Fluid Friction (Viscosity)

Fluid Friction (Viscosity)

In our daily life we observe that thin liquid, like water, alcohol, spirit etc. flow readily where as thick liquid, like coltar, castor oil, glycerin flow more slowly under similar conditions. The liquids of the second kind are said to be more viscous than the liquids of the first kind.

As there is friction between two solid surfaces tending to oppose their relative motion when one is made to slide over the other, there is also friction between two layers of even the same liquid (in general fluid) when they are in relative motion. This can be demonstrated by a simple experiment as described below.

Consider a liquid which flows on a smooth horizontal glass surface. We may suppose the liquid to be divided into different layers parallel to the fixed surface on which the liquid is flowing. It is found that different layers move with different velocities. For a streamline motion the layer in contact with the fixed surface is stationary. The velocity of the layers increases with the distance from the fixed surface in a perpendicular direction. The length of the arrow gives the velocity of the corresponding layers.