Non-Newtonian fluid

In the fields of fluid mechanics and physical chemistry, substances are often classified by how they flow. Most common liquids, like water, follow Newton's law of viscosity. This means they have a constant viscosity, which is a measure of how much a fluid resists flowing. In these Newtonian fluids, the relationship between shear stress and the shear rate is linear. However, many substances do not follow this predictable pattern. These are known as non-Newtonian fluids. In these substances, the viscosity is variable and depends on the stress applied to them.

To understand how these fluids work, we must look at the relationship between shear stress and shear rate. Shear stress is the force applied to the fluid, while the shear rate describes how fast the fluid is deforming. In a Newtonian fluid, the viscosity remains a constant coefficient of proportionality. In a non-Newtonian fluid, this coefficient cannot be defined because the viscosity changes. Some fluids change their thickness instantly when force is applied. Others exhibit time-dependent viscosity, meaning the thickness changes based on how long the force is held. Because of this complexity, scientists often use rheometers to study these materials. These devices measure rheological properties, such as how stress and strain rate tensors interact under different flow conditions.

Non-Newtonian fluids can be categorized into several distinct types based on their behavior. One major group is time-independent fluids. Within this group, we find pseudoplastic and dilatant flows. Pseudoplastic fluids are also called shear-thinning fluids. Their apparent viscosity decreases as the shear stress increases. A common example is ketchup, which becomes runnier when you shake the bottle. Another example is blood, which thins out to flow more easily through the body.

Dilatant fluids are the opposite; they are known as shear-thickening fluids. In these substances, the apparent viscosity increases when the shear rate increases. A famous example is "oobleck," a suspension of corn starch in water. If you stir oobleck slowly, it behaves like a liquid. If you strike it with great force, it thickens and acts like a solid.

There are also Bingham plastics, such as toothpaste or mayonnaise. These fluids require a finite yield stress before they begin to flow at all. While still, a Bingham plastic can even hold peaks or shapes on its surface.

Another way to classify these fluids is by their time-dependency. Thixotropic fluids are time-dependent shear-thinning fluids. Their viscosity decreases the longer they are subjected to stress. Examples include yogurt, peanut butter, and many types of paint. Thixotropic fluids are significantly different from purely pseudoplastic fluids because of this time element. On the other hand, rheopectic fluids are time-dependent shear-thickening fluids. Their apparent viscosity increases with the duration of the applied stress. Examples of rheopectic behavior include synovial fluid, printer ink, and gypsum paste.

Some materials exhibit even more complex characteristics, such as viscoelasticity. Viscoelastic substances, like Silly Putty or whipped cream, show a combination of elastic and viscous effects. They can behave like a liquid or a solid depending on the rate of strain. For instance, Silly Putty is a silicone polymer suspension that can flow, bounce, or break depending on the strain rate.

Plant resins are also viscoelastic solids. They may flow slowly to fit a container, but they can shatter like a solid if struck with enough force. This ability to shift between states makes them fascinating to study in continuum mechanics.

These substances are not just laboratory curiosities; they are essential to many systems. In biology, the viscosity of blood is a critical factor for circulation. In industry, the properties of paint must be carefully controlled so it flows from a brush but does not drip excessively from a wall. Even natural phenomena like magma, lava, and quicksand involve non-Newtonian mechanics. Understanding these complex flows helps scientists predict how everything from toothpaste to geological materials will behave in the real world.