Capillary forces in a confined isotropic-nematic liquid crystal.

We have investigated nematic capillary condensation in the isotropic phase of nematic liquid crystals 5CB (4-cyano-4(')-n-pentylbiphenyl) and 8CB (4-cyano-4(')-n-octylbiphenyl) confined to nanometer thick layers between two orienting surfaces. The capillary condensation was induced by decreasing the liquid crystal layer thickness using an atomic force microscope, and the onset of condensation was detected by monitoring the structural force on a confining surface. Very strong and long-ranged capillary forces were observed at temperatures close to the isotropic-nematic transition. We have analyzed the temperature dependence of the thickness of the liquid crystal layer, at which the condensation occurs, with a thermodynamic Kelvin equation and determined the interfacial tension between the isotropic and nematic phases. The separation dependence of capillary forces was analyzed within the Landau-de Gennes approach, including electrostatic interaction due to surface charging. The quantitative agreement between the measured and calculated force profiles is very good, and a single set of parameters is needed to describe a set of measured force profiles at different temperatures. Surface charge density, surface potential, and Debye screening length were determined directly from the observed surface forces.