Effects of Confinement and Shear on the Properties of Thin Films of Thermotropic Liquid Crystal

Abstract

The surface forces apparatus technique was used to measure the normal forces, thin film viscosity, and lateral (frictional) forces between two surfaces interacting across 4′-n-octyl-4-cyanobiphenyl (8CB), to determine the effects of confinement (film thickness) and shear (sliding velocity) on the ordering of the smectic-A and the nematic phases. The surface roughness and hydrophobicity were altered by different adsorbed surfactant monolayers to study the effects on the orientation and anchoring of 8CB. The positional order increases as the surface separation decreases, and the orientational ordering increases with increasing shear rate. The friction force in the planarly oriented nematic phase resembles results for alkanes, while the better ordered smectic-A phase exhibits lower friction forces. The liquid crystal orients perpendicularly to surfactant-coated surfaces. On a loose-packed surfactant layer, it becomes strongly anchored, which increases the resistance to sliding, while on a close-packed monolayer the friction force is low, but the liquid crystal is easily removed by the applied load or pressure.

Effect of nanoconfinement on liquid-crystal polymer chains

“…the short axis of a GB monomer. As for the choice of the exponents, we recall that by integrating the dispersion interaction between a LJ particle and an infinite slab formed by LJ centres, an overall particle-surface potential governed by a (9,3) law is obtained. In the literature, the particle-surface energy is usually modelled with a variety of repulsive and attractive exponents, like the (10, 4) exponents implemented by Steuer et al, 26 and the (9, 3) used by Caprion et al 27 Considering that most common materials mentioned in the Introduction are far from being completely smooth 9,20,21 and also the existence of electrostatic interactions [13][14][15][16][17][18][19] between surfaces and molecules, we have arbitrarily set the repulsive coefficient n to a softer value of 6 to allow for some surface penetration, while keeping m = 3.…”

“…4 On the other hand, untreated hydrophilic mica or a flat gold layer cause a "planar parallel" orientation 9 of the director of nematic LC. 9 In non-display applications an increasingly important technique for consolidating the specific LC molecular organizations obtained is that of polymerizing suitably chosen prepolymer-monomer LC mixtures. 22 This strategy for controlling surface interactions offers in principle the possibility of creating thin films of polymer with well defined molecular organization and optical or mechanical properties.…”

Friction control with nematic lubricants via external fields

“…LC systems can be characterized by the presence or absence of positional and/or orientational order of the elongated molecules, which is controllable by, e.g., tuning temperature or applying external electric or magnetic fields [15]. In the context of LC lubricants, also applied pressure and sliding velocity of the confining surfaces are expected to play a role [16][17][18][19][20]. On the other hand, lubrication properties of LCs should depend on their ordering [21][22][23].…”

A surface forces platform for dielectric measurements

“…These include force-distance relations of confined liquid crystals, 4 -6 especially as they depend on the relative orientation of the confining mica surfaces, and recently their linear and nonlinear shear rheology ͑including stick-slip transitions͒ as a function of relative orientation of the confining mica crystals. [6][7][8] It is desirable to develop additional experimental techniques, with which to complement the information gained from force-based measurement. Safinya and co-workers have investigated the structure and orientation of oriented liquid crystals using synchrotron x-ray scattering within a modified SFA but those experiments have been limited, to date, to film thickness 0.1 m or larger.…”

“…4,5,7,8 This laboratory has described elsewhere the force-distance relations for 5CB, with special attention given to the accompanying shear response, in the case of planar alignment, as a function of relative twist angle between the confining mica lattices. 6 …”