Comparing the global and molecular rheology of fluids, in situ, under high pressures

Abstract

This abstract is based on the authors previously published work: ‘In situ viscosity measurement of confined liquids’ and ‘Quantitative Viscosity Mapping Using Fluorescence Lifetime Measurements’ and work submitted for publication: ‘Comparing the molecular and global rheology of a fluid under high pressures’. The viscosity of pressure-sensitive liquids governs crucial physical phenomena. While data on lowpressure viscosity is easy to obtain using conventional rheometers, obtaining high-pressure rheology presents a significant challenge. This is particularly important for fluids operating in the elastohydrodynamic lubrication regime under high shear stresses where fluids are likely to experience shear thinning. Two new in situ viscosity measurement techniques have been developed and applied to a point contact with submicron film thickness. They involve the quantification of fluorescence lifetime for a viscosity-sensitive fluorophore or the fluorescence anisotropy of a conventional fluorophore. Spatial heterogeneity in viscosity caused by variations in pressure is measured in both Newtonian and nonNewtonian fluids. Fluorescence lifetime measurements were performed at pressures up to 1.2 GPa and shear rates up to 105 s-1. Fluorescence anisotropy measured viscosities quantitatively up to 6 Pa.s under Newtonian conditions. This work combines friction measurements, high-pressure rheometry and local molecular viscosity measurements and compares against well-known rheological models. Together viscosities ranging five orders of magnitude, at pressures up to 720 MPa, were examined. It is shown that friction measurements can be used to determine viscosity if adjustment is made to take into consideration the effect of pressure distribution, shear thinning and heating. Single exponential models cannot predict viscosity over the complete viscosity range. The initial compressibility and thus rapid decrease in pressure-viscosity coefficient limits the maximum viscosity under which the Barus equation can be applied. After the initial reduction in free volume, the pressure-viscosity response can be represented well by a single exponential relationship. The Roelands equation was shown to predict the results well up to 720 MPa and 630 Pa.s. The Hybrid model fits well but requires a large range of data making it suitable for interpolation but not extrapolation.