Opposing effects of confinement and confinement-induced shear-thinning on viscoelastic properties of liquid lubricant in nanometer-scale gaps

When a liquid lubricant is confined in nanometer-scale gaps, it exhibits characteristic viscoelastic properties that are different from those in the bulk state. Understanding the gap dependence of viscoelasticity in a confined state is important, particularly for the lubrication design of micromechanical devices and analysis of boundary lubrication, because tribological issues occur in nanometer-scale gaps. In the present study, we focus on the shear-thinning behavior of a lubricant that was induced by confinement and attempt to clarify the effect of shear-thinning on the gap dependence of viscoelasticity. We measured the viscoelasticity of the confined liquid lubricant in the gap range from 30 nm to a few nanometers for different oscillation frequencies using the fiber wobbling method. The experimental results revealed that shear-thinning occurred suddenly for gap widths of approximately 15 nm or less. As a result, the tendency of viscosity to increase during the confinement process had an inflection point at a gap width of approximately 15 nm.