The role of defects and boundary and volume inhomogeneities in the field-induced director reorientation in nematic liquid crystals confined between parallel plates

A deep understanding of the director dynamics in nematic liquid crystals confined between parallel plates is important for display technology as well as for basic science. Some aspects of this dynamics are still not clear, in spite of intense research and significant progress in the last thirty years. When a strong magnetic (or electric, with some precautions) field is applied to a well aligned nematic sample, depending on the angle between the director and the field one usually observes either a homogeneous or a spatially periodic (inhomogeneous) director reorientation to the new equilibrium position, parallel to the field, as shown by NMR observations well supported by the Leslie-Ericksen theory [A.F. Martins, P. Esnault and F. Volino, Phys. Rev. Lett. 57, 1745 (1986)]. These are collective modes (observed simultaneously over the whole sample) and the spatially periodic one originates in the long wavelength thermal fluctuations of the director orientation in equilibrium. Recent experiments [G.R. Luckhurst, A. Sugimura, B.A. Timimi and H. Zimmermann, Liquid Crystals 32, 1389 (2005)] have shown that in some practical cases the director reorientation in thin nematic samples confined between two parallel plates does not follows any of those two pictures. We focus on such experiments and present 2D computer simulations of the director dynamics in the presence of topological defects or boundary or volume inhomogeneities in the sample. Deuterium NMR spectra directly computed from the numerical results show good agreement with the experimental data. We propose that in addition to the above mentioned collective modes, a qualitatively different mode of reorientation, that we may call progressive mode, exists and may dominate the director reorientation in thin samples of low molecular mass liquid crystals