Viscosity is a fundamental property of fluids describing their internal resistance to flow, often informally called “thickness.” It represents the internal friction between layers of a fluid moving relative to one another. Fluids with high viscosity, such as honey or molasses, flow slowly because their molecules strongly resist moving past their neighbors. Conversely, fluids with low viscosity, like water, flow easily and quickly because their internal friction is minimal.
Temperature’s Governing Influence
Temperature is the most commonly observed factor affecting a fluid’s viscosity, though its effect is opposite for liquids and gases. In liquids, an increase in temperature causes viscosity to decrease. Added thermal energy increases the average kinetic energy of the molecules, allowing them to overcome the cohesive forces holding them together, resulting in reduced resistance to flow.
The relationship reverses for gases, where an increase in temperature causes viscosity to increase. Gas viscosity is primarily caused by the transfer of momentum between molecules during random collisions. When the temperature rises, molecules move faster and collide more frequently, leading to a higher rate of momentum transfer between the layers. This heightened molecular activity causes the gas to become more resistant to flow.
Internal Structure and Molecular Composition
The inherent makeup of a fluid dictates its baseline viscosity. The strength of the intermolecular forces (IMFs) between molecules is a primary determinant; stronger attractive forces mean molecules are more tightly bound and resist movement, leading to higher viscosity. For example, liquids capable of forming strong hydrogen bonds, such as water or ethylene glycol, tend to be more viscous than those held together only by weaker forces.
Molecular size and weight also play a significant role, as larger or heavier molecules generally increase a fluid’s viscosity. The shape of the molecule is equally important, particularly for organic substances. Long, chain-like molecules, such as those found in motor oils and polymers, tend to become entangled with their neighbors, which dramatically increases the friction and resistance to flow. Spherical molecules can roll or slide past one another with less entanglement, resulting in a lower viscosity compared to chain-like counterparts.
The Effect of Applied Force
Many common fluids, known as Newtonian fluids, maintain a constant viscosity regardless of how fast they are stirred or pumped. Fluids like water and simple oils maintain a linear relationship between the applied force (shear stress) and the resulting flow rate (shear rate). However, a vast number of fluids are classified as Non-Newtonian because their viscosity changes when a force is applied. This means their resistance to flow depends on the rate of shear.
Non-Newtonian fluids fall into two main groups. Shear-thinning fluids (pseudoplastic) become less viscous when a force is applied. Ketchup is a classic example; force aligns the suspended particles, reducing internal resistance and allowing it to flow easily. Conversely, shear-thickening fluids (dilatants) increase in viscosity and become thicker when stressed. A mixture of cornstarch and water is a well-known shear-thickening fluid, which acts like a liquid when slowly poured but becomes nearly solid when pressed quickly.
Pressure and Other Physical Influences
Pressure is another factor that influences viscosity, although its effect is often minimal for liquids under normal conditions. In most liquids, a substantial increase in pressure is required to produce a noticeable increase in viscosity. This is because molecules are already tightly packed, and compression slightly reduces molecular spacing, increasing internal friction. For gases, viscosity is generally independent of pressure over a wide range, only changing significantly at extreme pressures.
The presence of suspended particles or a high concentration of dissolved matter can dramatically increase a fluid’s viscosity. Adding solid particles or increasing the amount of solute increases the frequency of interactions between the particles and the surrounding fluid. This causes the overall resistance to flow to rise steeply, especially when the volume fraction of the suspended material is high. The flow behavior of these suspensions can also become complex, sometimes exhibiting Non-Newtonian characteristics.