J. Phys. II France
Volume 6, Numéro 6, June 1996
Page(s) 827 - 844
DOI: 10.1051/jp2:1996214
J. Phys. II France 6 (1996) 827-844

The Anchoring of a Cholesteric Liquid Crystal at the Free Surface

R. Meister, H. Dumoulin, M.-A. Hallé and P. Pieranski

Laboratoire de Physique des Solides, Bâtiment 510, Université Paris-Sud, 91405 Orsay Cedex, France

(Received 9 November 1995, revised 25 January 1996, accepted 4 March 1996)

The free surface of a cholesteric oligomer, examined by Atomic Force Microscopy, shows a periodic relief that is in relation to the half helical pitch. A thorough discussion of this relief and of the structure of the interfacial region which result from the interplay between the surface tension and the anchoring and distortion energies shows that the anchoring is strong but finite. Due to the anchoring at the free surface, the cholesteric is perturbed in an interfacial region, whose thickness is of the order of the half pitch. This region can be seen as made of two layers. The upper layer contains an array of defect lines whose spacing is larger than the half helix pitch. In the lower layer the pitch is adjusted to its equilibrium value by a progressive inclination of the helix axis. Such an interface exerts an oblique anchoring with respect to the helix axis.

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