J. Phys. II France
Volume 5, Numéro 9, September 1995
Page(s) 1297 - 1320
DOI: 10.1051/jp2:1995185
J. Phys. II France 5 (1995) 1297-1320

Undulation Instability of the Interface Between a Smectic-C Liquid Crystal and its Isotropic or Nematic Melt

J.B. Fournier and P. Galatola

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

(Received 22 February 1995, revised 18 May 1995, accepted 12 June 1995)

The spontaneous undulation instability of phase boundaries of smectics C is investigated both theoretically and experimentally. The driving mechanism is more efficient than that of Herring's instability of crystal surfaces, due to the coupling of a periodic distortion of the c-director with the interfacial undulation. The instability is studied experimentally in various geometries: free drops, capillaries and sandwiched cells. In the presence of a thermal gradient G, the undulation period is shown to scale as G-1/3, as predicted theoretically. The existence of a threshold reflattening the interface in the presence of strong thermal gradients is investigated. Upon varying the thickness of the smectic, a sequence of instabilities occurs which produces first the interfacial undulations, then focal conic defects as in the textural instability of the smectic-A-isotropic interface. In free drops, the undulation instability yields a richer phenomenology: arborescent patterns, splayed islands, spirals, trefoils, and eventually rows and triangular networks of focal conics.

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