Definition Of Viscosity

Definition Of Viscosity

Viscosity is a measure of a fluid's resistance to flow

More About Viscosity

  • Viscosity arises due to the internal friction between layers of fluid as they slip past one another while liquid flows
  • It describes the internal friction of a moving fluid
  • 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 contact 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 in passing from one layer to the next is called laminar flow
  • If we choose any layer in the flowing liquid 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 layer
  • This force is proportional to the area of contact of layers and velocity gradient. f∝A (A is the area of contact)
  • F ∝ du/dz : Where du/dz is the change in velocity with distance i.e. velocity gradient
  • • F ∝ A du/dz
  • F = η A
  • ‘η’ is proportionality constant and is called coefficient of Viscosity
  • Viscosity coefficient is the force when velocity gradient is unity and the area of contact 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 = (PaS = 1 kgm-1 S-1)
  • In cgs system the unit of coefficient of Viscosity is poise
  • 1 poise = 1gmcm-1 S -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 than 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


  • 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 forms a thin layer so that direct contact between the surfaces is prevented
  • 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