Transitions in the orientational order of liquid crystals induced by periodic patterned substrates I. H. Bechtold, 1 F. Batalioto, 2 L. T. Thieghi, 2 B. S. L. Honda, 2 M. Pojar, 2 J. Schoenmaker, 2 A. D. Santos, 2 V. Zucolotto, 3 D. T. Balogh, 3 O. N. Oliveira, Jr., 3 and E. A. Oliveira 2 1 Departamento de Física, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, Brazil 2 Instituto de Física, Universidade de São Paulo, P.O. Box 66318, 05315-970 São Paulo-SP, Brazil 3 Instituto de Física de São Carlos, Universidade de São Paulo, P.O. Box 369, 13560-970 São Carlos-SP, Brazil Received 16 March 2006; published 31 August 2006 The orientational order of liquid crystals LCs induced by periodic patterned substrates has been investi- gated with cells coated by azopolymer films that could be photoaligned in a controlled way. Two regimes were observed depending on the period of the patterns: i above 3.0 m the LC follows the direction imposed by the patterned substrate since the energy stored in the surface potential minimizes the elastic energy of the LC medium. ii For periods smaller than 1.0 m a homogeneous in-plane state was induced and the LC did not follow the orientation imposed by the surface. This in-plane transition could be explained qualitatively by a theoretical model based on the competition between the Frank-Oseen elastic energy and the phenomenological surface potential. The results also suggest an out-of-plane transition for the LC director as the period was reduced. These results agree with data in the literature for patterned substrates with completely distinct archi- tectures. This indicates that for a particular LC sample the overall behavior depends basically on the texture period instead of the texture architecture. The textures were characterized with a scanning near-field optical microscope SNOM, which allowed simultaneous morphological and optical images in the submicrometer range. DOI: 10.1103/PhysRevE.74.021714 PACS numbers: 61.30.Hn, 61.30.Dk, 61.41.+e, 68.37.Uv I. INTRODUCTION The development of experimental techniques to control liquid crystal LC alignment in the submicrometer scale has become of special interest in many technological areas, such as photonics and optoelectronics. Nanorubbing of polymers with an atomic force microscope AFM tip provides local alignment 1, which may produce surface patterns with neighboring orthogonal square domains and surface bistabil- ity 2. For example, the reduction in periodicity of the tex- ture to less than 0.8 m induced a uniform configuration of the LC director with a large pretilt of approximately 40° 2. This transition was theoretically investigated using a model based on the competition between the Frank-Oseen elastic energy and the phenomenological surface potential, which exhibited good agreement with the experiment 3. Polymers bearing azogroups azopolymers have also been used to in- duce LC alignment, by exploiting the photoalignment of azo- groups with linearly polarized light. Owing to a series of trans-cis-trans photoisomerization cycles, the azodyes tend to align perpendicularly to the polarization direction, thus creating a film anisotropy that is ultimately responsible for the orientation of the LC molecules. The magnitude of such interaction the anchoring energy depends on the concentra- tion of the chromophores in the film and on the irradiation time 4. A further use for LC alignment is with surface relief gratings SRGs photoinscribed on azopolymers, where the LC molecules tend to align along the grooves of the SRGs due to minimization of the elastic distortions close to the surface 5,6. Such photoinduced methods are considered “clean,” i.e., there is no mechanical contact with the surface during preparation. In addition, they are reversible and the anchoring energy can be easily controlled. In an earlier work, the preferred LC configuration for the director induced by flat periodic patterned substrates with different aligning directions was theoretically obtained. We observed that in the limit of large periods, this configuration depended on the aligning directions of the individual stripes and the ratio of the elastic constants K 1 / K 3 7. This model also predicts an orientational in-plane transition to a homo- geneous state as the period of the pattern is reduced. Here the in-plane transitions for the configuration described in Ref. 7 are experimentally and theoretically investigated, consid- ering the system initially in state II   ±60°  and reducing the period. The patterns have been recorded on polymers containing azogroups covalently attached to the main chain, by illuminating the film with a periodic pattern of construc- tive and destructive interference fringes. This resulted in al- ternating patterns with different orientation directions in the microsize domain. We changed the period of the patterns down to the submicrometer range for a fixed irradiation time. Such substrates were optically and morphologically charac- terized with a scanning near-field optical microscope SNOM. II. THEORETICAL MODEL Consider a nematic LC sample of thickness d bounded at both sides at z =0 and z = d by two identical, aligned pat- terned substrates, as depicted in Fig. 1. Due to this special FIG. 1. Schematic representation of the pattern periodic micro- textured substrate used in this work. PHYSICAL REVIEW E 74, 021714 2006 1539-3755/2006/742/0217146 ©2006 The American Physical Society 021714-1