Quantitative Viscosity Mapping Using Fluorescence Lifetime Measurements

Abstract

Lubricant viscosity is a key driver in both the tribological performance and energy efficiency of a lubricated contact. Elastohydrodynamic (EHD) lubrication produces very high pressures and shear rates, conditions hard to replicate using conventional rheometry. In situ rheological measurements within a typical contact are therefore important to investigate how a fluid behaves under such conditions. Molecular rotors provide such an opportunity to extract the local viscosity of a fluid under EHD lubrication. The validity of such an application is shown by comparing local viscosity measurements obtained using molecular rotors and fluorescence lifetime measurements, in a model EHD lubricant, with reference measurements using conventional rheometry techniques. The appropriateness of standard methods used in tribology for high-pressure rheometry (combining friction and film thickness measurements) has been verified when the flow of EHD lubricant is homogeneous and linear. A simple procedure for calibrating the fluorescence lifetime of molecular rotors at elevated pressure for viscosity measurements is proposed.

On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction

“…4 shows good agreement between the thermally corrected experimental data (_ g o 10 7 s À1 ) and the NEMD simulation data (_ g 4 10 7 s À1 ) for most of the fluids and applied conditions studied. In the experiments, there will be significant variation in the conditions within the macroscopic contact, 67,68 which cannot all be captured in a NEMD simulation limited to the nanoscale. 6 However, the general agreement found between the experimental and NEMD mean shear stress results for the fluids and conditions studied here suggests that NEMD simulations performed under average conditions for the contact are sufficient to reproduce the important trends.…”

“…Similar conclusions have been drawn from experimental comparisons between microscopic local viscosity measurements and more conventional global average viscosity measurements for contacts under EHL conditions. 67,68 Fig . 4 shows that the mean shear stress varies significantly with the applied load as well as the strain rate for all of the fluids studied.…”

Elastohydrodynamic lubricant flow with nanoparticle tracking

“…14 The viscosity of lubricants has been shown to increase locally with pressure in EHD conditions (known as the piezo-viscous effect) using uorescence-based techniques. [23][24][25][26] The diffusion of NPs is estimated to be negligible due to the high viscosities of our test solutions. Based on previous investigations, the uid may experience shear thinning, particularly at high pressures.…”

Estimation of rheological parameters for unstained living cells

“…The conducted experiments are rather illustrative than explorative. We have not so far dealt with linking the results to biology but, compared with the literature, e.g., [27,29,30], they seem to be promising.…”