Viscosity is the measure of a fluid’s resistance to flow. It quantifies the internal friction within the fluid. The relationship between shear stress ($\tau$) and velocity gradient (du/dy) in a fluid is described by Newton’s law of viscosity, which can be expressed as:

Where:

• $\tau$ is the shear stress,
• $\mu$ is the dynamic viscosity of the fluid,
• du/dy​ is the velocity gradient or shear rate.