When a liquid flows slowly over a fixed horizontal surface. For a laminar flow, the layer of the liquid in contact with the fixed surface remains at rest due to adhesion. The layer just above it moves slowly over the lower one, the third layer moves faster over the second one, and so on. The velocities of the layers of liquid increase with the increase in distance from the horizontal rigid surface.
The property by virtue of which a liquid opposes the relative motion between its layers is called the Viscosity of the liquid. Viscosity is a general property of a fluid. The frictional force acting between two solid surfaces resembles in many ways the viscosity of a liquid. Hence, viscosity is called internal friction of liquid.
Like friction, the viscous force is absent if a liquid is at rest. The main difference between the frictional force in solids and the viscosity in liquids is that the viscous force depends on the area of the liquid surface while the frictional force does not.
Co-efficient of Viscosity:
The coefficient of viscosity of a liquid is defined as the required tangential force acting per unit area to maintain unit velocity gradient between two liquid layers unit distance apart.
Units of Co-efficient of Viscosity:
In SI, the unit of Co-efficient of Viscosity is N.s.m-2 or Pa.s
Poise and Decapoise:
The co-efficient of viscosity of a liquid is 1 poise, when a tangential force of 1 dyn is necessary to maintain a relative velocity of 1 cm.s-1 between two parallel layers of the liquid 1 cm apart where each layer has an area of 1 cm2.
So, 1 poise is the CGS unit of the coefficient of viscosity.
1 poise = 1 Dyn.s.cm-2 = 1g.cm-1.s-1.
As, 1 kg.m-1.s-1 = 10g.cm-1.s-1 = 10 poise.
The SI unit of coefficient of viscosity is called 1 decapoise = 10 poise.
The coefficient of viscosity of a liquid is 1 decapoise, when a tangential force of 1 newton is necessary to maintain a relative velocity of 1 m.s-1 between two parallel layers separated by a distance of 1 m, where each layer has an area of 1 m2.