The interfacial tension γeq between nematic and isotropic phases of some cyanobiphenyls has been measured by using the sessile drop method. Values of γeq ranging from 0.7×10−2 to 1.8×10−2 erg/cm2 are obtained. In a recent paper we measured the optical reflectivity coefficients at the nematic–isotropic interface of 7CB. By using these coefficients and the measured value of γeq we can obtain the experimental value of the intrinsic thickness of the nematic–isotropic interface of 7CB. This thickness is compared with the one predicted by the Landau–De Gennes theory. A satisfactory agreement between experimental and theoretical values is found.

2014 Une nouvelle technique de mesure de la constante élastique de torsion des cristaux liquides nématiques est proposée. Cette technique est basée sur une mesure directe du couple exercé par le directeur sur les surfaces quand le nématique est soumis à un champ magnétique. Cette technique est appliquée à la mesure de la constante élastique K22 du cristal liquide 4-n-pentyl-4'-cyanobiphényl (5CB). Abstract 2014 A new experimental method to measure the twist elastic constant of nematic liquid crystals is proposed. The method is based on a direct measurement of the torque which the nematic liquid crystal exerts on the surfaces when a magnetic field is applied. The reliability of this technique is tested by measuring the twist elastic constant of the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). The experimental results are compared with previous measurements performed by other authors.

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2014 Nous étudions par réflectométrie optique les propriétés de l'interface nématique-isotrope du cristal liquide 7CB. Nous mesurons l'épaisseur de l'interface, l'angle polaire du directeur à l'interface et son énergie d'ancrage. Abstract 2014 The properties of the nematic-isotropic interface of the nematic liquid crystal 7CB are studied by optical reflectometry. It is found that at the interface the director is tilted with respect to the vertical axis. Measurements are reported of the tilt angle 03B8t, of the thickness of the interface, and of the anchoring energy.