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The Weissenberg Effect
Posted by Jesse Pritchard
Take a look at this video on the Weissenberg effect.
When stress is applied to a fluid, a certain amount of strain, or deformation, is observed. In particular, for a Newtonian fluid, viscous stresses that arise from the flow are linearly proportional to the local rate of deformation over time. It is determined from this that for Newtonian fluids (such as water and oil), the viscosity of the fluid does not depend on the applied stress. So when a rotational stress is applied (as in the video) at the bottom of a Newtonian fluid, below an air-liquid interface, the resulting flow is dominated by inertial and gravity-effects which causes the fluid to flow down toward the source of the stress and radially outward. For pseudoplastics (such as grease), a type of Non-Newtonian fluid where the viscosity decreases as the applied stress increases, applying the same rotational stress will decrease viscosity near the source, which causes inertia and gravity to no longer dominate the flow, causing the fluid to flow upward and bulge at the air interface.
A fun instance of this effect is known as 'rod-climbing'. Rotating a rod inserted into a pseudoplastic causes the polymer chains that make up the fluid to congregate around the regions of highest stress and orient themselves in the direction of shearing, meaning that chains closer to the rod (where stress is highest) are stretched less than those chains further away, and so occupy 'lower states of energy'. The chains' desire to reach these regions of low energy creates an inward normal force and so the fluid goes in the only direction is can - UP THE ROD.