Viscosity is a fundamental property of fluids, including both liquids and gases, that determines how they move. A viscous fluid exhibits a high resistance to flow, often informally described as its “thickness” or “stickiness.” This resistance dictates the speed and manner in which a fluid deforms or flows when subjected to an external force, such as pouring or stirring. The concept of viscosity is central to countless natural and industrial processes, from the flow of blood to the movement of motor oil in an engine.
Understanding Internal Friction
Viscosity is scientifically defined as the measure of a fluid’s internal friction. This friction arises from cohesive forces between molecules in a liquid or momentum transfer due to molecular collisions in a gas. When a fluid flows, it moves in theoretical layers that slide past one another, a phenomenon known as laminar flow.
The layer closest to a stationary surface effectively sticks to it, having near-zero velocity. As the distance from that surface increases, the velocity of the fluid layers increases, creating a velocity gradient. To move faster layers past slower layers, a shear stress, or tangential force, is required to overcome the internal friction.
This internal resistance acts as a drag force that opposes the motion. A fluid with high viscosity, like honey or tar, has strong internal friction, meaning a greater force is needed to maintain its movement. Conversely, a low-viscosity fluid, such as water or air, flows easily with minimal applied force.
Measuring Resistance to Flow
Viscosity is a quantifiable property that allows for precise comparison between different fluids. This resistance is quantified by measuring the relationship between the shear stress and the rate of shear strain within the fluid. The standard International System of Units (SI) for dynamic viscosity is the pascal-second (Pa·s), which represents the force per unit area required to move one layer of fluid past another at a specific speed.
An older, non-SI unit still commonly used is the poise (P), or the centipoise (cP), where one centipoise equals one millipascal-second. Dynamic viscosity, or absolute viscosity, measures the fluid’s direct resistance to flow. Kinematic viscosity is a related measure that factors in the fluid’s density and is expressed in units like square meters per second (\(\text{m}^2/\text{s}\)) or centistokes (cSt).
Specialized instruments called viscometers or rheometers are used to determine these values accurately. Different types of viscometers exist, including falling sphere viscometers, which measure the time it takes for a sphere to fall through a fluid, and rotational viscometers, which measure the torque required to spin an object within the fluid. These devices provide data necessary for quality control and engineering calculations, such as sizing pumps and pipelines.
Factors That Modify Viscosity
A fluid’s viscosity is significantly influenced by external conditions, with temperature being the most impactful factor. For nearly all liquids, an increase in temperature causes the viscosity to decrease. Heating a liquid increases the kinetic energy of its molecules, weakening cohesive forces and allowing them to slide past one another more easily. This is why motor oil flows more freely when an engine is hot than when it is cold.
The opposite effect is observed in gases, where an increase in temperature causes the viscosity to increase. In gases, viscosity is primarily due to the transfer of momentum during molecular collisions. Higher temperatures lead to faster molecular movement and more frequent collisions, which increases the internal friction and the viscosity.
Pressure also modifies viscosity, though its effect is less pronounced than temperature. For liquids, very high pressures can slightly increase viscosity because the molecules are forced closer together, increasing intermolecular resistance. The addition of solutes or suspended particles to a liquid also tends to increase its viscosity, as these foreign substances increase the overall internal friction.
Classifying Fluid Behavior
Fluids are classified based on how their viscosity responds to the rate of shear, or the speed at which they are stirred, pumped, or spread. The simplest category is Newtonian fluids, which are characterized by a constant viscosity regardless of the applied shear rate. Common examples include water, air, and most simple oils, where the relationship between shear stress and shear rate is linear.
Non-Newtonian fluids are more complex because their viscosity changes when a force is applied. These fluids do not have a single, constant viscosity value, as their resistance to flow depends directly on the force’s magnitude and duration. Two notable types of non-Newtonian behavior are shear-thinning and shear-thickening.
Shear-thinning fluids, also known as pseudoplastic fluids, decrease in viscosity as the shear rate increases. Ketchup and paint are classic examples; applying force causes the internal structure to temporarily break down, making the fluid flow more easily. Conversely, shear-thickening fluids, or dilatant fluids, exhibit an increase in viscosity when shear stress is applied. A common household example is a cornstarch and water mixture, which acts like a liquid when slowly poured but becomes nearly solid when hit rapidly.