Hydrodynamic effects on conformational changes in anisotropic fluids

Abstract

A phenomenological theory is presented for the dynamics of anisotropic fluids in which a discrete number of conformational changes may take place. The method is based on irreversible thermodynamics by treating conformational changes as internal relaxation processes. A second-order, symmetric and traceless tensor is taken as an order parameter describing the macroscopic alignment. This allows for the description of the isotropic and anisotropic phases with the same set of equations. Furthermore, a set of relaxation equations modelling the conformational changes as unimolecular reactions are proposed. In the isotropic phase, the theory reduces to the usual theory of relaxational processes in isotropic fluids. In general, however, the anisotropy of the material allows for a coupling between the rates of conformational change, hydrodynamic flow, and alignment. The effect of this coupling is illustrated for sound waves, where it implies a frequency and orientation dependent contribution to both the sound speed and attenuation. Experimental measurements in several liquid crystals are consistent with these results.