Onsager theory
The Onsager theory for the isotropic-nematic phase transition was developed by Lars Onsager (Ref. 1). In a 3-dimensional gas of hard rods there are two contributions to the entropy: a part due to translation and a part due to orientation. These two contributions are coupled.
References
- L. Onsager "THE EFFECTS OF SHAPE ON THE INTERACTION OF COLLOIDAL PARTICLES", Annals of the New York Academy of Sciences 51 pp. 627- (1949)