# What is Meant by Viscosity

Some fluids move faster than the other. For example, consider two bottles, one containing honey and other containing water. If you made a small hole at the bottom of the bottle, which bottle gets emptied first? The property here we are talking about is known as viscosity.

Viscosity is an important property of the fluid. It's defined as the measure of the resistance of the fluid to flow. It is an indication of internal friction.
A fluid having high viscosity experience high internal resistance to flow. They move slower than fluid having low viscosity. In the above example, water has a lower viscosity than honey. Hence, water gets emptied (or move) readily than honey.

The viscosity of fluid varies with temperature and pressure. Zero viscosity is observed at very low temperature and in superfluids. All other fluids have positive viscosity.

Read: Newton's Law of Viscosity and Equation

### The importance of fluid viscosity in various applications

1. Viscosity is considered during the selection of lubricants for machines. For example, high viscous oils are chosen for slowly moving parts while low viscous lubricants used for fast moving parts.
2. Viscosity data help to predict how fluid behaves in a particular condition; that helps in machine designs.
3. Viscosity holds the liquid where they placed. If the fluid has no viscosity (superfluid), they can flow forever without any internal resistance, and they can itself flow out of the container.
4. For some application, the viscosity must be precisely regulated to get desirable properties. For example, if the viscosity of paint is very low, it would run down the walls. If the viscosity is high, then it is tough to apply paint on the wall.
5. Convection of heat depends on the viscosity.
- Effect of temperature on viscosity of liquids and gases
- What does surface tension mean