title: Direct nanomechanical measurement of an anchoring transition in a nematic liquid crystal subject to hybrid anchoring conditions.
abstract: We have used a surface forces apparatus to measure the normal force between two solid curved surfaces confining a film of nematic liquid crystal (5CB, 4'-n-pentyl-4-cyanobiphenyl) under hybrid planar-homeotropic anchoring conditions. Upon reduction of the surface separation D, we measured an increasingly repulsive force in the range D = 35-80 nm, reaching a plateau in the range D = 10-35 nm, followed by a short-range oscillatory force at D < 5 nm. The oscillation period was comparable to the cross-sectional diameter of the liquid crystal molecule and characteristic of a configuration with the molecules parallel to the surfaces. These results show that the director field underwent a confinement-induced transition from a splay-bend distorted configuration at large D, which produces elastic repulsive forces, to a uniform planar nondegenerate configuration with broken homeotropic anchoring, which does not produce additional elastic forces as D is decreased. These findings, supported by measurements of the birefringence of the confined film at different film thicknesses, provide the first direct observation of an anchoring transition on the nanometer scale.
authors.affiliation: Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts 01854, USA. marina_ruths@uml.edu
authors.forename: Marina
authors.initials: M
authors.lastname: Ruths
authors.forename: Bruno
authors.initials: B
authors.lastname: Zappone