When water is poured into a glass, it falls freely and quickly, whereas if honey or maple syrup is poured over a pancake, it flows slowly and sticks to the surface of the container. This difference in the flow of different kinds of liquids arises due to the fluid friction between the liquid layers and the liquid and the surrounding material. This property of fluids is called fluid viscosity. In this example, water has a lower viscosity than honey and maple syrup.

The SI unit of viscosity is newton·second/meter2 (N·s/m²) or pascal·second (Pa·s). Viscosity is a temperature-dependent quantity that varies from one fluid to another by several orders of magnitude. For example, the viscositiesof air, water, and honey are 0.0181 mPa·s, 1.002 mPa·s, and 2000–10000 mPa·s, respectively, at 20 ℃. The viscosity of water decreases with an increase in temperature, calculated to be 0.282 mPa·s at 100 ℃.

There are two types of fluid flow, namely laminar flow and turbulent flow. In laminar flow, layers of the fluid flow in parallel without mixing. However, viscosity causes drag between layers as well as with the fixed surface. The velocity of the top layer is higher than the bottom layers. This creates a gradient in velocity, which decreases in the direction from the moving surface towards the fixed surface. In turbulent flow, high speed and obstruction cause the mixing of layers via eddies and swirls. Significant velocities occur in directions other than the overall direction of flow.

The smoke from a burning incense stick is an excellent example of fluid flow. At the tip of the incense stick, the smoke rises smoothly for a while showing laminar flow. Then the speed of the accelerating smoke reaches the point that it begins to swirl due to the drag between the smoke and the surrounding air, indicating turbulent flow.

This text is adapted from Openstax, University Physics Volume 1, Section 14.7: Viscosity and Turbulence.