Viscosity measurements were successfully performed at pressures up to 1.2 GPa and shear rates up to 10 5 s −1. The capability of the developed technique is verified by taking measurements in submicron thick films of two model fluids confined in a ball on flat contact. It involves the quantification of the fluorescence lifetime of a fluorescent dye that is sensitive to viscosity. This work describes a novel in situ viscosity measurement technique to fill this gap. There is a lack of suitable methods for measuring fluid viscosity under such conditions. This is particularly the case for fluids being sheared at high pressure under severe confinement, which experience very high shear stresses and often show extensive shear thinning. While viscosity data can easily be acquired using conventional rheometers, the results are not always applicable to fluids under engineering conditions. Engineering fluids frequently experience local conditions that change their bulk rheological properties. The viscosity of liquids governs crucial physical and engineering phenomena, ranging from diffusion and transport processes of nutrients and chemicals, to the generation of friction and the physics of damping.
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