Surface and smectic layering transitions in binary mixtures of parallel hard rods

12 pages, 13 figures.-- PACS nrs.: 64.70.M-, 61.30.Hn, 61.20.Gy, 64.75.Xc.-- ArXiv pre-print available at: http://arxiv.org/abs/1002.0512The surface phase behavior of binary mixtures of colloidal hard rods in contact with a solid substrate (hard wall) is studied, with special emphasis on the region of the phase diagram that includes the smectic A phase. The colloidal rods are modeled as hard cylinders of the same diameter and different lengths, in the approximation of perfect alignment. A fundamental-measure density functional is used to obtain equilibrium density profiles and thermodynamic properties such as surface tensions and adsorption coefficients. The bulk phase diagram exhibits nematic-smectic and smectic-smectic demixing, with smectic phases having different compositions; in some cases they are microfractionated. The calculated surface phase diagram of the wall-nematic interface shows a very rich phase behavior, including layering transitions and complete wetting at high pressures, whereby an infinitely thick smectic film grows at the wall via an infinite sequence of stepwise first-order layering transitions. For lower pressures complete wetting also obtains, but here the smectic film grows in a continuous fashion. Finally, at very low pressures, the wall-nematic interface exhibits critical adsorption by the smectic phase, due to the second-order character of the bulk nematic-smectic transition.We gratefully acknowledge financial support from Comunidad Autónoma de Madrid (Spain) under grants NANOFLUID, MOSAICO, and Grant No. S-0505/ESP-0299. This work was partly financed by Grant Nos. FIS2005-05243-C02-01, FIS2007-65869-C03-01, FIS2008-05865-C02-02, and FIS2007-65869-C03-C01 from Ministerio de Educación y Ciencia (Spain).Publicad