Experimental study of surfactant driven nematic liquid crystal (NLC) anchoring transitions at solid surfaces: Role of solid surface energy and anisotropic NLC - Solid interfacial energy

The fundamental understanding of nematic liquid crystal (NLC) alignment on solid surfaces is significant for the operation of liquid crystal displays and devices. We investigate the effect of structure and concentration of surfactants on the anchoring of NLC 4-cyano-4′-pentylbiphenyl (5CB) at 5CB-solid interface. 5CB molecules undergo an ordering transition from parallel to normal anchoring near the surfactant critical micelle concentration depending on the chemical structure of the surfactant tails. In contrast to the previous studies which point to the solubilization of 5CB in surfactant micelles, our experiments indicate that NLC anchoring transition occurs due to the ability of 5CB molecules to penetrate into the surface surfactant layer. The surfactant-driven 5CB anchoring is further related to the surface tension of 5CB (γ<inf>5CB</inf>), surfactant adsorbed substrate surface energy (γ<inf>sa</inf>), anisotropic 5CB-surface interfacial energy (Δ γ <inf>sl</inf>), and spreading coefficient (S). Our experimental results agree with the Creagh-Kahn criterion of predicting surface anchoring from relative values of γ<inf>sa</inf> and γ<inf>5CB</inf>. A change in the sign of Δγ<inf>sl</inf> and S is also observed along with the anchoring transition. Using a semi-empirical approach, we show that dipolar interactions dominate over dispersion interactions when 5CB molecules exhibit perpendicular anchoring at the solid-5CB interface.