We designed and synthesized new SmA* liquid crystals with different molecular architectures based on the chiral mesogen MR11, which was unsubstituted, or attached at one end and at both ends of a trisiloxane segment in the monosubstituted MSi3-MR11 and disubstituted DSi3-MR11 siloxanes, respectively. We studied the critical divergence of the magnetic susceptibility by measurements of the induced tilt angle Θind at the SmA*–SmC* transition. While all three SmA* liquid crystals presented a de Vries character, this was quite distinctive for DSi3-MR11 for which the critical exponent γ = 1.87 was maximal. X-ray diffraction experiments confirmed that the SmA*–SmC* transition occurred with essentially no layer shrinkage.

We have studied the dynamics of defects (disclinations and walls) which appear in a thin nematic cell with planar anchoring, after a quench from the isotropic phase. The transient texture can be analyzed in terms of ±1/2 disclinations and surface π-walls (corresponding to a π-rotation of the director in the plane of the cell). We determine the structure of the walls and explain the main features of the dynamics. The relaxation of the texture is largely dominated by surface effects, which control the relaxation trajectories and velocities of the disclinations, as well as the dynamics of π-walls.

We measure the electric-field-driven annihilation of nematic disclination pairs with strength ±1/2 in the 4−cyano−4′−n-pentylbiphenyl (5CB) liquid crystal. The use of a very weak azimuthal anchoring ensures a two-dimensional director field. The relaxation is governed by the formation of a π wall connecting the two opposite charge defects. The +1/2 disclinations move almost twice as fast as the −1/2 disclinations. The simple used geometry allows a quantitative comparison with numerical studies based on the hydrodynamics of the tensorial order parameter. The simulations show that in the π wall regime the symmetry breaking is due to the backflow and not to the elastic anisotropy.

Disclinations in nematic liquid crystals usually adopt a straight shape in order to minimize their elastic energy. Once created in the course of a nonequilibrium process such as a temperature quench from the isotropic to the nematic phase, the topologically stable disclinations of half-integer strength either annihilate each other in pairs of opposite strength or form topologically unstable disclinations of integer strength. In this article, we demonstrate that the annihilation process can be inhibited and the defects can be deformed by an applied electric field. We study the disclination lines in the deep uniaxial nematic phase, located at the boundary between two different types of walls, the so-called π wall (a planar soliton stabilized by the surface anchoring) and the Brochard-Léger (BL) wall stabilized by the applied electric field. By changing the electric voltage, one can control the energy of director deformations associated with the two walls and thus control the deformation and dynamics of the disclination line. At small voltages, the disclinations are straight lines connecting the opposite plates of the cell, located at the two ends of the π walls. The π walls tend to shrink. When the voltage increases above EF, the Fréedericksz threshold, the BL walls appear and connect pairs of disclinations along a path complementary to the π wall. At E>2EF, the BL walls store sufficient energy to prevent shrinking of the π walls. Reconstruction of the three-dimensional director configuration using a fluorescent confocal polarizing microscopy demonstrates that the disclinations are strongly bent in the region between the π and the BL walls. The distortions and the related dynamics are associated with the transformation of the BL wall into two surface disclination lines; we characterize it experimentally as a function of the applied electric field, the cell thickness, and the sample temperature. A simple model captures the essential details of the experimental data.

An array of edge dislocation forms spontaneously in a Grandjean-Cano wedge filled by a smectic liquid crystal. In the vicinity of the smectic A to smectic C transition, these defects are visible under the microscope [R. B. Meyer, B. Stebler, and S. T. Lagerwall, Phys. Rev. Lett. 41, 1393 (1978)]. This paper deals with their dynamics under controlled deformation (dilation and compression). First, we characterize several regimes of dislocation mobility occurring with increasing strain epsilon or strain rate epsilon. We relate these regimes to the interactions between screw and edge dislocations. We also show that screw dislocations give rise to loops of edge dislocations under sufficient strain, which strengthens the model of loop nucleation by helical instability of screw dislocations. Lastly, we discuss several models for the microscopic origin of the interactions between defects.