What is viscosity?

Viscosity is a measure of a fluid's resistance to flow or deformation. It quantifies the internal friction within a fluid as it moves or deforms under the influence of an external force, such as gravity or shear stress. In simpler terms, viscosity describes how "thick" or "sticky" a fluid is.

The viscosity of a fluid depends on its molecular composition, temperature, and pressure. Generally, fluids with high viscosity flow more slowly and resist deformation more strongly than fluids with low viscosity.

Viscosity is typically measured in units of Pascal-seconds (Pa·s) in the International System of Units (SI), although other units such as centipoise (cP) or millipascal-seconds (mPa·s) are also commonly used.

There are two main types of viscosity:

1. Dynamic Viscosity (Absolute Viscosity):

   • Dynamic viscosity measures a fluid's resistance to flow under an applied shear stress. It represents the ratio of the shearing stress to the velocity gradient in the fluid.
   • Mathematically, dynamic viscosity ($𝜇$) is expressed as: $𝜇=\frac{𝜏}{\frac{𝑑𝑢}{𝑑𝑦}}$ Where:
     • $𝜇$ is the dynamic viscosity.
     • $𝜏$ is the shearing stress.
     • $\frac{𝑑𝑢}{𝑑𝑦}$ is the velocity gradient in the direction of flow.

2. Kinematic Viscosity:

   • Kinematic viscosity is the ratio of dynamic viscosity to fluid density. It measures a fluid's resistance to flow relative to its density.
   • Mathematically, kinematic viscosity ($𝜈$) is expressed as: $𝜈=\frac{𝜇}{𝜌}$ Where:
     • $𝜈$ is the kinematic viscosity.
     • $𝜇$ is the dynamic viscosity.
     • $𝜌$ is the fluid density.

Viscosity plays a crucial role in various physical phenomena and engineering applications, including fluid dynamics, lubrication, heat transfer, and materials processing. Understanding the viscosity of a fluid is essential for predicting its behavior in different situations and designing systems that rely on fluid flow or deformation.