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Viscosity

The resistance of a fluid to any deformations that is caused by shear stress and tensile stress is known as viscosity. It also refers to the thickness of a fluid substance . A fluid with a lower viscosity is referred to as thin fluid and a fluid with a high viscosity is called a thick fluid. Viscosity can also be considered as the internal resistance that a fluid must overcome before it can flow. Fluidity is the lack of fluid resistance and the opposite of viscosity.
Viscosity in a fluid is caused by the friction between the molecules in the substance. Gasses and fluids consist of adjacent layers of molecules. When an external force or pressure acts upon the fluid, the friction between the adjacent layers of the substance will determine the deformation and flow response of the fluid.

A fluid’s viscosity is dependant on the temperature of the substance. Increasing the temperature will reduce the viscosity of the fluid and cause it to become thinner. Butter for example, is hard when kept in the refrigerator. It becomes soft at room temperature and melts to a watery consistency when heated. Cooking oil also thickens when it is kept in a refrigerator.