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How does viscosity affect the rate at which layers of fluid slide past each other?
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fluid mechanics
Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.
In fluid mechanics, viscosity acts as a form of internal friction between the molecules of a fluid, which directly opposes the movement of adjacent layers relative to one another.12
When a fluid undergoes laminar motion, layers of the fluid slide past each other at different velocities. Viscosity affects this process in the following ways:
- Resistance to Flow: Viscosity represents the resistance to a change in shape or movement. As one layer of fluid slips over another, the fluid exerts a frictional force that opposes the development of velocity differences between these layers.23
- Momentum Transport: The sliding motion is impeded by the migration of individual molecules between layers. A faster-moving layer is slowed down by the arrival of molecules from a slower layer, and vice versa. Viscosity is the proportionality constant between the magnitude of the shearing stress and the rate at which momentum is transported between these adjacent layers.4
- Shear Stress: For Newtonian fluids, the shear stressāthe force per unit area required to cause this slidingāis directly proportional to the rate of shear strain (the rate at which the velocity changes between layers).23
Essentially, higher viscosity increases the internal friction, making it more difficult for fluid layers to slide past each other and thereby opposing the flow.12