Viscosity
When two fluid layers move relative to each other, a friction force develops between them and the slower layer acts to slow down the faster layer.
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Source: Wikipedia
Viscosity
When two fluid layers move relative to each other, a friction force develops between them and the slower layer acts to slow down the faster layer.
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Source: Internal
What is Viscosity, and why does it matter?
This concept appears everywhere in physics. Once you understand it, a wide range of natural phenomena start to make sense.
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Source: Wikipedia
Deep dive: Viscosity
When two fluid layers move relative to each other, a friction force develops between them and the slower layer acts to slow down the faster layer. This internal resistance to flow is described by the fluid property called viscosity, which reflects the internal stickiness of the fluid. In liquids, viscosity arises from cohesive molecular forces, while in gases it results from molecular collisions. Except for the case of superfluidity, there is no fluid with zero viscosity, and thus all fluid flows involve viscous effects to some degree.
For liquids, it corresponds to the informal concept of thickness; for example, syrup has a higher viscosity than water. Viscosity is defined scientifically as a force multiplied by a time divided by an area. Thus its SI units are newton-seconds per metre squared, or pascal-seconds.
For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's center line than near its walls. Some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity.
In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. However, the dependence on some of these properties is negligible in certain cases. For example, the viscosity of a Newtonian fluid does not vary significantly with the rate of deformation.
Zero viscosity (no resistance to shear stress) is observed only at very low temperatures in superfluids; otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. A fluid that has zero viscosity (non-viscous) is called ideal or inviscid.
For non-Newtonian fluids' viscosity, there are pseudoplastic, plastic, and dilatant flows that are time-independent, and there are thixotropic and rheopectic flows that are time-dependent.
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