2014 On a construit un appareil pour mesurer les viscosités de cisaillement. ~1, ~2, ~3 et ~12 de Miesowicz. Celles-ci sont déterminées lors de l'ecoulement d'un liquide nématique dans un capillaire rectangulaire qui fait partie d'un appareil thermostatisé. L'ensemble est situé entre les pôles d'un électro-aimant pour contrôler le directeur n. De plus, on a déterminé l'angle d'alignement par écoulement en mesurant la variation de la biréfringence dans l'échantillon entre la situation où n est fixé parallèle à l'écoulement et celle où il est aligné par l'écoulement. On donne des résultats pour des ensembles complets de coefficients de viscosité de p-methoxy-p'-butylazoxybenzène et de p, p'-dibutylazoxybenzene. Les résultats sont bien en accord avec des rapports visco-élastiques obtenus par des techniques de diffusion de la lumière. La relation d'Onsager-Parodi est bien vérifiée compte tenu de la précision expérimentale. Abstract. 2014 To determine the Miesowicz viscosity coefficients a shear-flow setup has been built, which allows the measurement of ~1, ~2, ~3 and ~12. These coefficients are determined by pressing the liquid crystal through a rectangular capillary which forms part of a thermostated setup placed between the poles of a magnet in order to control the director n. In addition, the flow-alignment angle 03B80 is determined from the change in optical path difference between the ordinary and extraordinary components of polarized light, comparing the situation with n aligned along the flow-direction and with flow-alignment, respectively. Results are given for the complete set of viscosity coefficients of p-methoxy-p'-butylazoxybenzene and p, p'-dibutylazoxybenzene. The results show good agreement with visco-elastic ratios found with light-scattering techniques. Within the experimental accuracy the Onsager-Parodi relation is well fulfilled. Tome 44 N° 2 FEVRIER 1983
The static and dynamic behavior of the Frederiks transition for twist of a uniform planar nematic sample in an external magnetic field has been studied by monitoring the associated conoscopic figure. In particular, the decay of this figure after the field has been switched off is considered in detail in order to obtain results for the twist viscosity γ1. Results for the twist elastic constant K2 and for the viscosity γ1 are reported for the homologous series of p, p′-dialkylazoxybenzenes. K2 is found to be smaller than both K1 and K3, while K2 varies little along the series. The temperature dependence of γ1 can be described by a combination of an activated process and a term depending on the orientational order parameter.
A setup is described to determine the flow alignment angle () 0 in nematic liquid crystals. Results for compounds with and without a strongly polar terminal cyano group indicate that the antiparallel dipole correlation associated with cyano groups has no influence on ()o. The influence of the occurence ofa smectic-A phase on ()o is investigated on three homologous series. The results show that () 0 is a very sensitive probe for smectic-like short-range order. With increasing chain lengths a decrease of ()o is found, until finally no flow alignment exists anymore. Interestingly, a similar behavior is found in a homologous series without smectic phases.
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