# What are fluids?

The term fluid is used to designate all matter composed of molecules that attract each other through a very weak force of attraction. This prevents them from maintaining a specific, stable shape, differentiating them from solid materials that do have this property.

Physically, fluids can be liquids or gases since neither of these can retain a stable form. However, the difference between them is that the former adapts to the shape of its container, while the latter has less attraction between its particles, so they can be compressed therefore lacking both volume and shape.

## What characteristics do fluids have?

Fluids, regardless of what they are, all share certain physical properties, such as:

• Shape-shifting: no restorative force that requires them to recover the mass’s original composition, differentiating them from a malleable solid, which does have restorative forces.
• Molecular dispersion: they can move from one state to another, and due to their fluidity, this dispersion can be concentrated if an external variable, such as temperature, is applied.
• Density: they can have a higher or lower density depending on the amount of matter in a fluid’s volume. The density is usually represented in kg/m3.
• Viscosity: this has to do with the resistance that fluids have against being shaped, thus limiting their fluidity.
• Volume: the space occupied by a fluid in a given space, found with the height, width, and length. Liquid fluids usually take the shape of their container. Gases, on the other hand, have no volume or shape, so they will expand as much as possible in the space containing them.
• Pressure: the force that the fluid’s mass exerts on bodies immersed in it. For instance, the pressure on a seabed is much higher than on land.

## How are fluids classified?

Fluids can be:

1. Real fluids: these are all viscous or compressible; the position of their molecules can change continuously.
2. Newtonian fluids: these have a constant viscosity, which doesn’t vary depending on any force applied to them. Newton’s laws of motion explain the behavior of this type of fluid.
3. Non-Newtonian fluids: these do not have a unique viscosity since it depends on the force exerted on them, as well as temperature changes, so they can be in an intermediate state between a liquid and a solid.
4. Superfluids: these are characterized by the fact that they can flow under the slightest force applied to them without any resistance or friction.

## What is the difference between liquid fluids and gaseous fluids?

While both liquids and gases are considered fluids, for a  liquid, intermolecular forces facilitate the movement of particles. They also have a fixed volume. When a liquid is poured into a container, it takes up a volume less than or equal to that of the container.

On the other hand, gases have separate particles in motion, which collide with each other and scatter; this is why they don’t have a defined volume. This way, they tend to occupy the largest possible volume of the container and are highly expandable.

Gases are also compressible since their volume decreases when forces are applied to them. Liquids, however, don’t decrease in volume when a very large force is exerted on them, so they are considered incompressible. Liquid fluids also usually exert pressure (known as hydrostatic pressure) on bodies submerged in them or on the walls of container vessels.

## What is fluid mechanics?

Fluid mechanics is a branch of continuum mechanics and studies the behavior and movement of fluids, both at rest (fluid statics) and in motion (fluid dynamics), as well as their interaction with the container or contour limiting them.

## What are some examples of fluids?

• Blood plasma: a non-Newtonian fluid whose viscosity increases with applied shear stress but which stops increasing progressively as velocity increases.
• Helium: lacks viscosity and is considered a superfluid.
• Mercury: a Newtonian fluid, the only liquid metal at room temperature.