Optical and mechanical responses of liquid crystals under confinement

Abstract

The optics of liquid crystals (LCs) lay an important foundation for LC displays and the mechanics of LCs are the backbones of LC elastomers that are promising materials for artificial muscles. Despite broad prospects for applications, it is still a challenge to precisely measure both optics and mechanics at the nanoscale. Here both optical and mechanical responses are simultaneously probed by the Surface Forces Apparatus to understand how optical anisotropies of LCs interact with the birefringence of the mica, and how mechanical anisotropies of LCs interact with anchoring conditions and the confinement. Optically, the birefringence of nematics adds complexities to the two intrinsic birefringent mica surfaces for multiple-beam interference. The phase retardation by multiple birefringent layers is a result of composition by the phase retardation from each layer and their relative intersection angles, which is intuitively understood by the parallelogram rule that is similar to the geometrical composition of forces but with double intersection angle. The simulation based on 4x4 matrices is used to reconstruct the interaction of fringes and to compare the deviation of average wavelengths in the same chromatic order and isotropic wavelengths generated by the average refractive indices. Mechanically, LC behaviours result from the competition among surface anchoring, elasticity of LCs and confinements. During the retraction of surfaces, the neck of cholesteric layers is broken by the innermost circular dislocation defect that serves as a bulk crack with the opening mode of fracture, producing periodical twist transitions and structural forces. During the approach of surfaces, three regimes, constrained, stick-slip and sliding-slip, of cholesteric mechanical windings are observed with the time evolution of the surface anchoring. The onset of three regimes and the retardation of twist transitions results from the balance between the twist elastic torque and the frictional surface torque, namely the anchoring torque and the viscous torque, which is analogous to friction torque in rotational friction. The deviation of the anchoring angle on surfaces provides evidence of interfacial ruptures, with tearing or sliding mode, described by the paradigm of fracture mechanics for the onset from static frictions to kinetic frictions. This thesis sheds light on the understanding of boundary effects on permeative flows, frictions, fractures, yield stress materials, adhesions and biomechanics.