Influence of the Geometry on the Transient and Steady Flow of Lubricating Greases

Thermorheological behaviour of a lithium lubricating grease

“…On the contrary, a minimum in the flow curve appears, in the same shear rate range, at temperatures above 60°C. As has been previously pointed out [12], this behaviour in which a non-monotonic evolution of the shear stress with shear rate is observed, corresponds to a dynamically nonstable region [22,23] and may be related to a nonhomogeneous field of velocities during the viscometric flow of the lubricating greases, such as wall slip and shear banding, favouring the fracture of the sample at sufficiently high shear rates. This fact makes the experimental results highly dependent of the geometry used [12].…”

“…As has been previously pointed out [12], this behaviour in which a non-monotonic evolution of the shear stress with shear rate is observed, corresponds to a dynamically nonstable region [22,23] and may be related to a nonhomogeneous field of velocities during the viscometric flow of the lubricating greases, such as wall slip and shear banding, favouring the fracture of the sample at sufficiently high shear rates. This fact makes the experimental results highly dependent of the geometry used [12]. The visualization of the lubricating grease flow in Couette geometries, using the NMR velocity imaging technique, confirms the non-homogeneity of the flow and the existence of discontinuities in the strain rate field [24].…”

“…Stress or strain sweep tests, at the frequency of 1 Hz, were previously performed on each sample to determine the linear viscoelasticity region. Viscous flow tests were carried out in a shear rate range of 10 )4 to 10 2 s )1 , using serrated plate-and-plate geometries (35 mm and 25 mm diameters, 1 mm gap) to prevent wall slip [12]. Flow curves were obtained by applying a multi-step ramp of increasing shear rates/ stresses and waiting 180 s in each step, which is enough to almost attain the steady-state condition in the range of shear rate/stress studied.…”

Mixing rheometry for studying the manufacture of lubricating greases

“…Oscillatory tests were performed inside the linear viscoelastic region using a cone-plate geometry (4 • , 40 mm), in a frequency range comprised between 0.01 and 100 rad/s. Steady-state viscous flow tests, in a range of 10 −4 -10 2 s −1 , were performed using a serrated plate-plate geometry (25 mm, 1 mm gap, relative roughness 0.4) to prevent slip effects (Balan and Franco, 2001). At least two replicates of each test were performed on fresh samples.…”

Wall Slip Phenomena in Oil-in-Water Emulsions: Effect of Some Structural Parameters

“…The use of NMR microscopy to image the velocity profiles confirms a discontinuity in the flow curve (7,13,14). Similarly, as has been detected in several systems (4,5,7,15,16), wall depletion phenomena are generally confined to a certain range of shear rates associated with constant values of shear stress or, in other words, around a critical stress at which a sudden drop in viscosity takes place. However, in complex systems, such as emulsions or suspensions, this phenomenon has until now not been fully understood from a microstructural point of view.…”