Thermal conductivity measurements have been performed in nematic and smectic liquid crystalline phases by the forced Rayleigh light-scattering technique. Results show clearly that the thermal transport anisotropy is governed by the molecular shape anisotropy and by the molecular orientation but is independent of the smectic layer ordering.

Thin films (0,2 – 10μm) of a nematic liquid crystal NLC: 4-4′-pentylcyanobiphenyl (5CB) have been spread on large surfaces of water.

The orientation of the molecules has been examined between crossed polarizer und analyzer und by studying the average retardation of white light as a function of film thickness. Ath _T ≌ 2.2μm a Frederiksz transition takes place owing to the balance of elastic and surface forces acting on the film and determining the molecular orientations in bulk at the surface as a function of films thickness. From this variation andh _T , an anisotropy of surface tension of 5CB of the order of 10⁻⁶ Jm⁻² is found. This weak anisotropy determines also the variation of the structural disjoining pressure und the film stability which is discussed at length. A thermodynamic potential for this films is suggested.

L'étude de la structure d'un film mince de cristal liquide nématique (le 4-4'-pentyl-cyanobiphenyl) sur l'eau permet de déterminer l'orientation des molécules dans le volume et aux interfaces et d'en déduire la variationδy =y _∥ -y _I de la tension superficielle selon que les molécules seraient orientécs parallélement (y ∥) ou perpendiculairement (y _I) à la surface libre.

De l'emistence et de la stabilité de trous dans le film mince, on déduit Fordre de grandeur de la tension de ligneϰ. Cette tension de ligne est comparée aux tensions de ligne de déformation élastique du cristal liquide.

A sensitive optical grating technique has been developed for flash photolysis experiments. The sample is excited by an optical fringe pattern obtained by interfering two light waves derived from a pulsed high power laser. Due to the photochromatic modifications within the sample, a spatially periodic refractive index and/or absorption coefficient distribution is set up, producing a phase and/or amplitude grating respectively. This grating is easily observed through the Bragg diffraction of a second, low intensity, continuous laser beam. The relaxation time of this grating is directly related to the intramolecular relaxation time of the photoexcited states. Preliminary experiments on the cis-trans photoisomerism of an azo dye shows that optical density changes of less than 10⁻⁴ can be detected, below the limit of resolution with classical transmittance methods.

Binary mass diffusionmeasurements have been performed in fluid media doped with photochromic dye molecules. A periodic concentration of tagged molecules is created by illuminating the sample with a fringe intensity pattern. The photo induced change of refractive index and/or absorption of the dye molecules creates an optical grating which is observed through Bragg diffraction of an auxiliary laser beam. When the flash excitation is switched off, this grating will relax since photoexcited molecules will diffuse into nonphotoexcited zones and vice versa. The main advantages of this method are that: (1) the duration of the experiment is strongly reduced compared to classical tracer methods since diffusion lengths, defined by the fringe spacing, are small (1–100 μm); (2) the possibility of studying anisotropicdiffusion is readily available; and (3) small sample volumes are required. Results are reported on the diffusion of methyl red in a homogeneously aligned sample of MBBA. In the nematic phase, the diffusion is faster along the local optical axis than perpendicular, the anisotropic ratio D _∥/D _⊥ being 1.6±0.1 at 22 °C, in good agreement with previous tracer data. The temperature dependence yields an activation energy of 5.8±0.7 kcal/mole for D _∥ and 6.0±0.8 kcal/mole for D _⊥. In the isotropic phase, the activation energy is found to be 10±1.5 kcal/mole.