Photoacoustics Applied to Liquid Crystals

Abstract

Liquid crystals are composed of organic molecules of asymmetric shape which do not melt in a single stage from the crystalline state to an isotropic liquid.1,2 For the most common type of liquid crystals the shape anisotropy is prolate. In this case the liquid crystalline phases are characterized by orientational order of the long molecular axes of the rod-like molecules. A variety of mesophases with symmetries and properties intermediate between those of a crystal and a normal isotropic liquid can be present. Differences in the orientational and spatial ordering of the molecules define the mesophases. The nematic (N) phase has the translational symmetry of a fluid but a broken rotational symmetry characterized by long-range orientational order produced by the alignment of the long molecular axes along a unit vector called the director. In the nematic phase the centers of mass of the molecules are, however, still randomly distributed. This is not the case anymore for the many different kinds of smectic phases which show layered structures described by a one-dimensional density modulation.3 One of the most common smectic phases is the smectic A phase which has a layered structure with layer planes perpendicular to the director. Within the layers there is no long-range order in the position of the centers of mass of the molecules. In the related smectic C phase there exists a tilt angle between the direction of the normal to the layers and the director. Here also in the layers there is no long-range positional order of the centers of mass of the molecules. For other types of smectics, however, positional order within the layers can be present.3