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Journal ArticleDOI

Surface tension of some liquid crystals in the cyanobiphenyl series

01 May 2001-Liquid Crystals (Taylor & Francis Group)-Vol. 28, Iss: 5, pp 793-797
TL;DR: In this article, the pendant drop method was used to measure the surface tension of alkylcyanobiphenyls (nCBs) in the nematic liquid crystal and isotropic phases.
Abstract: 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.
Citations
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Journal ArticleDOI
TL;DR: In this paper, the authors describe the emergence of a set of experimental observations that revolve around the ordering of liquid crystals at interfaces to immiscible aqueous phases, including the self-assembly of surfactants, lipids, proteins and synthetic polymers at these interfaces.

158 citations

Journal ArticleDOI
TL;DR: In this article, a variety of ordered structures, including hexagonal structures and chains, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (LC).
Abstract: 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.

145 citations

Journal ArticleDOI
TL;DR: The interpretation of the 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.
Abstract: 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.

64 citations

Journal ArticleDOI
TL;DR: In this paper, the authors derived a supplemental evolution equation for an interface between the nematic and isotropic phases of a liquid crystal when flow is neglected, based on the notion of configurational force.
Abstract: We derive a supplemental evolution equation for an interface between the nematic and isotropic phases of a liquid crystal when flow is neglected Our approach is based on the notion of configurational force As an application, we study the behavior of a spherical isotropic drop surrounded by a radially oriented nematic phase: our supplemental evolution equation then reduces to a simple ordinary differential equation admitting a closed-form solution In addition to describing many features of isotropic-to-nematic phase transitions, this simplified model yields insight concerning the occurrence and stability of isotropic cores for hedgehog defects in liquid crystals

56 citations

Journal ArticleDOI
TL;DR: The model indicates that periodic arrays of straight oily streaks have been observed so far only for 8CB on crystalline surfaces such as mica or MoS2 due to a particularly strong anchoring of the liquid crystal on such surfaces.
Abstract: Using 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 microm and h=0.8-17 microm. 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 MoS2 . Our model indicates that this specificity is due to a particularly strong anchoring of the liquid crystal on such surfaces.

54 citations

References
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Journal ArticleDOI
TL;DR: In this paper, a simple phenomenological theory is outlined, based on the hypothesis that this excess order resides solely in the degree of alignment of the molecules, the conventional nematic order parameter being larger near the surface than in the bulk.
Abstract: The surface tension γ has been measured for two cyanobiphenyls (5CB and 8CB) at temperatures near the nematic-isotropic transition temperature T c, using the Wilhelmy plate method. Reproducible results were obtained, but only by the exercise of considerable patience: for reasons that are discussed at the end of the paper, it took up to four days for equilibrium to be re-established whenever the temperature was changed. The results suggest that in both materials the equilibrium value of γ increases discontinuously (by approximately 0·3 dyne cm−1) on heating through T c, and that (dγ/dT) is positive both for the isotropic phase just above T c and for the nematic phase just below it. A positive value of (dγ/dT) implies excess order in the surface layers. A simple phenomenological theory is outlined, based on the hypothesis that this excess order resides solely in the degree of alignment of the molecules, the conventional nematic order parameter being larger near the surface than in the bulk. The pre...

126 citations

Journal ArticleDOI
TL;DR: In this article, the free surface of a homologous series of low molecular weight liquid crystals (nCB, n=5,6,7,8) is studied by reflection ellipsometry.
Abstract: The free surface of a homologous series of low molecular weight liquid crystals 4‐cyano‐4′‐n‐alkyl‐biphenyls (nCB, n=5,6,7,8) is studied by reflection ellipsometry. On approaching the bulk isotropic–nematic transition temperature TNI from the isotropic phase, the growth of a nematic layer at the surface is observed. The director orientation in the layer is identified as homeotropic. The ellipsometric data are analyzed in terms of a perturbation calculation. It is shown that for 7CB and 8CB very close to TNI, where the layer has a thickness d≥50 A, both d and the order parameter S0 at the surface can be determined. In all other cases only one parameter, which corresponds to the nematic coverage, can be extracted from the data. Results are discussed in terms of a wetting process. Partial wetting by the nematic phase is observed for n=6,7,8. On approaching TNI from lower temperatures where the bulk is in the nematic phase a small increase of excess surface order is measured which is interpreted as a homeotropic nematic wetting layer possessing higher order than the nematic bulk.

50 citations

Journal ArticleDOI
TL;DR: In this article, a drop of liquid crystal is suspended at the tip of a capillary tube inside a completely enclosed thermostatic chamber filled with an inert gas and is maintained in equilibrium with its saturated vapour throughout the experiment.
Abstract: Equilibrium measurements of surface tension have been carried out on nematic compounds using the pendant drop method. A drop of the liquid crystal is suspended at the tip of a capillary tube inside a completely enclosed thermostatic chamber filled with an inert gas and is maintained in equilibrium with its saturated vapour throughout the experiment. The surface tensiontemperature characteristic for all the compounds studied show anomalous behaviour. The slope in the nematic phase is initially negative but changes sign near the nematic-isotropic transition for p-azoxyanisole, p-anisaldazine and p-methoxy benzylidene-p'-n-butylaniline, whereas for p-azoxyphenetole the change of slope occurs in the isotropic phase. The original monotonically decreasing trend is regained in all the cases at higher temperatures in the isotropic phase. The data on p-azoxyanisole, p-azoxyphenetole and p-anisaldazine confirm the earlier measurements of Ferguson and Kennedy1 on the same compounds. The observed features ar...

46 citations

Journal ArticleDOI
03 Jul 1998-Langmuir
TL;DR: In this article, high-resolution surface tension measurements have been carried out on the free-standing films of more than 30 selected liquid-crystal compounds in their smectic phases.
Abstract: High-resolution surface tension measurements have been carried out on the free-standing films of more than 30 selected liquid-crystal compounds in their smectic phases. Surface tension (γ) values falling into major categories of 27, 24, 21, 13, and 11.5 dyn/cm have been obtained. The dependence of γ on specific terminal groups or molecular structures will be discussed. Our results provide important information about the molecular origin of surface tension.

42 citations