Springer LINK: The European Physical Journal B - Abstract Volume 5 Issue 1 (1998) pp 99-110

The European Physical Journal B

ISSN: 1434-6028 (printed version)
ISSN: 1434-6036 (electronic version)

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Abstract Volume 5 Issue 1 (1998) pp 99-110

Phases hexagonales colonnaires thermotrope et lyotrope : défauts observés par cryofracture et caractère anomal du thermotrope

B. Yahiaoui (1), A. Gharbi (1), M. Kléman (2) (a), J. Malthête (3)

(1) Laboratoire des Cristaux Liquides et des Polymères, Département de Physique Faculté des Sciences de Tunis, 1060 le Belvédère, Tunisie
(2) Laboratoire de Minéralogie-Cristallographie (Laboratoire associé au CNRS (UMR 7509)), Universités de Paris VI et Paris VII, Tour 16, 4 place Jussieu, Case 115, 75252 Paris Cedex 5, France
(3) Section de Recherche, Institut Curie (UMR CNRS 168), 11 rue Pierre et Marie Curie, 75231, Paris Cedex 05, France

Reçu : 26 mai 1997 / Révisé : 20 Janvier 1998 / Accepté : 27 avril 1998

Abstract: Electron microscopy observations of replicas of freeze-fractured samples of two columnar hexagonal phases of different nature (a lyotropic one, the inverse AOT in water; a thermotropic one, $\rm C_{8}HET$) yield very different results: most defects at microscopic scales are screw dislocations in the lyotropic phase, longitudinal edge dislocations in the thermotropic phase. A possible way to interpret these differences is as follows: in the lyotropic the Lamé coefficients $\lambda$ and µ and the bend modulus K₃ would not display any anomaly compared to expected values; in the thermotropic the shear modulus µ would be ten times smaller than the compressibility modulus $\lambda$, while K₃ would still be comparable to (but larger than) the bend modulus of a small molecules liquid crystal. We present an elementary theoretical model of the latter case which could explain the anomalous measurements of K₃ and of the longitudinal compressibility $ B_{\parallel}$ (Ref. [#!ref10!#]) without contradicting more recent measurements of (Refs. [#!ref17!#,#!ref22!#]). Essentially, the $\rm C_{8}HET$ hexagonal phase would be a phase with defects (longitudinal dislocations) akin to an hexatic phase but with some differences.

PACS. 61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures - 61.30.Jf Defects in liquid crystals

(a) email: kleman@lmcp.jussieu.fr

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