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When a liquid crystal is confined between two closely spaced surfaces, the structural forces are exerted on confining surfaces due to the breaking of the translational and orientational order across the liquid crystal-solid interface. This allows for a direct analysis of the orientational and translational order profile across the interface by measuring the separation dependence of the interfacial forces. The structural forces, relevant to the nematic-solid interfaces are considered, such as the pre-nematic, pre-smectic, capillary condensation and the Casimir force. The experimental technique of the Atomic Force Spectroscopy of interfacial forces at the isotropic to nematic phase transition is presented, with sub-nanometer, milli-Kelvin and piconewton precission [1]. The results of our studies of the temperature dependence of interfacial forces of nematic liquid crystals 5CB, 8CB and 12CB on DMOAP-silanated glass and sapphire are presented [2-6]. We have observed that these interfaces are clearly formed of the first molecular layer, which is smectic-like. It is followed by a partially ordered region that shows pre-nematic order in 5CB, pre-smectic order in 8CB and well developed layer-by layer order in 12CB. In many cases capillary condensation of a nematic and even smectic order is observed. In the second part, we present studies of the inter-particle forces in nematic colloids using the laser tweezers. The dispersions of solid particles in the liquid crystal show several novel classes of anisotropic forces between inclusions, which are extremely strong, anisotropic and of long-range. These forces lead to a broad variety of colloidal assemblies in liquid crystals, which cannot be observed in isotropic solvents: colloidal wires, assembled by entangled topological defects, superstructures in the mixtures of large and small colloidal particles and a broad variety of 2D [7] and 3D nematic colloidal crystals. In all cases, the colloidal binding energy is several orders of magnitude stronger compared to water based colloids.
[1] I. Musevic, G. Slak, R. Blinc, Rev.Sci.Instr. 67, 2554(1996). [2] K. Kocevar, A. Borstnik, I. Musevic, S. Zumer, Phys. Rev. Letters 86, 5914(2001). [3] K.Kocevar, R.Blinc, I.Musevic , Phys. Rev. E62, R3055(2000). [4] K.Kocevar, I.Musevic, Phys. Rev. E64, 051711(2001). [5] K.Kocevar, I.Musevic, Phys. Rev. E65, 030703(R)(2002). [6] K.Kocevar, I.Musevic, Liquid Crystals 28, 599(2001). [7] I. Musevic, M. Skarabot, U. Tkalec, M. Ravnik, S. Zumer, Science 313, 954(2006) |
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