LIGHT SCATTERING PROM THE FREE SURFACE OF A NEMATIC LIQUID CRYSTAL

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Abstract
We have measured the spectrum of light scattered by surface waves thermally excited on the free surface of a nematic liquid crystal in a magnetic field at several scattering angles. We have fitted the data by the theoretical spectrum that we have derived using the fluctuation-dissipation theorem and the Eriksen-Leslie hydrodynamic theory. For the range of frequencies and scattering angles of interest, the asymetric part of the Leslie viscous stress tensor is shown to be negligible : one is left with 3 independent shear viscosities as for any uniaxial incompressible system. A fourth parameter affecting the spectral shape is the surface tension σ which is supposed isotropic. The theoretical spectrum depends on the orientation of the molecules with respect to the wave vector q of the fluctuations, as we have observed. We present the results obtained for two liquid-crystals : P. A. A. at 122 °C and M. B. B. A. in the temperature range 350 to 52 °C (the transition temperature tc being 45 °C). Our results for P. A. A. are consistent with those obtained previously with more conventional techniques. Our measurements for M. B. B. A. yield the value of the surface tension and of the three viscosities versus temperature. σ exhibits a discontinuity at tc. The viscosity data are compared to those deduced from viscosimetry or ultrasonic methods

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