Viscocity and Applications of viscosity


The property by virtue of which the relative motion between the layers of a liquid is maintained is called viscosity. We can also say viscosity is the resistance to flow.

Let us consider a liquid flowing over a horizontal surface. The layer in contact with the surface is at rest. The top most layer have the maximum velocity. The intermediate layers have intermediate velocity.

To maintain this relative motion of the layers, an external force must be acting on the liquid. Otherwise the liquid will come to rest due to internal frictional forces acting between the layers of the liquid. These internal frictional forces that bring the liquid to rest are known as viscous force and this property is known as viscosity.

Applications of viscosity

To reduce friction between the two moving surfaces which are in contact, a drop or two of some oil is introduced between the two contact surfaces. The oil enters into the depressions in the contact surfaces and form a thin layer so that direct contact between the surfaces is prevented. Now, the viscous force of the oil in between the surfaces (lubricant) reduces the friction between the two moving surfaces because the fluid friction is always less than the friction between the solid surfaces. However a good viscous lubricant can only reduce friction, but cannot completely eliminate the friction.

Generally, for high speeds and low pressures, highly viscous oils such as Mobil oil, grease, etc. are used. For low speeds, oils of lower viscosity are used. It may be interesting to note that air also acts as a sort of lubricant in reducing friction. The friction between two metallic surfaces in vacuum is found to be greater than that in air.

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