Viscosity is a fundamental property of fluids that describes their internal resistance to flow, often informally called “thickness.” High-viscosity fluids like honey flow much slower than low-viscosity fluids like water. At a molecular level, viscosity is a manifestation of the internal friction created when layers of fluid move past one another. Most common liquids display a predictable relationship between the force applied and the resulting flow, but a peculiar class of substances behaves in a much more dynamic way.
Viscosity and Non-Newtonian Fluids
The way a fluid’s viscosity reacts to an applied force, or shear stress, determines its classification. Fluids that follow Isaac Newton’s law of viscosity are known as Newtonian fluids, which include substances like water or simple oils. For these fluids, the viscosity remains constant regardless of how fast they are stirred or moved, provided the temperature is stable.
Fluids that deviate from this constant relationship are classified as non-Newtonian fluids because their viscosity is not fixed; it changes depending on the shear rate, which is the speed at which the fluid is deformed. Shear thinning is the most common type of non-Newtonian behavior observed in complex fluids. This phenomenon is specifically defined as a reduction in a fluid’s apparent viscosity as the applied shear rate increases.
The fluid appears thick and resistant to flow when undisturbed, but becomes noticeably thinner and flows more easily when a force is applied. Shear-thinning fluids, also called pseudoplastic fluids, will pour more readily the harder or faster it is agitated. The observed change is a direct result of the internal structure of these complex materials responding to mechanical stress.
The Physical Mechanism of Shear Thinning
The structural components of shear-thinning fluids are typically large, anisotropic molecules, such as long-chain polymers or colloidal particles suspended in a liquid. When the fluid is at rest, these large internal structures are randomly oriented and highly entangled, forming a dense, chaotic network. This tangled state causes high viscosity and resistance to flow when the fluid is stationary. The internal friction between the many random contact points slows the movement of the fluid layers.
When an external force is applied, initiating flow, the shear stress begins to influence the orientation of these complex structures. The long molecular chains or elongated particles start to disentangle and align themselves longitudinally, parallel to the direction of the flow. The alignment dramatically decreases the number of physical entanglements and the hydrodynamic drag between the molecules or particles. With less internal friction and fewer structural obstacles, the fluid layers can slide past one another much more easily, which is perceived as a significant drop in viscosity. Once the mechanical stress is removed, the random Brownian motion of the molecules causes them to gradually return to their initial, entangled state, and the fluid’s viscosity quickly recovers.
Practical Examples and Relevance
Shear thinning is a property deliberately engineered into many common products for practical utility. Modern wall paint, for example, must be thick enough to cling to a brush and not drip off the wall when stationary, yet it must flow easily when the brush applies the shear stress. The shear force from the brushing action lowers the paint’s viscosity, allowing a smooth, even coating. The viscosity immediately recovers upon removal of the brush to prevent running or sagging.
Ketchup is a classic food example; the fluid is viscous and difficult to pour when it is at rest in the bottle. Shaking or striking the bottle applies the necessary shear stress, which temporarily decreases the viscosity and allows the condiment to flow easily. Biological fluids also exhibit this behavior, most notably human blood, which is a suspension of cells and plasma. The shear thinning property allows blood to reduce its resistance as it moves through the body’s smaller vessels. Other everyday products like whipped cream, cosmetics, and hydrogels for bioprinting rely on this property to transition between a storage-stable, thick state and a low-viscosity, functional state for application or extrusion.