Viscosity is a fundamental property of any fluid—liquid or gas—that quantifies its innate resistance to flow or deformation. This physical characteristic determines how easily a substance moves when a force is applied, such as when a liquid is poured or stirred. The informal concept of a liquid being “thick” directly correlates with its viscosity; a higher viscosity means the fluid is more resistant to movement, leading to a slower flow rate. For instance, a liquid with high viscosity will drain from a container far more slowly than a thin liquid with low viscosity.
The Scientific Meaning of Resistance to Flow
A fluid’s resistance to movement is scientifically explained by internal friction, which occurs between adjacent layers of the moving fluid. When a fluid is set in motion, its layers slide past one another at different speeds, creating a velocity gradient across the flow. The resulting friction between these layers is known as shear stress, and viscosity defines the magnitude of this resistance.
At a molecular level, this internal friction is governed by the cohesive forces and the physical entanglement between the fluid’s molecules. Fluids composed of large, complex molecules or those with strong intermolecular attractions exhibit greater resistance because their molecules cling together more tightly. This strong attraction requires a greater force to separate the layers and allow them to flow relative to one another.
For example, long-chain hydrocarbon molecules found in heavy oils can easily become entangled, significantly increasing the fluid’s internal resistance. This interaction explains why some substances are inherently more sluggish than others, even when they have similar densities. Understanding this mechanism of resistance is central to rheology, the study of the flow of matter.
Measuring Viscosity
Scientists use specialized tools to measure viscosity quantitatively, primarily the viscometer or, for more complex fluids, the rheometer. These instruments operate by measuring the force, or torque, required to move an object through the fluid or by timing the fluid’s flow rate through a precise opening.
The standard international unit for dynamic viscosity, sometimes called absolute viscosity, is the pascal-second (Pa·s). Another commonly encountered unit is the poise (P), or more frequently, the centipoise (cP), which is one-hundredth of a poise. Water at 20 degrees Celsius has a dynamic viscosity of approximately one centipoise, making it a common reference point for comparison.
Scientists also measure kinematic viscosity, which is the dynamic viscosity divided by the fluid’s density. This measurement is often expressed in units called centistokes (cSt) and is useful for measuring flow under the influence of gravity. While dynamic viscosity represents the actual internal force resisting flow, kinematic viscosity provides a measure of how quickly a fluid will flow when only its own weight is driving the movement.
Viscosity in Everyday Substances
The contrast between high and low viscosity fluids is easily observed in common household substances. Low-viscosity fluids, such as water or rubbing alcohol, flow rapidly and offer minimal resistance to stirring or pouring. In contrast, high-viscosity liquids, like molasses or motor oil, exhibit significant internal friction, resulting in a noticeably slow, drawn-out flow.
Temperature is a primary external factor affecting a liquid’s viscosity. For most liquids, viscosity decreases dramatically as the temperature rises, a change driven by increased molecular kinetic energy. Higher temperatures cause the molecules to move faster, weakening the cohesive forces and allowing the layers to slide past each other more easily.
A practical example is honey, which is highly viscous when cold, pouring sluggishly from a jar. When the same honey is warmed, its viscosity decreases substantially, causing it to flow much more freely. This temperature dependence is an important consideration in many industrial applications, such as the formulation of engine lubricants, where viscosity must be carefully controlled across a wide range of operating temperatures.