Viscosity measurements of glycerol in a parallel-plate rheometer exposed to atmosphere

Abstract

Glycerol is a hygroscopic fluid that spontaneously absorbs water vapour from the atmosphere. For applications involving glycerol, care must be taken to avoid exposure to humidity, since its viscosity decreases quickly as water is absorbed. We report experimental measurements of the viscosity of glycerol in a parallel-plate rheometer where the outer interface is exposed to atmosphere. The measurements decrease with time as water is absorbed from the atmosphere and transported throughout the glycerol via diffusion and advection. Measured viscosities drop faster at higher relative humidities, confirming the role of hygroscopicity in the transient viscosities. The rate of viscosity decrease shows a non-monotonic relationship with the rheometer gap height. This behaviour is explained by considering the transition from diffusion-dominated transport in the narrow-gap regime to the large-gap regime where transport is dominated by inertia-driven secondary flows. Numerical simulations of the water absorption and transport confirm this non-monotonic behaviour. The experimental viscosity measurements show unexpectedly fast decreases at very small gap heights, violating the parallel-plate, axisymmetric model. We propose that this drop-off may be due to misalignment in the rheometer that becomes non-negligible for small gaps. Theoretical considerations show that secondary flows in a misaligned rheometer dominate the typical secondary inertial flows in parallel-plate rheometers at small gaps. Finally, simulations in a misaligned parallel-plate system demonstrate the same sharp drop-off in viscosity measurements at small gap heights. This modelling can be used to estimate the gap height where misalignment effects dominate the transient glycerol viscosity measurements.