A non-Newtonian fluid is a substance whose resistance to flow, known as viscosity, changes dramatically when a force or stress is applied to it. Unlike common fluids, the thickness of a non-Newtonian substance is not constant but depends on the mechanical action it experiences, such as stirring, shaking, or squeezing. This behavior means these fluids do not conform to the principles of classical fluid dynamics established by Isaac Newton. Their unique flow properties make them a subject of study in rheology, which is the study of the flow and deformation of matter.
Understanding Viscosity and Newtonian Fluids
Viscosity is a measure of a fluid’s internal friction or its resistance to continuous flow. A fluid with high viscosity, like honey, flows slowly, while a fluid with low viscosity, like water, flows quickly. Temperature can change a fluid’s viscosity, but applied force normally does not.
A Newtonian fluid maintains a constant viscosity regardless of the rate of force applied, referred to as the shear rate. If you double the force used to stir water, the water will flow twice as fast, but its inherent thickness remains the same. Water, gasoline, and simple oils are classic examples of Newtonian fluids, maintaining a linear relationship between the force applied (shear stress) and the resulting flow rate (shear rate).
The Defining Feature: Viscosity Changes with Stress
The fundamental difference for a non-Newtonian fluid is that its viscosity is not a fixed property but is variable. This means the apparent thickness of the fluid changes when it is subjected to mechanical stress. When a force is introduced, the internal structure of the fluid begins to reorganize, which alters its resistance to movement.
The mechanism involves structural components within the fluid, typically suspended particles or long-chain molecules. These components interact differently depending on the velocity of the applied force. When the fluid is at rest, these internal components may be randomly oriented or loosely entangled.
Once a force is applied, the fluid’s microstructure realigns or rearranges itself in response to that stress. This rearrangement causes the viscosity to either increase, making the fluid seem thicker, or to decrease, making it appear thinner. This non-linear relationship between shear stress and shear rate is the hallmark of non-Newtonian behavior.
Two Primary Types of Non-Newtonian Behavior
The behavior of non-Newtonian fluids is primarily categorized into two types based on how their viscosity changes with increasing shear stress. The most common type is shear-thinning behavior, also called pseudoplasticity. In these fluids, the apparent viscosity decreases as the shear rate increases, meaning the fluid becomes runnier the harder it is pushed or stirred.
The shear-thinning effect occurs because the long, entangled polymer chains or elongated particles align themselves parallel to the direction of flow when a force is applied. This alignment reduces internal friction and resistance, allowing the fluid to move more freely. Once the force is removed, the particles return to their random, entangled state, and the fluid’s viscosity increases again.
The opposite phenomenon is shear-thickening behavior, also known as dilatancy. In this type, the apparent viscosity increases with increasing shear rate, causing the fluid to become thicker or even solidify under stress. This effect is observed in highly concentrated suspensions of rigid particles.
When a sudden, strong force is applied to a shear-thickening fluid, the particles are forced together into an arrangement that minimizes the space between them, a process called hydrocluster formation or jamming. This rapid packing traps the liquid between the particles, momentarily transforming the mixture into a solid-like state with high resistance to flow. The fluid instantly returns to its liquid state when the stress is relieved.
Everyday Examples of Non-Newtonian Fluids
Many familiar household and biological substances exhibit non-Newtonian behavior, demonstrating these flow characteristics in daily life. Ketchup is a classic example of a shear-thinning fluid; it resists pouring when still, but a quick shake (applying shear stress) momentarily lowers its viscosity, allowing it to flow easily. Paint is also designed to be shear-thinning, flowing smoothly off a brush when applied but rapidly thickening on the wall to prevent drips.
Other common shear-thinning examples include blood, which is crucial for efficient flow through the body’s vascular system, and many cosmetic products like lotions and shampoos. These materials are formulated to be thick when at rest but thin out instantly when squeezed or rubbed. This property allows for easy dispensing and application.
An easily demonstrable example of a shear-thickening fluid is a mixture of cornstarch and water, often called “Oobleck.” If you slowly sink your finger into the mixture, it acts like a liquid, but if you quickly punch the surface, it feels hard like a solid. This principle is sometimes utilized in specialized protective gear, such as impact-resistant armor, which instantly stiffens upon a sharp blow.