Surface energy dissipation in homeotropic nematic layers: the role of the flexoelectric and surfactant desorption

Homeotropic nematic layers of MBBA have been studied by a phase-sensitive flexoelectric spectroscopy method. They have been oriented by films of dilauroyl phosphatidyl choline (DLPC) and cetyl trimethyl-ammonium bromide (CTAB) self-assembled onto the cell glass plates. Using continuum theory, the transmitted light versus excitation frequency spectral shapes have been derived in terms of space variation of the nematic parameters (elastic constant, flexoelectric coefficient, rotational viscosity, and birefringence). These viscoelastic spectra contain information about the surface dissipation of the orientational energy for different aligning films that partially desorb from the surface and dissolve in the nematic, producing a gradient of the surfactant. A new type of flexoelectric effect dependent on the space derivative of flexo-coefficients has been identified. This effect consists of a bulk flexo-torque source that substantially influences the apparent liquid crystal anchoring. Static and dynamic cases have been analyzed for a steplike surfactant distribution. Resulting spectra have been successfully compared with the experiment, yielding information about the surfactant gradient. These results provide new insights on the interfacial physics of nematic liquid crystals and solid surfaces, where flexoelectricity and desorption play a fundamental role.