Biaxial Nematics

Abstract

The biaxial nematic phases are completely anisotropic liquids which present translational disorder like the ordinary liquids, but with the molecules statistically oriented along the three directions of space. They appear to be the generalisation of the uniaxial nematics, and may be understood as having a second (and then a third) director, the three directors being perpendicular to one another. The biaxial nematics have been experimentally discovered only recently in lyotropic mixtures (Saupe, 1980), and in thermotropic and polymer compounds (1986). They thus have been the subject of few theoretical and experimental studies. However we can mention some interesting physical properties already evidenced: —The disclination lines in the biaxial nematics are observed to make spontaneous zig-zags with a definite angle. (The explanation is that the order on the defect lines is lowered to the uniaxial order, making the lines to bear a director which couples to the biaxial bulk, and orients to a preferred angle if referred to it). —The phase transition between the uniaxial and biaxial nematic phases is second order (because this transition may be reduced to the 2D-plane perpendicular to the uniaxial director). This allows in principle, one to approach the phase transition as close as possible. Different experimental means, like light scattering and light interferences, have been employed to study this particular transition. —Dissolving a small amount of a chiral compound in a biaxial nematic has led one to obtain the biaxial cholesteric phase. This phase presents only one cholesteric pitch, with its cholesteric axis along a director while the other chiral axes remain frustrated. —Indirect informations on the shape of the micelles and the structures they build. have been obtained in the lyotropic nematic phases by means of high resolution X-ray measurements associated to light scattering experiments. These short insights give some ideas on the physical properties of the biaxial nematic phase. They also show that this domain of the Liquid Crystals is not closed yet, and that progresses are expected for the next years.