  Glycerol is a hygroscopic fluid that spontaneously absorbs water vapor 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 on the transient viscosities. The rate of viscosity decrease
shows a non-monotonic relationship with the rheometer gap height. This behavior
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 behavior. 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 modeling can be used to
estimate the gap height where misalignment effects dominate the transient
glycerol viscosity measurements.
