What Is Fluids;types, properties, characteristics and examples

We explain what fluids are, how they are classified and some examples. Also, what are its general characteristics and physical properties.

What are fluids?

Matter composed of molecules weakly attracted to each other is called fluid , so that it does not have the ability to hold its specific shape, but rather acquires that of the container in which it is contained. In this it is distinguished from solids , whose particles do not change position so easily, but resist displacement.

In principle, both gases and liquids can be classified as fluids , since neither retains its specific shape. But there are differences between them, since gases have even less attraction between their particles, which allows them to be compressed, something that with liquids cannot be done . Despite this, the principles of fluidity (static and dynamic) apply to both.

See also: Separation of mixtures

Why do fluids flow?

How much they flow will depend on the viscosity of the fluids themselves.

Fluids flow because the force that holds their particles together is strong enough to hold them together, but not to maintain a certain rigidity or to maintain a shape memory.

That is, the fluids do not have a certain , fixed shape , but they take the shape of whatever supports them: a glass, a bucket, a plate, or a U-shaped tube.

Then, as the fluid particles must be kept together but cannot resist change, the action of some continuous force on them (such as gravity ) causes them to continuously deform until they move from place, being able to flow from one container to another, from a container to the ground , etc. How much they flow in that direction will depend on their viscosity .

What examples of fluids are there?

Some simple examples of fluids are: the water , oil, air , alcohol, volcanic magma (lava) , tomato sauce, the paint , the noble gases (neon, xenon, krypton, helium, etc.), the blood , wet mixtures of water with flour or water with cement.

How are fluids classified?

Non-Newtonian fluids flow as more or less dense liquids.

Fluids can be of three types:

  • Newtonian fluids. Those who submit to the laws of simple mechanics, as established by Isaac Newton in his studies . They are, if you will, simple and ordinary fluids, like water.
  • Superfluids. Also called “perfect fluids”, they are characterized by a total lack of viscosity, that is to say, to flow at the lowest applied force without offering resistance, that is, without friction. This type of fluid is of synthetic origin.
  • Non-Newtonian fluids. It is an intermediate type between fluid and solid, depending on its temperature and shear stress conditions. Thus, it will not have a unique viscosity, but will depend on the forces that impact it: if it is subjected to a sudden force , it will react as a solid, offering resistance; while if it is left to rest it will flow as a more or less dense liquid.

What are its physical properties of fluids?

Density is the indicator of how much mass is in a body.

Fluids have the following physical properties:

  • Viscosity. It is the friction that fluids offer when their particles are set in motion by some force and that tends to impede fluidity. For example, a substance such as tar is highly viscous and will flow much slower and more difficultly than a low-viscosity substance such as alcohol or water.
  • Density. It is an indicator of how close the matter is, that is, how much mass is in a body. Fluids have a higher or lower density, according to the number of particles in the same volume of fluid.
  • Volume. This is the amount of three-dimensional space that the fluid occupies in a given region, considering length, height, and width. Liquids have a specific volume, while gases have the volume of the container that contains them.
  • Pressure. Fluid pressure is the force that its mass exerts on the bodies within it: an object that falls to the bottom of a lake will have the weight of the entire volume of water on it, which translates into greater pressure than being on the surface. On the seabed the pressure is many times greater than that of the Earth’s atmosphere , for example.
  • Capillarity. This intermolecular cohesion force of the fluids allows them to rise through a capillary tube, against gravity, since their internal attraction is much greater than the attraction of their particles by the tube material. This is due in part to surface tension.

What is Pascal’s Law?

This principle, discovered by Blaise Pascal in the 17th century , dictates that a pressure change applied to a liquid enclosed in a container is transmitted equally to all points of the fluid and all the walls of the container.

This law is known as the Pascal Principle and is extremely useful in hydraulics , which uses fluids as a mechanical tool to achieve movement.

What is surface tension?

The liquid’s molecules attract enough to exert some resistance.

Surface tension is a unique property of liquids, allowing it to resist penetration of its surface by a light object , keeping it completely out of the liquid, as is the case with insects that can move or stay above water.

This is because the liquid has a resistance to increase its surface , that is, the molecules of the liquid attract enough to exert some resistance to displacement.

What is thrust?

When an object or a body is under a fluid, for example, submerged in water, its weight is a force that, by gravity, pulls it down, overcoming the pressure that the fluid exerts on it at all its submerged points and measured against a similar force exerted by the column of fluid under the body, known as thrust.

If an object thrown into the water sinks, it is because its weight overcomes the thrust with which the liquid counteracts its mass; whereas if the object remains floating, it is because the thrust is equal to or greater than its own weight.

This is why it costs less to lift objects underwater than from the surface: the force of the fluid’s outward movement must be added to our force.

What differentiates liquid fluids from gases?

The gases are devoid of volume and can be compressed.

Liquid and gaseous fluids are not the same. The former have their own volume and are incompressible , unlike the latter, devoid of volume and compressible (in fact, this is how liquefied gases are made: they are compressed until they are forced to change to a liquid).

By varying the temperature and pressure, as is known, a fluid can be forced to switch between these two states , without losing its fluidity.

What is Hydraulics?

This is the name given to the branch of physics that studies the behavior of liquids based on their specific properties, subjecting them to forces and conditions that allow their behavior to be predicted and used conscientiously in favor of obtaining a result.

What is fluid mechanics?

Fluid mechanics studies their reaction to the environment and to forces.

