# Viscosity

## Viscosity

When you pour a bottle of water on someone’s head, it flows through their hair and over their face. But when you pour honey on someone’s head, it takes its own sweet time in flowing through that person’s head. This is because of the property of the fluids called viscosity. Honey is thicker than water, means honey more viscous than water.

What is Viscosity?

The property of a fluid by virtue of which an internal frictional force acts between its different layers which opposes their relative motion is called viscosity. These internal frictional forces are called viscous force.

Viscosity is the measure of the resistance of a fluid to gradual deformation by shear or tensile stress. Viscous forces are intermolecular forces action between the molecules of different layer of liquid moving with different velocities.

$$F\,\,=\,\,-\,\eta A\frac{dv}{dx}$$,

$$\eta \,\,=\,\,-\,\,\frac{F}{A\left( \frac{dv}{dx} \right)}$$.

Where,

η = Co – efficient of Viscosity,

$$\frac{dv}{dx}$$ = Velocity gradient,

A = Area of cross – section.

SI unit of η is Ns/ m² or Pascal seconds. Its dimensional formula is [ML⁻¹T⁻¹].

Knowing about the coefficient of viscosity of different oils and its variation with temperature helps us to select a suitable lubricant for a given machine. Viscosity is due to the transport of momentum. The value of viscosity for ideal liquid is zero.

The viscosity of liquids decreases with increase in temperature while for gases, it increases with increase in temperature. Dynamic or Absolute, Kinematic and Bulk are the different types.