Biaxial nematic and smectic-A boundaries in thin planar samples of 8OCB aligned by rubbed polyimide *
Centre de Recherches Paul Pascal - Av. A. Schweitzer,
F-33600 Pessac, France
Accepted: 2 September 1996
The birefringence of planar 8OCB samples has been measured as a function of temperature at thicknesses decreasing from 7.2 to . The first curve () is perfectly superimposable to the result of conoscopic measurements done with a homeotropic uniaxial sample, from a minimum of 0.08 close to the isotropic phase up to 0.19 in the smectic-A phase. For thinner planar samples, one gets an increasing birefringence (up to 35% more in both phases with a new plateau at 0.26). This means that the bulk uniaxial phase is surrounded by biaxial boundaries (a few thick) where the extreme indices of refraction lie parallel to the plates. A mean-field model of long-range interaction is proposed that takes into account the observed features.
PACS: 61.30.-v – Liquid crystals / 68.45.Gd – Wetting / 78.20.Fm – Birefringence (including stress birefringence, flow birefringence, etc.)
© EDP Sciences, 1996