Direct Nanomechanical Measurement of an Anchoring Transition in a Nematic Liquid Crystal Subject to Hybrid Anchoring Conditions

A Surface Forces Apparatus was used 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 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 visualization of an anchoring transition at the nanometer scale.