Surface breaking in lyotropic nematic liquid crystals induced by a magnetic field.

The twist deformation induced by a magnetic field H in a lyotropic liquid crystal is analyzed. Our results show the existence of three regimes for the surface deformation related to two critical values for the external magnetic field. The first, for low magnetic field, is an elastic one in which the surface deformation disappears when the field is removed. In a second regime, a permanent deformation of the lyotropic liquid crystal is induced by the distorting field. This regime is characterized by a threshold for H, called H(*)(c). In the third regime, the sample is uniformly oriented along H. Also this regime has a well defined threshold value, H(**)(c), and the alignment remains even when the distorting field is removed. To interpret the experimental data recently published, we proposed a phenomenological model according to which there is an elastic coupling between the surface layer of thickness l and the bulk. This elastic coupling has a saturation for a given value of the deformation of the bulk with respect to the surface layer.