Collective Phenomena in Liquids: Liquid Crystals and Superfluidity

Abstract

A crystal melts when the thermal energy which tends to create disorder, becomes greater than the intermolecular interaction energy which stabilizes the periodic structure of the crystal, ensuring its cohesion. The structure is then broken up and the molecular positional order is destroyed. The molecules are then free to move randomly. On the contrary, if the molecules are rod-shaped, a different process can be observed. At a certain temperature (melting point of the crystal), the thermal energy can be sufficient to destroy the positional order, but still insufficient to oppose the intermolecular forces responsible for the orientational order. A mesomorphic phase is then obtained: the molecules can conserve a preferred orientation within the liquid. Finally, at a higher temperature (melting point of the mesomorphic phase), this ordered liquid phase will give rise to the isotropic phase of the normal liquid, when the thermal energy will be sufficient to overcome the contribution of the potential energy which favors alignment of the molecules. In crystals formed from quasispherical molecules, each molecule occupies a well-defined place in the lattice; the centers of gravity of the molecules are located in a three-dimensional periodic lattice — the molecules have positional order. In a crystal, rod-shaped molecules also have positional order, but they also have the same direction at any point; there is in addition orientational order. This positional order breaking induces the appearance of the liquid crystal phase.