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

• Published in: Langmuir Vol. 28; no. 22; pp. 8371 - 8383
• Main Authors: Ruths, Marina, Zappone, Bruno
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 05.06.2012
• Subjects: Chemistry; Exact sciences and technology; films (materials); General and physical chemistry; liquid crystals; mica; nanotechnology; surface tension; Confinement; Separation; Support; Film; Force; Liquid crystals; Solid; Thickness; Chemical reduction; Oscillation; Birefringence; Curved surface; Anchoring; Diameter
• ISSN: 0743-7463; 1520-5827; 1520-5827
• DOI: 10.1021/la204746d

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.