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How do fluids respond to shear stress?
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fluid mechanics
Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.
When subjected to shear stress, a fluid undergoes a continuous and irrecoverable change in shape, a process known as flow. Unlike an elastic solid, which can maintain a shearing force while held in a deformed position, a fluid cannot sustain such a force when at rest.1
The response of a fluid to shear stress is governed by its viscosity, which acts as internal friction between molecules, opposing the movement of neighboring portions of the fluid relative to one another.2
- Newtonian Fluids: For many fluids, the shear stress is directly proportional to the rate of shear strain (the rate of deformation). This linear relationship is defined by the coefficient of viscosity.32
- Non-Newtonian Fluids: Some fluids, such as certain shampoos or egg whites, do not follow this linear relationship. In these cases, the stress may depend on the fluid's deformation history or the rate of shear in complex ways.31
In essence, when a fluid is forced to move, its viscosity resists the development of velocity differences, requiring a continuous application of force to maintain the flow.32