Viscosity measures the resistance a fluid offers to flow and deformation. It results from internal friction between layers of fluid moving relative to one another. Dynamic viscosity, denoted by the Greek letter mu (μ), quantifies the force needed to move one fluid layer over another. For Newtonian fluids like water and air, the relationship between the shearing stress and the rate of shearing strain is linear, meaning their viscosity remains constant regardless of the applied stress. Non-Newtonian fluids, such as ketchup, toothpaste, and quicksand, exhibit a viscosity that changes with varying shear rates.

Temperature significantly affects viscosity. Higher temperatures decrease viscosity for liquids because the intermolecular forces weaken, allowing the fluid to flow more easily. An example is heated syrup, which pours more readily than when it is cold. Conversely, higher temperatures increase viscosity in gases, as molecular activity and momentum exchange between layers rise, similar to how warm air feels thicker in a sauna.

Accurately predicting fluid behavior requires considering these temperature effects, which is crucial in designing systems like pipelines and car engines. In pipelines, the oil must flow smoothly, while in car engines, the correct oil viscosity ensures proper lubrication and efficiency. This understanding helps engineers design and operate systems more effectively under varying temperatures.