In situ viscosity measurement of confined liquids

Abstract

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.Engineering fluids frequently experience local conditions that change their bulk rheological properties.While viscosity data can easily be acquired using conventional rheometers, the results are not always applicable to fluids under engineering 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. There is a lack of suitable methods for measuring fluid viscosity under such conditions. This work describes a novel in situ viscosity measurement technique to fill this gap. It involves the quantification of the fluorescence lifetime of a fluorescent dye that is sensitive to viscosity.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. Viscosity measurements were successfully performed at pressures up to 1.2 GPa and shear rates up to 10 5 s À1 . Spatial heterogeneity in viscosity caused by variations in pressure within the thin fluid film could be observed using the technique. It was also possible to detect differences in the rheological responses of a Newtonian and a non-Newtonian fluid.These first in situ high pressure, high shear viscosity measurements demonstrate the versatility of the proposed technique in providing information on the viscosity in conditions where contemporary techniques are insufficient. More importantly it highlights the complexity of the rheology of engineering fluids and provides a means of verifying existing theories by performing in situ measurements.Information on local viscosity is crucial for understanding the physics of confined fluids and to facilitate improvements in engineering technology. Fig. 8 (a) Variation of ThT lifetime in (a) glycerol and (b) IGEPAL; confined in a point contact at various shear rates. All lines are for guidance only. 99592 | RSC Adv., 2015, 5, 99585-99593 This journal is

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

“…There have been previous NEMD, 74 and experimental investigations, 75 ('shear banding') in liquid systems composed of more structured molecules are known to occur for more normal lubricant liquid like systems, [34][35][36]85,86 so this feature appears to be generic to the liquid state under these conditions.…”

“…27,31,32 Some of the previously observed NEMD steady states have been detected experimentally. For example, Plug-Slip, 34,35 and Central Localization, 36 steady states have been detected using a novel methodology based on phosphorescence imaging. Martinie and Vergne, 37 also gave evidence that the LJ shear localised states are present in real lubricants under traction experiment conditions despite the simplicity of the computer model.…”

Quantitative Viscosity Mapping Using Fluorescence Lifetime Measurements

“…Details on its choice can be found in [22]. High dye concentration is required due to the ultra-thin films studied in this work and the dyes' low 14.5 and 13.6…”

“…Details of the fluorescence lifetime set-up used to measure ThT fluorescence lifetime in model lubricants can be found in [22] and is briefly described here. The optical rheometer described in Sect.…”

“…This technique has been recently adapted and applied to probe local lubricant viscosity in EHD contacts with microscale spatial precision [22]. However, quantitative measurements were only achieved in glycerol.…”