Dynamical behaviour of holographic gratings with a nematic film --Polymer slice sequence structure

We have studied the dynamical response of a new kind of holographic grating to a pulsed electric field. The structure, named POLICRYPS, consists of a sequence of homogeneous polymer slices separated by liquid-crystal films. We have found that both the rise and fall response times depend on the pulse duration and amplitude of the applied electric field. We propose a phenomenological explanation for this behaviour, outlining the possibility of getting response times of the order of 100 s by properly choosing the pulse shape. Results are very interesting for developments in which POLICRYPS gratings could be proposed as basic elements in photonic devices for telecom applications, and induce to a deeper analysis of the microscopic interaction between liquid crystal and polymer interfaces in these samples.Received: 5 February 2004, Published online: 14 September 2004PACS: 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 42.40.Eq Holographic optical elements; holographic gratings - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions     

Stability of thin nematic films

We discuss the peculiarity of thin nematic films on solid substrates with a free surface, underlining the differences with what is usually seen in dewetting. We review the thermodynamic basis of the coupled phase/thickness separation that has previously been shown experimentally. We give new experimental evidences for the origin of the coupling force chosen in our previous theoretical model. This additional information contributes to the discussion raised by the article of Ziherl and Zumer in this issue [19].Received: 3 December 2003PACS: 68.15. + e Liquid thin films - 64.70.Md Transitions in liquid crystals - 61.30 Surface phenomena in liquid crystals including anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions and wetting transitions     

Static and dynamic electro-optic properties of a SmC* phase in surface stabilized geometry and dispersed in the polymer matrix

Comparative electro-optical measurements have been made on a ferroelectric liquid crystal (FLC) in surface stabilized geometry and confined to an ellipsoidal cavity within a polymer matrix. The static and dynamic electro-optical characteristics were measured for both systems and show qualitatively similar behaviours. A fast switching and important bistability were observed and characterized as a function of the applied electric field strength. The switching time between the two stable states of the surface stabilized cell was found to be longer than that found for the composite films. We argue that the faster switching dynamic of the FLC in cavities is due to the enhance of the rotational mobility of the molecules, probably (and partly) because of the soft anchoring character of the molecules at the cavity walls. Using a collective switching model in the high field regime, which assume a linear coupling between the spontaneous polarization and the local cavity electric field, we give an estimate of the rotational viscosity of the FLC molecules in the droplets.Received: 5 October 2003, Published online: 5 February 2004PACS: 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions - 77.80.Fm Switching phenomena     

Morphology and structure of thin liquid-crystalline films at nematic-isotropic transition

We review the main features of very thin nematic liquid-crystalline films on solid substrates, focusing on 5CB on oxidized silicon wafers. By discussing the theoretical aspects of the observed structures, we show that the phenomena at work include isotropic capillary condensation and that the coexistence of isotropic and nematic terraces in thin films is a result of the interplay of several mechanisms. Further theoretical as well as experimental efforts are needed to completely understand the wetting behavior of these systems.Received: 1 August 2003PACS: 68.08.Bc Wetting - 68.15. + e Liquid thin films - 61.30.Hn Surface phenomena in liquid crystals including anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions     

Stripe instability in thin films of smectic liquid-crystal polymers

Two-dimensional nanostripes are formed in thin films of side-chain liquid-crystalline polymer films when the material enters the smectic phase. The structure is investigated using transmission electron microscopy. Electron diffraction patterns show that the chain molecules are mostly aligned in the film plane and the average molecular director is parallel to the direction of the stripes. We discuss factors affecting the stripe amplitude and periodicity, such as the film thickness and the temperature of annealing in the nematic phase, and suggest a possible mechanism for their formation. We propose that an equilibrium instability occurs due to a competition between the layer-aligning effect of the substrate and the planar director alignment, forcing smectic layers perpendicular to the film surface. The stripes decorate the overall patterns of nematic director in the polymer film and provide a means of high-resolution imaging for observation of textures and disclinations.Received: 10 June 2003, Published online: 19 August 2003PACS: 68.37.Lp Transmission electron microscopy (TEM) (including STEM, HRTEM, etc.) - 61.30.Vx Polymer liquid crystals - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions - 68.55.Nq Composition and phase identification     

Thickening of a smectic membrane in an evanescent X-ray beam

We report an unusual thickening of smectic membranes under the influence of X-ray irradiation below the critical angle. In the case of a four-layer film the thickness was found to grow at the footprint of the beam, reaching within minutes tens of layers. The effect is attributed to the localized energy dissipation of the evanescent wave. The island thus created is not connected to the meniscus and after the beam is switched off the film returns to its original state. A possible explanation is given in terms of a local disrupture of the tension of the smectic membrane.Received: 22 July 2004, Published online: 1 October 2004PACS: 61.30.-v Liquid crystals - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions - 52.25.Os Emission, absorption, and scattering of electromagnetic radiation     

