Anisotropic Shear Viscosity of Photoaligned Liquid Crystal Confined in Submicrometer-to-Nanometer-Scale Gap Widths Revealed with Simultaneously Measured Molecular Orientation

Abstract

In the context of the use of liquid crystals (LCs) as lubricants and lubricant additives, this study investigates the anisotropic shear viscosity of LCs confined in nanometer-sized gap widths subject to both shearing and photoalignment. The photoalignment is achieved using anisotropically dimerized polyvinyl cinnamate (PVCi) films coated on substrates. We simultaneously measure the viscosity and order parameter of a liquid crystal (4-cyano-4'-pentylbiphenyl) confined and sheared in the gap range of 500 nm down to a few nm. We achieve this simultaneous measurement using an original method that combines a highly sensitive viscosity measurement and a sensitive birefringence measurement. When the LC is sheared in the same direction as the photoalignment (parallel shearing), the order parameter, which is around 0.3 in the bulk state, increases up to around 0.4 at a gap width of less than 50 nm and the viscosity is smaller than half the bulk viscosity. We consider that this increase in the order parameter is due to the highly ordered photoaligned LC layer near the PVCi film, and the viscosity decrease is due to shear thinning of this layer enhanced by both confinement and molecular ordering. In addition, we observe a gradual decrease in viscosity starting at a gap of less than around 300 nm in the parallel shearing. Based on the apparent slip model, we show that the LC layer near the PVCi film can also cause this gradual viscosity decrease. In contrast, when the LC is sheared in the direction perpendicular to the photoalignment direction (perpendicular shearing), the viscosity increases as the gap decreases. We speculate that this is due to the rotational motion of the LC molecules caused by the competing effect of shear alignment and photoalignment. We believe our findings can significantly contribute to a better understanding of the confined LCs utilized for lubrication.

Probing of Reorganization Dynamics within the Different Phases of Themotropic Liquid Crystals

“…Outcome of this study will help researchers in rational designing of optical switchable windows, optical devices for telescope and other liquid crystal devices where efficiencies of devices are largely controlled by the orientational dynamics of the LCs molecules ,. Reorganization time of LCs molecules, obtained in this study, can be correlated with the response time of rearrangements of LCs molecules against an external electric/magnetic field in a birefringence LC cell . Optical studies using polarized microscope reveal different phases with distinct textures within LCs.…”

“…[1,[14][15][16] Reorganization time of LCs molecules, obtained in this study, can be correlated with the response time of rearrangements of LCs molecules against an external electric/magnetic field in a birefringence LC cell. [17][18][19] Optical studies using polarized microscope reveal different phases with distinct textures within LCs. Thermotropic LCs used in the present study exhibit several phases, namely: nematic, smectic and so on.…”

Molecular behaviors in thin film lubrication—Part one: Film formation for different polarities of molecules

“…5CB liquid crystals have great solubility in common base oils because of their similar structure. It has been studied as a lubricant and additive, and shows good potential in reducing friction and externally controlling the lubricating performance [19]. Furthermore, 5CB molecules arrange in an orderly manner under the effect of surface adsorption [17,19] and flow alignment [12], which makes it an excellent model for studying the molecular orientation of the lubricant.…”

“…In the field of tribology, much attention has been focused on lubricants confined in nano-sized gaps. Itoh et al [19] studied the orientation parameter and viscosity of 5CB confined and sheared in a gap of size ranging from 500 nm to a few nm using a surface force apparatus (SFA). Zhang et al [20] explored the molecular orientation and friction performance of 5CB confined in the contact region with a gap size of tens of nanometers.…”

Evaluation of 1,3-diketone as a novel friction modifier for lubricating oils

“…Gao et al [21] reported that the adsorption of 5CB on the sliding steel surfaces play an important role in the friction reduction. Itoh et al [22] also found that the slip of the 5CB layers adjacent to the substrate leads to low friction and high load capacity.…”