INTEMA   05428
INSTITUTO DE INVESTIGACIONES EN CIENCIA Y TECNOLOGIA DE MATERIALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Computational Study of Morphological Instabilities in Directional Nematic Growth
Autor/es:
N. M. ABUKHDEIR; E.R. SOULÉ; A. D. REY
Lugar:
Boston, USA
Reunión:
Conferencia; MRS 2008 fall meeting; 2008
Institución organizadora:
MRS 2008 Fall meeting proceedings
Resumen:
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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.