Mass diffusion measurements in liquid crystals by a novel optical method

Binary mass diffusion measurements 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 anisotropic diffusion 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.