Viscosity is a property shared by all fluids—both liquids and gases—that governs how they move and deform. It is a fundamental characteristic measuring a fluid’s resistance to being stirred or poured, often described as its “thickness.” Understanding viscosity has immense practical consequences, from formulating paint consistency to ensuring optimal performance of motor oil in a car’s engine.
Defining Viscosity: The Concept of Internal Friction
Viscosity is scientifically defined as a fluid’s resistance to flow and shear stress. When a fluid moves, different layers within it travel at varying speeds, causing molecules to exert a drag force on one another. This opposition to relative motion between adjacent fluid layers is known as internal friction, which is the physical basis of viscosity.
A highly viscous liquid, such as molasses, exhibits high internal friction because its molecules strongly resist sliding past one another. Conversely, a low-viscosity fluid like water has little internal friction, allowing its layers to slip by easily and resulting in rapid flow. This resistance is directly related to the cohesive forces between a liquid’s molecules; stronger intermolecular forces result in higher viscosity.
Understanding Viscosity Measurement and Units
Viscosity is quantified using a specialized instrument called a viscometer. Scientists distinguish between two primary types of viscosity: dynamic and kinematic. Dynamic viscosity, also known as absolute viscosity, measures the force required to make a fluid flow at a certain rate. Its standard SI unit is the pascal-second (Pa·s), though the centipoise (cP) is also frequently used.
Kinematic viscosity is the ratio of dynamic viscosity to the fluid’s density. This measurement indicates how fast a fluid will flow under the force of gravity alone. The CGS unit for kinematic viscosity is the Stoke, with centistokes (cSt) being the more practical unit for common liquids.
Variables That Change a Liquid’s Viscosity
The viscosity of a liquid is not constant and can be significantly altered by outside factors, particularly temperature. For most liquids, viscosity and temperature have an inverse relationship: as temperature increases, viscosity decreases. Heating a liquid gives molecules greater kinetic energy, helping them overcome intermolecular forces and allowing layers to slide more freely. This is why motor oil is thicker when an engine is cold but thins out once the engine reaches operating temperature.
Molecular structure also plays an important part in a liquid’s inherent viscosity. Liquids composed of long, chain-like molecules, such as heavy oils or polymers, tend to have higher viscosities because these chains can become tangled. This molecular entanglement creates greater resistance to flow than that found in liquids with small or spherical molecules. Furthermore, the presence of strong intermolecular forces like hydrogen bonds significantly increases viscosity, as these attractions make it harder for molecules to separate and move.