Viscosity Totally Explained

Everything about Viscosity totally explained

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid. The study of viscosity is known as rheology.

Etymology

The word "viscosity" derives from the Latin word "" for mistletoe. A viscous glue was made from mistletoe berries and used for lime-twigs to catch birds.

Viscosity coefficients

When looking at a value for viscosity, the number that one most often sees is the coefficient of viscosity. There are several different viscosity coefficients depending on the nature of applied stress and nature of the fluid. They are introduced in the main books on hydrodynamics and rheology.

Dynamic viscosity determines the dynamics of an incompressible Newtonian fluid;
Kinematic viscosity is the dynamic viscosity divided by the density for a Newtonian fluid;
Volume viscosity determines the dynamics of a compressible Newtonian fluid;
Bulk viscosity is the same as volume viscosity
Shear viscosity is the viscosity coefficient when the applied stress is a shear stress (valid for non-Newtonian fluids);
Extensional viscosity is the viscosity coefficient when the applied stress is an extensional stress (valid for non-Newtonian fluids).

Shear viscosity and dynamic viscosity are much better known than the others. That is why they're often referred to as simply viscosity. Simply put, this quantity is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction, to the change in velocity of the fluid as you move down in the fluid (this is what is referred to as a velocity gradient). For example, at room temperature, water has a nominal viscosity of 1.0 × 10^-3 Pa∙s and motor oil has a nominal apparent viscosity of 250 × 10^-3 Pa∙s. ť Extensional viscosity is widely used for characterizing polymers.

Volume viscosity is essential for Acoustics in fluids, see Stokes' law (sound attenuation)

Newton's theory

In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force. Isaac Newton postulated that, for straight, parallel and uniform flow, the shear stress, τ, between layers is proportional to the velocity gradient, ∂u/∂y, in the direction perpendicular to the layers. ť

au=eta fracdA_j,

Further Information

Get more info on 'Viscosity'.

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