The internal friction that arises due to this difference in velocity is termed viscosity. Fluids with high viscosity (like honey or oil) flow more slowly, while fluids with low viscosity (like water or air) flow more readily. Viscosity is sometimes referred to as the "thickness" of a fluid.
In liquids, viscosity arises from cohesive molecular forces, while in gases it results from molecular collisions. [1][2] Except for the case of superfluidity, there is no fluid with zero viscosity, and thus all fluid flows involve viscous effects to some degree. [1]
This constant is called the dynamic, or absolute, viscosity and often simply the viscosity. Fluids that behave in this way are called Newtonian fluids in honour of Sir Isaac Newton, who first formulated this mathematical description of viscosity. The dimensions of dynamic viscosity are force × time ÷ area.
Viscosity is a physical quantity that describes a fluid’s resistance to flow. It is a property that resists the relative displacement of the different layers of the fluid.
Effect of Pressure on Viscosity: The coefficient of viscosity of liquids rises as pressure increases, although there is no relationship to explain the phenomenon thus far.
Informally, viscosity is the quantity that describes a fluid's resistance to flow. Formally, viscosity is the ratio of shearing stress to velocity gradient.
Viscosity looks at the difference in the rate of flow or deformation between between molecules a certain distance from a surface and those at the liquid-surface interface.
The viscosity of a liquid will drop as the temperature rises, but the viscosity of a gas will increase. To put it another way, the viscosity of a liquid will decrease as the temperature rises, whereas the viscosity of gases will increase as the temperature rises.