CONICET | Buscador de Institutos y Recursos Humanos

The morphological stability of the nematic/isotropic interface of 5CB (pentyl-cyanobiphenyl) in a temperature gradient is studied via computational 2-D simulations. A Landau-de Gennes type quadrupolar tensor order parameter model for the first-order isotropic/nematic transition is used. An energy balance, taking anisotropy into account, was derived and incorporated into the time-dependent model. The evolution of a small amplitude sinusoidal interface, representing a planar interface with a small perturbation to the planar shape, was analyzed for different wavelengths of the perturbation and in the presence of different temperature gradients. Dispersion diagrams (growth/shrinkage velocity of the perturbation vs. wavelength), are constructed in the small-amplitude regime and compared with a sharp-interface linear stability theory. It was found that, besides the shape instability, there is a texturing process. When the director is initially homeotropic at the nematic/isotropic interface, a defect forms and then is shed from the interface into the bulk nematic phase, giving rise to a region in the interface where the orientation becomes planar. This can be explained taking into account that the planar orientation is energetically more favorable than the homeotropic orientation. A similar defect formation and shedding mechanism has been observed in simulation of nematic spherulite growth [Wincure, B. M. and Rey, A. D.; Nano Letters, 2007, 7, 1474], due to the presence of strong orientation gradients along the interface. When the director is initially planar and in the same plane that the perturbation, a second nematic region is formed, also with a planar orientation but perpendicular to the plane of the perturbation. This second region is nucleated at the interface and then grows into the bulk. This can be explained considering that the homeotropic orientation parallel to the perturbation implies a distortion in the director as it follows the shape of the interface, while if the orientation is perpendicular to the plane of the perturbation there is no distortion. This type of phenomena has been observed in directional growth experiments [Bechhoefer, J. in Pattern Formation in Liquid Crystals, eds. Buka, A. and Kramer L., Springer, N.Y., 1996], although it was related to an anchoring conflict between the nematic isotropic interface and the plates, while in the present case is related with the distortion energy.

enviar mensaje