Films nanométriques de cristaux liquides étudiés par mesure de force SFA et AFM

Des forces structurales sont mesurées pour des cristaux liquides nématiques (CLN) confinés en films nanométriques, à l'aide d'un appareil pour la mesure des forces de surface (SFA). Dans un CLN micellaire, on a étudié la stratification aux parois, en termes de variation de la longueur de corrélation entre la phases calamitique et la discotique. Dans des CLN biphenyls, on a considéré différentes conditions d'ancrage aux parois: homéotrope, planaire et hybride planaire/homéotrope. La configuration résultante est d'abord identifiée par des mesures interferométriques. Les courbes de force sont en suite comparées à la théorie élastique pour les CLN. L'accord est bon dans le cas planaire. La force en géométrie hybride ne s'explique pas par la même théorie, car elle presente une composante attractive inattendue. Des phases smectiques ont été étudiées aussi par AFM, en mode "Spectroscopie de Force". L'épaisseur et la compressibilité des couches smectiques sont mesurés avec une résolution comparable à d'autres techniques moins rapide et plus coûteuses.Using a surface force apparatus (SFA) and an atomic force microscope in "Force Spectroscopy" mode (SP-AFM) we have studied the behaviour under nanometric confinement of nematic and smectic liquid crystals (LC). First, we have considered the confinement-induced layering in a lyotropic LC composed of biaxial micellae, organized in different nematic phases depending on the temperature: calamitic C, biaxial Bx and discotic D. We have characterized the layering in term of the layer thickness and of the strength and the range of the induced order. We observe some variation between the C and D phase. The D-phase deviates more than the C phase from the theoretical behaviour. For strongly birefringent LC, the interferometric method usually employed to measure the film thickness (FECO) is no longer valid. We have developed a numerical approach to identify the LC configuration across the confinement using the FECO data. We have used these data as a starting point to interpretate the force profiles obtained for two nematic biphenyls, subjected to different anchoring conditions: homeotropic, twisted planar and hybrid planar/homeotropic. The force profiles are compared to a model, including the nematic elasticity and the anchoring energy at the surfaces. The agreement is good for the planar samples, if we consider a very high anchoringenergy that is particularly high. For hybrid anchoring conditions, the force does not follow the same kind of model. We do not observe the anchoring transition to a uniform-director configuration, predicted for small thicknesses. We measure a strong attraction for a thickness of about 100 angstroms, probably due to tensor order gradients across the confinement. Using the SP-AFM we have measured the layer thickness and compressibility of two smectic A and C* materials, with a resolution comparable to that of other techniques, more specific but also slower and more expensive

Modeling the optical properties of self-organized arrays of liquid crystal defects

International audienceLocal full Mueller matrix measurements in the Fourier plane of a microscope lens were used to determine the internal anisotropic ordering in periodic linear arrays of smectic liquid crystal defects, known as 'oily streaks'. We propose a single microstructure-dependent model taking into account the anisotropic dielectric function of the liquid crystal that reproduces the smectic layers orientation and organization in the oily streaks. The calculated Mueller matrix elements are compared to the measured data to reveal the anchoring mechanism of the smectic oily streaks on the substrate and evidence the presence of new type of defect arrangement. Beyond the scientific inquiry, the understanding and control of the internal structure of such arrays offer technological opportunities for developing liquid-crystal based sensors and self-assembled nanostructures

Self-organized arrays of dislocations in thin smectic liquid crystal films

International audienceCombining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of smectic liquid crystal 8CB with thickness 100-300 nm confined between air and a rubbed PVA polymer substrate which impose hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations control the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring condition over the entire polymer substrate and whose thickness does not depend on the one of the film. The strength of the planar anchoring appears to be high, larger than 10 −2 J/m 2 , compensating for the high energy cost of creating an additional 2D defect between an horizontal smectic layer and perpendicular ones. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated

Polymer Adhesion: Seeking New Solutions for an Old Problem

Polymer adhesion is ubiquitous in both the natural world and human technology. It is also a complex multiscale phenomenon, such that the solution of adhesion problems requires a convergence of chemistry, physics, and engineering. In this Perspective, we provide an overview of some of the fundamental concepts that have emerged in the field of polymer adhesion, discuss recent work, and identify challenges in three specific areas: (a) theories and simulations, with an emphasis on problems involving chain scission; (b) experimental methods for measuring forces and characterizing interfaces at the molecular scale; and (c) strategies inspired by living organisms to generate underwater adhesion

One-dimensional patterns and topological defects in smectic liquid crystal films

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Surface-frustrated periodic textures of smectic-A liquid crystals on crystalline surfaces

International audienceUsing polarizing optical microscopy we studied thin films and droplets of smectic-A 4-cyano-4(')-n-octylbiphenyl (8CB) liquid crystal deposited in air on crystalline surfaces of muscovite mica that induce monostable planar anchoring. The competition with the homeotropic anchoring at the 8CB-air interface leads to the formation of one-dimensional (1D) patterns composed of straight, parallel defect domains that are organized in periodic arrays over areas as large as several mm(2). We have developed a simple model which identifies the arrays with self-assembled ``oily streaks,'' comprising straight disclination lines and curvature walls. The model reproduces the observed monotonic increase of the period p with the film thickness h in the range p=1-4 mu m and h=0.8-17 mu m. For higher values of h we observed a sharp transition to a 2D lattice of fragmented focal conic domains. Despite the apparent generality of our model for hybrid planar-homeotropic anchoring conditions, periodic arrays of straight oily streaks have been observed so far only for 8CB on crystalline surfaces such as mica or MoS(2). Our model indicates that this specificity is due to a particularly strong anchoring of the liquid crystal on such surfaces

Self-assembly of structural defects and nano-objects in liquid crystal films

Scuola di Dottorato in Scienzae Tecnica Bernardino Telesio, Ciclo XXVII, a.a. 2015-2016Università della Calabri

Twist transitions and force generation in cholesteric liquid crystal films

The response of a cholesteric liquid crystal film to mechanical confinement is theoretically investigated considering planar anchoring conditions on the limiting surfaces and pure twist distortion of the liquid crystal director. We evaluate the total twist angle and normal force acting on the surfaces as a function of the film thickness. Assuming the Rapini-Papoular functional form for the surface anchoring energy, we show that the surface twist angle undergoes discontinuous jumps or continuous transitions as a function of the film thickness and anchoring strength. The transitions take place at well-defined film thicknesses, related to the intrinsic periodicity of the cholesteric liquid crystal, and produce oscillations in the normal force and position of the optical band-gap as the film thickness is varied