It is one of the characteristic properties of liquids. Viscosity is a measure of resistance to flow which arises due to the internal fraction between layers of fluid as they slip past one another while liquid flows. Strong intermolecular forces between molecules hold them together and resist movement of layers past one another.

When a liquid flows over a fixed surface, the layer of molecules in the immediate contract of surface is stationary. The velocity of upper layers increases as the distance of layers from the fixed layer increases. This type of flow in which there is a regular gradation of velocity in passing from one layer to the next is called laminar flow. If we choose any layer in the flowing liquid in the below figure, the layer above it accelerates its flow and the layer below this retards its flow.

If the velocity of the layer at a distance dz is changed by a value du then velocity gradient is given by the amount du/dz. A force is required to maintain the flow of layers. This force is proportional to the area of contact of layers and velocity gradient i.e.,
‘η’ is proportionality constant and is called coefficient of viscosity. Viscosity coefficient is the force when velocity gradient is unity and the area of contract is unit area. Thus ‘η’ is measure of viscosity. SI unit of viscosity coefficient is 1 newton second per square metre (N s m-2) = pascal second (Pa s = 1 kg m-1 s-1). In CGS system the unit of coefficient of viscosity is poise (named after great scientist Jean Louise Poiseuille).

1 poise = 1 g cm-1 s-1 = 10-1 kg m-1s-1

Greater the viscosity, the more slowly the liquid flows. Hydrogen bonding and van der Waals forces are strong enough to cause high viscosity. Glass is an extremely viscous liquid. It is so viscous that many of its properties resemble solids. However, property of flow of glass can be experienced by measuring the thickness of windowpanes of old buildings.

These become thicker at the bottom that at the top.

Viscosity of liquids decreases as the temperature rises because at high temperature molecules have high kinetic energy and can overcome the intermolecular forces to slip past one another between the layers.