Frustrated rotations in nematic monolayers

Tabe and Yokoyama found recently that the optical axis in a chiral monolayer of a ferronematic rotates when water evaporates from the bath: the chiral molecules act as propellers. When the axis is blocked at the lateral walls of the trough, the accumulated rotation inside creates huge splays and bends. We discuss the relaxation of these tensions, assuming that a single dust particle nucleates disclination pairs. For the simplest geometry, we then predict a long delay time followed by a non-periodic sequence of bursts. These ideas are checked by numerical simulations.PACS: 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions - 61.30.Jf Defects in liquid crystals     

Enhancement of the orientational order parameter of nematic liquid crystals in thin cells

We report measurements of birefringence of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to ) and thick (7 to cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to ) of ( , where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far ( C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.Received: 22 February 2004, Published online: 24 May 2004PACS: 61.30.-v Liquid crystals - 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitionsSurajit Dhara: Present address: Department of Physics, Birla Institute of Technology and Science, Pilani 333 031, India.     

Surface and bulk interchange energy in binary mixtures of chain molecules

The interchange (interaction) parameter, controlling the phase behaviour of a binary mixture, is determined for the bulk and the surface of binary mixtures of different types of chain molecules, using surface tensiometry and a mean-field theory. For all mixtures and concentrations studied an identical behaviour is observed at the surface, depending only on the square of the reduced chain length mismatch , where and are the difference in and average of the number of carbons of the two components.Received: 21 December 2003, Published online: 25 March 2004PACS: 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions - 68.35.Md Surface thermodynamics, surface energies - 68.03.Cd Surface tension and related phenomena     

Structure and thermodynamics of associating rods solutions

Physical gelation in rod-like associating polymer solutions is analyzed by computer simulations and by mean field models. The structural gelation (SG) transition is determined by a rapid change in the cluster weight average. The analytically determined SG transition is in good agreement with the simulation results. At low temperatures we observe a peak in the heat capacity, which maximum is associated with phase segregation. The SG transition is sensitive to the number of associating groups per rod but nearly insensitive to the spatial distribution of associating groups around the rods. Phase segregation is strongly dependent on both the number and distribution of associating groups per rod. We find negligible overall nematic order near the SG transition and only partial local nematic order of the clusters at the phase segregation transitions.Received: 23 May 2003, Published online: 5 February 2004PACS: 61.30.Cz Molecular and microscopic models and theories of liquid crystal structure - 64.70.Md Transitions in liquid crystals     

Anchoring and structural transitions as a function of molecular length in confined liquid crystals

Using deuteron nuclear magnetic resonance to study liquid crystals confined to cylindrical pores, an anchoring transition has been found. The transition exhibits an unexpected sharp dependence of the anchoring strength on cyanobiphenyl liquid crystal molecular length. A structural transition from a parallel axial to a planar radial configuration occurs due to an anchoring transition from planar to weakly homeotropic orientation at the walls. The anchoring strength is at a minimum near the decylcyanobiphenyl (10CB) liquid crystal length. Long chain liquid crystal configurations depend on thermal cycling and on the equilibrium atmosphere leading to a bistable SmA structure. Orientational order wetting in the isotropic phase also depends on molecular length.     

Blobs,channels and “cigars”: Morphologies of liquids at a step

We have studied the equilibria of liquid droplets wetting a step edge with an opening angle by a combination of analytical and numerical methods. Depending on the wetting properties of the substrate walls and on the liquid volume, different locally or globally stable liquid morphologies are found. Complete spreading of the liquid along the bottom edge of the step is observed at equilibrium contact angles satisfying . If the contact angle exceeds a threshold value the liquid exists in a blob-like configuration. Surprisingly, we find an intermediate regime at a sufficiently high liquid volume and in a range of contact angles , in which cigar-shaped configurations arise in addition to the blob. We close the paper by a detailed discussion of the stability of this novel liquid morphology.PACS: 47.55.Dz Drops and bubbles - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions - 68.08.Bc Wetting     

Quartic coupling and its effect on wetting behaviors in nematic liquid crystals

Based on the fact that rubbed groove patterns also affect the anchoring of liquid crystals at substrates,a quartic coupling is included in constructing the surface energy for a liquid crystal cell.The phase diagram and the wetting behaviors of the liquid crystal cell,bounded by surfactant-laden interfaces in a magnetic field perpendicular to the substrate are discussed by taking the quartic coupling into account.The nematic order increases at the surface while it decreases in the bulk as a result of the introduction of quartic substrate-liquid crystal coupling,indicating that the groove anchoring makes the liquid crystal molecules align more orderly near the substrate than away from it.This causes a different wetting behavior:complete wetting.     

