydrodynamic relaxation time of distorted spherical lamellar droplets gives an estimation of the interfacial tension value. Results confirm the validity of the model and the dependence on membrane volume fraction is explained by a simple scaling law.