Fluid

In the world of physics, a fluid is a substance that can flow. This category includes both liquids and gases. A fluid is defined by its ability to move and deform under an applied shear stress. Shear stress is an external force that tries to make parts of a substance slide past each other. Because fluids have a zero shear modulus, they cannot resist these sliding forces. This means they will continuously change shape when such a force is applied.

To understand how a fluid works, we must look at how it reacts to different types of stress. Solids respond to shear stress with a spring-like restoring force. This means a solid can often return to its original shape after being pushed. Fluids behave very differently. Instead of resisting the force itself, they only resist the rate at which they deform. This resistance is often described in a frictional manner.

Fluids also react to normal stresses, which we call pressure. A fluid can handle compressive stress, which is known as positive pressure. It can also handle tensile stress, which is known as negative pressure. This is a key difference from solids. While both solids and liquids have tensile strength, exceeding it causes different results. In solids, it leads to permanent deformation or fractures. In liquids, it causes a phenomenon called cavitation.

Scientists classify fluids into different types based on their specific behaviors. One way to group them is by their relationship between shear stress and the rate of strain. Newtonian fluids follow Newton's law of viscosity. In these fluids, the stress is directly proportional to the rate of strain. However, non-Newtonian fluids do not follow this rule. In a non-Newtonian fluid, the stress is not proportional to the rate of strain or its derivatives.

Another way to classify fluids is by their compressibility. A compressible fluid is one that changes its volume or density when pressure is applied. This can also happen when the fluid becomes supersonic. In contrast, an incompressible fluid does not change its volume when pressure or flow velocity changes. Water and oil are examples of incompressible fluids. It is important to note that in the real world, perfectly Newtonian or incompressible fluids do not actually exist. Scientists use these terms as theoretical assumptions to help solve complex problems.

Some substances can even appear to be both solids and fluids. For example, Silly Putty is a non-Newtonian fluid that acts like a solid when a sudden force hits it. Pitch is another substance with extremely high viscosity. Because it is so thick, it can behave like a solid, as seen in the pitch drop experiment. Even in particle physics, the concept of a fluid is extended to include matters other than just liquids and gases.

The study of these materials is called fluid mechanics. This field is divided into two main branches. Fluid statics looks at fluids that are not moving. Fluid dynamics focuses on fluids that are in motion. To model these complex movements, scientists use the Navier–Stokes equations. These are a set of partial differential equations. They are based on the conservation of mass, energy, and both linear and angular momentum.

Understanding fluids is vital across many different scientific fields. In medicine and biology, the term "fluid" refers to the liquid constituents of the body. When doctors suggest drinking fluids, they are talking about liquid replacement. In the field of hydraulics, the term refers to liquids with specific properties used to move things. From the air we breathe to the blood in our veins, fluids are essential to how the world works.