Jean-Marc Gilli

Interface dynamics in liquid crystals

Abstract:We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns which have been both experimentally and theoretically pointed out.

New Aspect of the Voltage/Confinement Ratio Phase Diagram for a Confined Homeotropic Cholesteric

We have investigated the behavior of a cholesteric mixture, consisting of the negative dielectric anisotropy liquid crystal MBBA and a chiral dopant S811(1%b.w.). In the phase diagram a.c. applied voltage versus confinement ratio, in addition to the transition previously observed, before reaching the domain of the transition TIC(Translationnary Invariant Cholesteric)–modulated TIC, we have observed a new stability domain for the topological bubble. These bubbles are now surrounded by the TIC domains and necessarily associated to an hyperbolic − 1 defect of the in plane director component.We have also obtained experimentally topological defect chains, formed by topological dipoles similar to the ones formed in 2 compound materials: liquid crystal/solid spheres or liquid crystal/liquid droplets.

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

Abstract We will show here that, in a two-dimensional liquid crystal system where disclinations are present, the elastic torque cannot balance alone an external torque; a hydrodynamic flow must indeed occur so that the system can reach a stationary state. We suggest calling such flows topological flows. We will indicate how they are produced by the Ericksen forces.

Compared Study of a Quenched Blue Phase by Direct Transmission Electron and Atomic Force Microscopy

Abstract A side chain mesomorphic cooligomer presents a large blue phase temperature domain. The cooling down of the sample from BPI allows to quench coloured crystallites below the glass transition of this material. Observations by transmission electron microscopy on microtome cuts of these crystallites reveal the presence of mono- and bi-periodic domains corresponding to the respective quench of cholesteric and BPI phases at room temperature. The use of an atomic force microscopy technique directly on the cuts observed by electron microscopy, and the detection of the same textures by both methods reveal unambiguously that the TEM contrast originates from strong, “cut-induced”, surface relief. The nature of this corrugation is discussed using numerical simulations derived from a Ginzburg-Landau approach.

Cladis' orbiting disclinations in smectic films submitted to a torque

Abstract We study here the behaviour of S=+1 and S=-1 disclinations in SmC films submitted to a rotating electric field. In agreement with Cladis et al. experiments we observe that a S=+1 disclination submitted to large enough torque leaves the centre of the film and is orbiting around a target-like pattern. A single S=-1 disclination submitted to a torque behaves differently: independently of the applied torque, it stays in the centre of the film. We explain this difference as due to the presence (S=+1) and the absence (S=-1) of energy barriers with respect to the phase winding.

Textures and instabilities in liquid crystals and mesomorphic polymers

Abstract An aromatic polyester of monomeric units similar to PAA separated by flexible spacers show electrohydrodynamic instabilities in external electric field. As in the case of small molecule liquid crystals, conductive and dielectric modes are observed using polarizing microscopy. Slow kinetics due to the higher viscosity allow the observation of the evolution to turbulence. A new periodic texture in the dielectric domain with a frequency dependent periodicity is also observed in polymeric samples The Freedericksz transition in the case of homeotropic anchorage is discussed.

Experimental Study of One‐Dimensional Spinodal Decomposition in Liquid Crystals

The coarsening dynamics of a zigzag wall formed in nematic liquid crystals under external fields is investigated experimentally. The vertexes of zigzag can be considered as kinks in a one‐dimensional order parameter system and the geometrical constrain associated to the necessary equal length sum of zig and zag segments impose a conserved quantity in this Cahn‐Hilliard type problem. We consequently use this experiment to investigate the spinodal decomposition in a one dimentional conserved order parameter system. It is found that, as expected by the theories for one‐dimensional kink dynamics, the characteristic length of the system L(t) increases logarithmically in time and the dynamical scaling law holds in the coarsening process.

Observation of One-Dimensional Spinodal Decomposition in a Nematic Liquid Crystal

The coarsening dynamics of kinks in a one-dimensional conserved order parameter system is investigated experimentally in a nematic liquid crystal of negative dielectric anisotropy under a magnetic field. Without an external electric field, a straight splay-bend wall is formed in the cell perpendicular to the magnetic field. When an electric voltage, higher than a critical value, is applied to the liquid crystal cell, the straight wall spontaneously tilts locally toward the magnetic field direction and forms a zigzag line. Then the zigzag pattern starts to coarse. Considering the vertexes of the zigzag as kinks in a one-dimensional conserved Ising system, the coarsening dynamics is discussed. It is found that, as expected by the theories for one-dimensional kink dynamics, the characteristic length increases logarithmically in time and the dynamical scaling law holds in the coarsening process.

Hidden smectic properties of a chiral side-chain co-oligomer

We recently showed that a side-chain industrial co-oligosiloxane presents a quenchable enlarged blue phase behaviour at the cholesteric-isotropic phase transition. In this paper, we present the results of a structural study based on X-ray diffraction, differential scanning calorimetry and optical measurements. In particular, the smectic A organisation is demonstrated in the lower temperature domain, which was hitherto understood as a cholesteric phase. A structural model for this phase is proposed on the basis of the analysis of the anisotropic scattering of stretched fibers. Our results also suggest that the observed glass transition is indeed a rather complex phenomenon, which seems to involve not only the freezing of the main chains, but also smectic correlations at the side-chain level. Moreover, the calorimetric study indicates that, notwithstanding the conservation of the processed films optical properties, low kinetic reorganisations occur at room temperature.