Newtonian fluid

Some liquids and gases flow in a steady way.

Water and air are like this. They move the same way every time. This stays true even if you push them hard.

Other things do not work this way. Some get stiff when you stir them. Some get thin and runny.

This steady flow is named after Isaac Newton. He was a famous scientist.

It is easy to study these steady things. They help us understand how the world moves.

Some liquids and gases flow in a steady way. We call these Newtonian fluids. They are named after Isaac Newton. He was a famous scientist.

When these fluids move, they have a special way of acting. They have a trait called viscosity. Viscosity is how much a fluid resists flowing. In a Newtonian fluid, this resistance stays the same. It does not matter how fast you stir them. It does not matter how hard you push them. The way they flow stays constant.

Water and air are good examples. They act this way in our daily lives. Other liquids do not follow this rule. These are called non-Newtonian fluids. Some get stiff when you stir them fast. This is called a dilatant fluid. Other fluids get thinner when you move them. This is called a pseudoplastic fluid. Non-drip paint is one example. It gets thinner when you use it. Blood is another type of fluid that is not Newtonian. Scientists use Newtonian fluids to make math models. These models help them study how the world moves.

Have you ever wondered why water flows so smoothly? Scientists study how liquids and gases move using special rules. One very important group is called Newtonian fluids. These fluids are special because they have a steady way of flowing. Their thickness, or viscosity, stays the same no matter how hard you stir them. This makes them very helpful for scientists to study. They are the easiest way to create math models for how things move.

To understand these fluids, we look at how they change shape. When a fluid flows, it feels forces from the parts around it. This creates something called viscous stress. This stress causes the fluid to slowly deform, or change its shape, over time. In a Newtonian fluid, this stress is always linked to the strain rate. The strain rate is just a way to measure how fast the shape is changing. Because this link stays constant, the fluid acts very predictably.

We know about these fluids because of a famous scientist named Isaac Newton. He was the first person to use math to explain this relationship. He used a special kind of math called a differential equation. This helped him show how the rate of shape change relates to the force applied. His work allowed people to name these fluids after him. Even today, his ideas help us understand how everything from air to oil moves.

There are many real examples of these fluids in our world. Water and air are the most common ones we see every day. Other examples include alcohol, glycerol, and thin motor oil. Most liquids made of small molecules act this way. However, not every fluid is Newtonian. Some fluids, like blood, are actually non-Newtonian. Others, like oobleck, get stiffer when you move them quickly.

Think about how different liquids feel when you touch them. If you stir water fast, it still feels like water. This is because its viscosity does not change with speed. But if you stir a non-Newtonian fluid, it might act very differently. Non-drip paint is an example that gets thinner when you use it. This is called a pseudoplastic fluid. Knowing these differences helps engineers build better machines and tools.

A Newtonian fluid is a type of liquid or gas that flows in a very predictable way. In these fluids, the internal forces that cause them to flow are directly linked to how fast they are being deformed. This relationship is known as a linear correlation between viscous stress and the local strain rate. The strain rate describes how the shape of a fluid element changes over time. Because this connection remains constant, Newtonian fluids are the simplest mathematical models used to study viscosity. Viscosity is essentially the fluid's resistance to flowing or changing shape.

To understand the mechanism, we must look at how forces act on a fluid element. As a fluid moves, its parts endure viscous stress forces from the surrounding material. These forces cause the fluid to undergo deformation, which is a change in shape. Scientists use a mathematical tool called a strain rate tensor to measure this change. In a Newtonian fluid, the viscous stress and the strain rate are related by a constant viscosity tensor. This means the fluid's thickness does not change based on how much stress is applied or how fast it moves.

There are different ways to categorize these fluids based on their properties. If a fluid is isotropic, its mechanical properties are the same in every direction. For an isotropic Newtonian fluid, the complex viscosity tensor simplifies into just two real coefficients. One coefficient describes the resistance to continuous shear deformation, which is sliding layers of fluid past each other. The other describes the resistance to continuous compression or expansion. This distinction is important when scientists calculate how fluids behave under different pressures.

We can also look at these fluids through the lens of compressibility. In an incompressible flow, the volume of the fluid elements remains constant. For these fluids, the stress is often described by the dynamic viscosity, which relates shear stress to the velocity gradient. However, in a general compressible case, the math becomes more complex. Here, scientists must account for bulk viscosity, which relates to how the fluid handles changes in volume. This is especially important when studying things like sound waves or shock waves.

The study of these fluids is named after the scientist Isaac Newton. He was the first to use a differential equation to describe the relationship between shear strain rate and shear stress. This mathematical rule is often called the Newton law of viscosity. His work provided the foundation for modern fluid mechanics. By using his equations, researchers can predict how different substances will move in various environments. This has been vital for everything from engineering engines to understanding the atmosphere.

While many substances act like Newtonian fluids, many others do not. Non-Newtonian fluids are substances where the viscosity changes depending on the force applied. For example, oobleck is a dilatant fluid, meaning it becomes stiffer when it is vigorously sheared. On the other hand, non-drip paint is a pseudoplastic fluid, which becomes thinner when it is sheared. Other complex examples include blood, molten polymers, and many solid suspensions. These fluids do not follow the simple linear rules that Newton first described.

Common examples of Newtonian fluids include water, air, alcohol, and glycerol. Thin motor oil also behaves this way under the conditions we encounter in everyday life. Most single-phase fluids made of small molecules tend to be Newtonian. Scientists use the power law model to compare these different behaviors. In this model, a power law index of exactly one represents a Newtonian fluid. If the index is less than one, the fluid is pseudoplastic, and if it is greater than one, the fluid is dilatant.

Understanding these fluid types is essential for many scientific fields. In thermal hydraulics, engineers use specific equations of state to manage how fluids move under pressure and temperature changes. In acoustics, the second viscosity coefficient helps explain how sound waves are absorbed by a fluid. Even the Stokes hypothesis is used to simplify calculations by assuming certain viscosity terms disappear in specific scenarios. By distinguishing between Newtonian and non-Newtonian behaviors, we can better master the physical world around us.