Self-assembly of amphiphiles, polymers and proteins at interfaces between thermotropic liquid crystals and aqueous phases

We demonstrate a variety of ordered patterns, including hexagonal structures and chains, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (LC). The surface placement introduces a new type of particle interaction as compared to particles entirely in the LC bulk. Namely, director deformations caused by the particles lead to distortions of the interface and thus to capillary attraction. The elastic-capillary coupling is strong enough to remain relevant even at the micron-scale when its buoyancy-capillary counterpart becomes irrelevant.

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Ordered droplet structures at the liquid crystal surface and elastic-capillary colloidal interactions.

Through the combination of three different, complementary techniques (optical microscopy, x-ray diffraction and atomic force microscopy), we reveal the deformations inside thin smectic films frustrated between two interfaces imposing antagonistic anchorings. We study the model system, 4-n-octyl-4'-cyanobiphenyl (8CB) between MoS2 and air, which is characterized by the competition between homeotropic anchoring at air and planar unidirectional anchoring on the substrate, with thicknesses varying around 0.3 microm. Optical microscopy and x-ray diffraction demonstrate the continuous topology of smectic layers between the interfaces, which are stacked into periodic flattened hemicylinders. These latter are one-dimensional (1D) focal conic domains which form an optical grating in the smectic film, of a period ranging from 1 to 2.5 microm. The interpretation of our results through an energetic model, associated with the atomic force microscopy (AFM) measurements, shows the presence below a critical thickness of a new type of curvature wall between neighboring hemicylinders.

Optical gratings formed in thin smectic films frustrated on a single crystalline substrate.

The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematic–nematic phase transition at 376 K. The phase below this temperature has been identified...

Twist-bend nematic phase of the liquid crystal dimer CB7CB: orientational order and conical angle determined by 129Xe and 2H NMR spectroscopy

Cholesteric blue phases are liquid crystalline phases in which the constituent rod-like molecules spontaneously form three-dimensional, helical structures. Despite theoretical predictions that they are composed of cylindrical substructures within which the liquid crystal molecules are doubly twisted, real space observation of the arrangement of such structures had not been performed. Through transmission electron microscopy of photopolymerized blue phases with controlled lattice plane orientations, we report real space observation and comparison of the lattice structures of blue phases I and II. The two systems show distinctly different contrasts, reflecting the theoretically predicted, body centred and simple cubic arrangement of the double-twist cylinders. Transmission electron microscopy also reveals different tendencies of the two blue phases to align on unidirectionally rubbed surfaces. We thus show that TEM observation of alignment-controlled, photopolymerized liquid crystals can be a powerful tool to investigate complex liquid crystalline order.

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Double-twist cylinders in liquid crystalline cholesteric blue phases observed by transmission electron microscopy.

Alignment properties of rubbed films of polyvinyl imidazole were studied by observing with the polarizing microscope the features of the nematic liquid crystal (LC) droplets onto these layers. It was found that the observing of the droplets can reveal the direction of easy axis on the alignment substrate not only in the case of LC having the director normal to the LC/air interface but also in the case of the director tangent to this interface, when no disclination line could be seen. We observed that some of the LCs are oriented parallel to the rubbing direction, but the other LCs are perpendicularly oriented. This peculiar behavior is at variance to that found on rubbed layers of polyvinyl alcohol. The role of the end-groups in the structure of the LCs on the alignment properties is discussed.

Peculiar orientation of nematic liquid crystals on rubbed polyvinylimidazole

An analysis of the permitted structures for a cholesteric liquid crystal based on the continuum theory as a function of the anchoring conditions and the thickness of the liquid crystal layer is presented. A new type of bistability is found. The stability domains of various structure types are given.

Stability of some structures in a cholesteric liquid crystal layer for tilted anchoring

An evaluation of the tilt angle variation observed recently when unrubbed polyvinylic alcohol is deposited on obliquely evaporated SiO layers is presented. The method of calculation supposes that t...

Influence of the microtopography of the SiO covered by PVA layers on the tilt angle of liquid crystals

A new method of processing the experimental data in the interference method for the determination of birefringence of liquid crystals which allows obtaining results in good agreement with those from prism or wedge methods, is presented.



On presente un nouveau mode d'interpreter les donnees experimentales par la methode interferometrique de determination de la birefringence des cristaux liquides qui permet d'obtenir des resultats en bonne concordance avec des methodes de la prisme ou du coin.

On the Determination of Birefringence of Liquid Crystals by the Interference Method

We report improvements in the static drop shape method used to determine the temperature dependence of interfacial tension at the liquid crystal-isotropic liquid interface. The drop was formed above a capillary tip in glycerol, the density of which was higher than that of the liquid crystal. Images of the steady drop were recorded with a digital capture system that allowed reading up to 2000 points on the drop contour. The data were processed by an improved interpolation technique to trace the drop profile and a specially written program based on the Runge-Kutta integration technique of specific differential equations. As an example, we investigated an alkylcyanobiphenyl in the nematic and isotropic phases at the interface with glycerol. Upon increasing temperature, an upward jump was observed at the nematic-isotropic transition; anomalous temperature dependences were found on both sides of this transition. The results were compared with those in the literature.

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R. Moldovan

Papers

Peculiar orientation of nematic liquid crystals on rubbed polyvinylimidazole

Stability of some structures in a cholesteric liquid crystal layer for tilted anchoring

Influence of the microtopography of the SiO covered by PVA layers on the tilt angle of liquid crystals

On the Determination of Birefringence of Liquid Crystals by the Interference Method

Improved method for determining the temperature dependence of interfacial tension at liquid crystal-isotropic liquid interface