Viscosity, often informally referred to as a fluid’s “thickness,” is a fundamental physical property that quantifies its resistance to flow. This resistance arises from internal friction when layers of the fluid move relative to each other, dictating how easily a fluid can be poured, pumped, or spread. Water’s viscosity is a standard baseline reference point in science and engineering because it is one of the most common and well-characterized fluids on Earth. Understanding this property is crucial for fields ranging from fluid dynamics and hydraulic design to biology and climate science.
Understanding Fluid Viscosity
Viscosity measures the internal resistance a fluid exhibits against an applied shear force, which causes the fluid to deform or flow. This property arises from the cohesive forces between the molecules within the fluid, which resist the movement of one molecular layer sliding over another. The greater this internal friction, the higher the viscosity, meaning the fluid flows more slowly.
Scientists distinguish between two primary types of viscosity: dynamic and kinematic.
Dynamic Viscosity
Dynamic viscosity, also known as absolute viscosity, measures the force required to make the fluid flow at a specific rate. The standard International System (SI) unit for dynamic viscosity is the Pascal-second (Pa·s). It is frequently measured in centipoise (cP), where one centipoise equals one millipascal-second (mPa·s).
Kinematic Viscosity
Kinematic viscosity is the dynamic viscosity divided by the fluid’s density. This measurement is often more relevant in situations where gravity drives the flow, such as observing how long it takes a fluid to drain. The SI unit for kinematic viscosity is square meters per second (\(\text{m}^2/\text{s}\)), though centistokes (cSt) are commonly used.
The Measured Viscosity of Water
Water is used as a convenient benchmark for viscosity due to its common occurrence and consistent properties. Scientists measure its dynamic viscosity under specific, controlled conditions to establish a standard value. The accepted standard measurement is taken at a temperature of \(20^{\circ}\text{C}\) and under standard atmospheric pressure.
Under these standard conditions, the dynamic viscosity of pure water is approximately \(1.0016\) millipascal-seconds (\(\text{mPa}\cdot\text{s}\)). This value is almost exactly one centipoise (1 cP), which is why centipoise is a widely adopted unit for measuring the viscosity of many liquids. In SI units, this translates to \(0.0010016\) Pascal-seconds (\(\text{Pa}\cdot\text{s}\)).
This precise and reproducible value makes water an ideal reference substance for calibrating viscometers and comparing the flow properties of other fluids.
How Temperature and Pressure Affect Water’s Viscosity
Temperature is the single most significant factor that influences the viscosity of liquid water. As temperature increases, the viscosity of water decreases substantially, allowing it to flow more freely. For example, the dynamic viscosity of water drops by more than six times between the freezing point (\(0^{\circ}\text{C}\)) and the boiling point (\(100^{\circ}\text{C}\)).
This inverse relationship is explained by the behavior of water molecules and their hydrogen bonds. Water molecules are linked by weak, temporary hydrogen bonds, creating a structured network that resists the relative motion of fluid layers. At lower temperatures, these bonds are more stable and numerous, making the liquid more resistant to flow.
When the temperature rises, the increased thermal energy imparts kinetic energy to the molecules, causing them to move more rapidly. This motion constantly breaks and reforms the hydrogen bonds, effectively weakening the internal cohesive forces. With less internal resistance from this weakened molecular network, the fluid’s layers slide past each other more easily, resulting in lower viscosity.
The effect of pressure on water’s viscosity is far less pronounced than that of temperature for most common applications. At normal pressures, water is largely incompressible, meaning high pressure causes only a minor increase in its viscosity. Significant changes in viscosity due to pressure are generally only observed under extreme conditions, such as those found deep within the Earth or in specialized industrial processes.
Water’s Role Compared to Other Common Fluids
Water’s viscosity is relatively low compared to other common fluids, which allows it to flow easily. Highly viscous fluids like honey or motor oil flow much more slowly because their molecules have stronger or more complex internal interactions. For instance, common table honey can have a dynamic viscosity of around \(10,000\) centipoise, making it roughly ten thousand times more viscous than water.
Motor oils, composed of long-chain hydrocarbons, exhibit significantly higher viscosities, with a common SAE 30 motor oil measuring between \(150\) and \(200\) centipoise. Conversely, fluids like acetone or ethanol have dynamic viscosities slightly lower than water, while gases like air have extremely low viscosities, typically in the range of only \(0.018\) centipoise.
This low viscosity is biologically advantageous, allowing water to serve as an efficient solvent and transport medium in living organisms. Water’s flow characteristics enable the rapid circulation of blood and the efficient transport of nutrients and waste throughout biological systems. In engineering, water’s low viscosity means less energy is required to pump it through pipelines compared to thicker industrial fluids. Its predictable flow behavior is also foundational for modeling fluid dynamics in various applications, from designing hydraulic systems to understanding river flow.