Induction of orientational order in the isotropic phase of a nematic liquid crystal

The orientational properties of an isotropic dense liquid composed by anisotropic molecules, such as a liquid crystal in an isotropic phase, is studied. Using a Langevin-like equation it will be shown that the rotational motion of each molecule can be divided in two elements describing two kinds of physical motion. The first describes the Brownian rotational motion and another the coherent rotation induced by the external fields. It will be shown that, even at the isotropic phase, an order parameter describing the mean degree of alignment of the molecules around a given point can be defined. This order parameter also separates the order coming from the coherent motion from the order generated by the anisotropy in the thermal fluctuations. At the end the proposed model is compared with an experiment and it is shown that the coherent motion is enough to explain the experimental results.