Orientational diffusivities measured by Rayleigh scattering in a lyotropiccalamitic nematic (N-C) liquid crystal phase: the backflow problem revisited

Using a light-beating technique we have measured the damping time of therma l fluctuations of the nematic director fur the so called cylindrical or cal amitic nematic (N-C) phase of the lyotropic system K-laurate/decanol/D2O. B y varying the scattering angle in suitable geometries we have been able to estimate the orientational diffusivities associated to the three pure defor mations of splay, twist and bend. A former measurement made in the disk-lik e N-D phase of the same system yielded a large deviation between the splay and twist diffusivities. The effect was then attributed to induced flows, o r backflow, which could be responsible for the reduction of the splay visco sity. In fact, this is the analogous effect, for disks? to the one recogniz ed since long time ago arriving for rod-like molecules in a classical nemat ic, though in this case it is associated with bend deformations. The analog y comes about thanks to the interchange of thr rule played by disks and cyl inders for, respectively, splay and Lend fluctuations. The measurements rep orted here provide a new test on the applicability of the backflow model to a nematic system composed of micelles, that is, aggregates made of amphiph ilic (surfactant) molecules, in its cylindrical-like variant, i.e. the Nc p hase. In addition, the comparative study made here with the previous result s existing in the literature for the No phase, allows us to conjecture on s tructural issues concerning lyotropic nematics.