Rayleigh Scattering of Light by Liquid Crystals

Abstract

Liquid crystals are rather peculiar compounds constituted of long shaped organic molecules. Between the isotropic and the crystalline states they show intermediate phases called nematic, smectic-A, smectic-C. In all these “mesophases” the rod-like molecules are arranged with a long range orientational order. As early as 1937 Chatelain¹ observed a remarkable scattering of light by the nematic phase: the scattered intensity was typically 10⁴ times larger than in the isotropic phase and the light was strongly depolarized. To explain this observation the old concept of “swarms” was used. This concept was abandoned only thirty years later with the interpretation of de Gennes²: the scattering properties of nematics are in fact due to orientational fluctuations of the molecules. These fluctuations, excited by thermal energy, are controlled by the elastic properties of the nematic. In other “mesophases” a different elasticity gives rise to other kind of orientational fluctuations: thus can be understood a reduced scattering for smectics-A and an intense scattering again for smectics-C. Here I will first present briefly the most common phases of liquid crystals and their elastic properties. The readers interested in a more detailed description should refer to reference 3. I will then distinguish the various types of orientational fluctuations responsible for light scattering. The change from one type to the other can be continuous near second order or weakly first order phase transitions: thus the more fundamental problem of phase transitions can be tackled in the last section.