Boundary lubrication with aqueous solutions of silicone-based amphiphilic block copolymer aggregates: effect of concentration

Correlation between ordering and shear thinning in confined OMCTS liquids

Despite the extensive research that has been conducted for decades on the behavior of confined liquids, detailed knowledge of this phenomenon, particularly in the mixed/boundary lubrication regime, remains limited. This can be attributed to several factors including the difficulty of direct experimental observations of the behavior of lubricant molecules under non-equilibrium conditions, the high computational cost of molecular simulations to reach steady state, and the low signal-to-noise ratio at extremely low shear rates corresponding to actual operating conditions. To this end, we studied the correlation between the structure formation and shear viscosity of octamethylcyclotetrasiloxane confined between two mica surfaces in a mixed/boundary lubrication regime. Three different surface separations corresponding to two-, three-, and five-layered structures were considered to analyze the effect of confinement. The orientational distributions with one specific peak for n=2 and two distributions, including a parallel orientation with the surface normal for n>2, were observed at rest. The confined liquids exhibited a distinct shear-thinning behavior independent of surface separations for a relatively low shear rate, \dot{\gamma}<10^{8} s^{-1}. However, the shear viscosities at \dot{\gamma}<10^{8} s^{-1} depended on the number of layered structures. Newtonian behavior was observed with a further increase in the shear rate. Furthermore, we found a strong correlation between the degree of molecular orientation and the shear viscosity of the confined liquids. The magnitude of the shear viscosity of the confined liquids can primarily be determined by the degree of molecular orientation, and shear-thinning originates from the vanishing of specific orientational distributions with increasing shear rate.

Two-Dimensional Cellular Patterning on a Polymer Film Based on Interfacial Stiffness

The mechanical properties in the outermost region of a polymer film strongly affect various material functions. We here propose a novel and promising strategy for the two-dimensional regulation of the mechanical properties of a polymer film at the water interface based on an inkjet drawing of silica nanoparticles (SNPs) underneath it. A film of poly(2-hydroxyethyl methacrylate) (PHEMA), which exhibits excellent bioinertness properties at the water interface, was well fabricated on a substrate with a pattern of SNPs. X-ray photoelectron spectroscopy and atomic force microscopy confirmed that the surface of the PHEMA film was flat and chemically homogeneous. However, the film surface was in-plane heterogeneous in stiffness due to the presence of the underlying SNP lines. It was also noted that NIH/3T3 fibroblast cells selectively adhered and formed aggregates on the areas under which an SNP line was drawn.