10
Phenomenological Theory
The molecular level mean statistical models introduced above, although simplifying the atomic interactions by mean field modeling of the liquid crystal, actually are still quite complicated, and the calculated thermodynamic parameters are at variance with experimental results. The problems lay with the oversimplification of the molecular structure and the neglect of intermolecular interactions, but further consideration of a more accurate molecular model and the intermolecule interaction would bring hopeless complications. Thus the French physicist Pierre-Gilles de Gennes finessed the complexities with a more direct and practical approach to the investigation of liquid crystals.
The phenomenological approach to the phase transitions is independent of the details of molecular structure and molecular interaction, and as such can be said to include the short-range as well as long-range interactions by a “synthesis” of the experimental observations with their concomitant thermodynamic parameters through the employment of coefficients that are adjustable within the confines of the logic of physics, such as symmetry and self-consistency. After accordingly formulating and fitting the coefficients arising from the synthesis, as will be seen, an elegant description of the liquid crystal nematic to isotropic phase transition and dielectric anisotropy is the result.
The de Gennes phenomenological theory, based as it is on the symmetry inherent in long structures, ...