Surface ordering transitions at a liquid crystal-solid interface above the isotropic smectic-A transition

The degree of orientational order induced by confining cylindrical surfaces is monitored via deuteron nuclear magnetic resonance linesplitting and linewidth above the smectic-A to isotropic phase transition. The orientational order strongly depends on the length of the surfactant coupling molecule, on the surface coverage, and on the liquid crystal. Continuous and stepwise growth of orientational order and surface-induced orientational order transitions found in the isotropic phase are explained in terms of a simplified model of surface-induced layering and molecular self-diffusion.     

Ratchet effect in faceting

The paper deals with a new phenomenon, named ratchet effect, envisioned theoretically as a likely consequence of metastability of crystal facets and expected to occur upon a temperature cycling. In experiments, Pn3m lyotropic crystals surrounded by the isotropic L1 phase in the mixture C₁₂EO₂/water are used. At equilibrium, the Pn3m/L1 interface contains small (111)-type facets in coexistence with rough surfaces. In agreement with theoretical expectations, it is shown that upon a saw-tooth-shaped temperature cycling, facets are growing until the rough surfaces are completely eliminated. A model of the ratchet effect is proposed.Received: 8 January 2004, Published online: 25 March 2004PACS: 64.70.Md Phase transitions in liquid crystals - 68.35.Md Surface thermodynamics, surface energies - 61.30.-v Liquid crystals     

New investigation on the tetragonal liquid-crystalline phase or SmQ

Chiral liquid crystals, for which twist and smectic order strongly compete, can exhibit complex architectures. This is the case of the SmQ phase which shows a high degree of 3D order (tetragonal or hexagonal) as well as a liquid signature. We have combined X-ray scattering and optical observations on a new compound exhibiting a SmQ phase to discriminate between the two models proposed by A.M. Levelut et al. (J. Phys. II 7, 981 (1997)) at the molecular level for the phase with the I4₁22 space group.Received: 9 March 2004, Published online: 10 November 2004PACS: 61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures - 61.30.Mp Blue phases and other defect-phases     

Anchoring transitions with polar and non-polar nematic liquid crystals on incompletely oxidized silane substrates

The active oxygen gas arising from a plasma reactor is used to realize progressive chemical modifications onto silane coatings that could be particularly interesting as alignment layers for liquid crystal display applications. Depending on the oxygen density grafted onto the substrate, these alignment layers provide different zenithal anchoring angles, or pretilt angles, with anchoring transitions, for polar and non-polar nematic liquid crystals as 5CB and MBBA, respectively. The anchoring transitions are found to be smoother with the polar nematics. Such a behavior is discussed in terms of the differential wetting model by adding a cosine term to the interaction energy between the nematic and the substrate. A local justification is proposed for this symmetry breaking term. Received: 18 May 1998     

Theory of the incommensurate smectic-A phases in dimesogenic liquid crystals

A mechanism assuming a mismatch between the basic smectic periodicity and the amplitude and phase modulations periods of the mass density wave order parameter is proposed, for describing the incommensurate smectic-A structure found in dimesogenic liquid crystals. The different sequences of phases found in this family of compounds are described theoretically.PACS: 61.30.Cz Molecular and microscopic models and theories of liquid crystal structure - 64.70.Md Transitions in liquid crystals     

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     

Dewetting of polymeric films: unresolved issues

Since the early seminal theoretical work by Brochard and coworkers, and experiments by Reiter over a decade ago, considerable progress has been made toward the development of a comprehensive picture of the equilibrium and dynamic behavior of unstable thin polymeric films. Generally, theoretical work has carefully guided the design of many experiments conducted in this field. Recent experimental findings, however, raise new questions that could probably not have been foreseen by theory and now need to be revisited. In this paper we highlight three problems in two general areas, (1) the use of the effective interfacial potential describing film substrate interactions and (2) the dynamics of dewetting and the associated connection to slip phenomena and fingering instabilities. We suggest that in addition to experiments, analytical theory and simulations will play a critical role toward elucidating the ultimate goal of a universal picture of equilibrium and dynamic behavior of instabilities in thin films.Received: 1 August 2003PACS: 68.15. + e Liquid thin films - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions