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.

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.

Surface tension of some liquid crystals in the cyanobiphenyl series

On the Dielectric Behavior of Unaligned Samples of 4‐n‐Octyl‐4′‐cyanobiphenyl (8CB)

Optical birefringence measurements for a calamitic lyotropic liquid crystal containing sodium lauryl sulphate/decanol/water are presented. A value of the order of 10-3 was obtained, much smaller than that of thermotropic liquid crystals. The order parameter temperature dependence is evaluated using an extrapolating method and taking into accounts the influence of the micelle shape. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

Experimental results for the temperature dependence of the density of propyl – cyanobiphenyl (3CB), butyl – cyanobiphenyl (4CB) and hexyl – cyanobiphenyl (6CB) are presented. The results are compared with previous results for temperature dependence of other members of the alkyl cyanobiphenyl series (nCB). The deviation of the density from linear temperature dependence is discussed in terms of the recently discovered corresponding rule for nematic liquid crystals. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

A new method for producing tilt angle in liquid crystal cells is proposed. The method requires a SiO vacuum deposition layer which is subsequently covered with a polyvinyl alcohol (PVA) layer, obtained by dipping in a solution. It was possible to obtain tilt angles between 7 and 30° by changing the concentration of PVA in the solution.

R. Moldovan

Papers

Surface tension of some liquid crystals in the cyanobiphenyl series

On the Dielectric Behavior of Unaligned Samples of 4‐n‐Octyl‐4′‐cyanobiphenyl (8CB)

Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

Temperature dependence of the density of some liquid crystals in the alkyl cyanobiphenyl series

Orientational properties of PVA coated SiO films