Unlike hydraulics, this branch of physics is interested in all fluids, not just liquids , as well as their reaction to the environment that limits them and to the shear forces to which they are subjected.

This study starts from the principle that fluids are governed by two constants : a) the conservation of mass and momentum and b) the first and second laws of thermodynamics.

The properties of fluids



The properties of fluids

A fluid can be defined as an easily deformable system, which therefore does not have its own shape but assumes that of the container that contains it. The material is presented, normally, in three states of aggregation: the solid, the liquid and the gaseous state; solids are characterized by a defined volume and shape, liquids have their own volume, but not their own shape, while gases have neither their own shape nor volume. Liquids and gases collectively belong to the category of fluids, with the difference that liquids, compared to gases, are characterized by much more intense cohesive forces between the constituent molecules.


The liquid molecules are in mutual contact, although they can flow over each other, while the gas molecules are separated from each other. For this reason, liquids generally have a higher density than gases (density, is defined as the ratio between the mass and volume of the fluid).


The physical quantity that measures the resistance that the particles of a fluid encounter in flowing over each other is viscosity, which can be considered as the internal friction of the fluid molecules. Viscosity also occurs when a fluid flows on a solid surface, or a solid moves within a fluid, and is greater for liquids than for gases. Viscosity depends on temperature: in gases it increases with temperature, since the thermal motion between the gas particles increases, while in liquids temperature and viscosity are inversely proportional, because increasing the temperature decreases the cohesion between the molecules. Viscosity depends on the speed of the fluid in motion and can be described by a law due to Newton. Consider a fluid contained in a container,v is the difference in speed of the two layers andx their distance, the force, F , which opposes the sliding, called viscous resistance, is given by:

where A is the contact surface anda proportionality factor called the viscosity coefficient, different from one fluid to another.

In the International System the viscosity coefficient is measured in Ns / m 2 , but the unit called poise (symbol P) is more used, where 1 P = 0.1 Ns / m 2 . The unit of the International System is therefore the decapoise (daP), where 1 daP = 1 Ns / m 2 . The fluids that perfectly follow Newton’s law, given by the above formula, are called Newtonians (water, glycerin, alcohol, mercury etc.), while in some liquids a different behavior is observed (for example, the dependence on the time of viscosity) . Viscosity is measured with instruments called viscometers, which exploit the flow of fluids in capillary tubes of very small diameter, or the falling motion of mass and diameter balls known in containers that contain the test substance.


In the case of fluids the dynamic concept of force is no longer sufficient. By applying a force on a point of a fluid, unlike what happens for a solid body, the molecules of the fluid flow on each other, but the fluid as a whole does not undergo an acceleration. To obtain the same dynamic result as a force in a fluid, the force must be distributed to all points on the surface of the fluid: for example, a mass of water can be moved by pushing it with the entire surface of the hands. For this purpose a new quantity is introduced, the pressure, p , defined as the ratio between the value of the force F , perpendicular to the surface S , and the surface itself:

Pressure has the dimensions of a force per unit area and its unit of measurement in the International System is pascal (symbol Pa), where 1 Pa = 1 N / 1 m 2 .

It can be shown that in a fluid the pressure is transmitted uniformly to all its points. This discovery is due to the French scientist Blaise Pascal (1623-1662), in whose honor the unit of measurement of pressure was named. Consider an instrument consisting of a cylinder that contains a fluid, closed by a plunger, into which a balloon filled with air is inserted: by exerting pressure on the plunger, the balloon becomes smaller, while maintaining its shape unchanged. This means that the pressure exerted on the fluid through the piston acted on every point of the surface of the balloon, perpendicular to the surface itself.

Compressibility of fluids

The compressibility of a fluid is defined as its ability to decrease in volume when subjected to external pressure. If a fluid occupies a volume V at a given pressure p and is subjected to a compressionp , its volume will undergo a decreaseV given by:

where k represents the compressibility coefficient. The compressibility coefficient depends on the pressure and for liquids it is generally very small, while it is much higher for gases. Liquids are therefore almost incompressible, while gases are easily compressible.

Surface tension

Surface tension is the cohesive force exerted between the surface molecules of a liquid. It is due to the fact that, while on a molecule internal to the liquid the forces exerted by the other molecules are symmetrical in all directions, on those on the surface only lateral and internal forces of the liquid act. Therefore, on the molecules that are on the surface acts a resulting non-zero force directed downwards, precisely the surface tension, which causes the surface of the liquid, to a certain extent, to behave like an elastic membrane. The formation of drops and bubbles is due to surface tension. The surface tension tends to minimize the surface of a drop and for this reason the soap bubbles are spherical (in fact, for the same volume, the sphere is the solid with a smaller surface). Bubbles can also form in water, but since pure water has a much greater surface tension than soapy water, bubbles would have such small dimensions that they cannot be observed. The surface tension decreases with increasing temperature.

One of the effects of surface tension is the formation of menisci on the free surface of a liquid contained in a container: due to the surface tension, the surface of the liquid is not perfectly flat, but tends to take on a characteristic curved shape (called meniscus ), with a concavity (concave meniscus) or convexity (convex meniscus) facing upwards, depending on whether the liquid wets or does not wet the walls of the container, i.e. depending on whether the adhesion forces between the molecules of the liquid and the vessel or cohesive forces between the molecules of the liquid.

If the liquid is contained in a very thin tube, with an internal diameter of less than 0.1 mm, called the capillary, the surface tension forces are very evident. In capillaries, in fact, the free surface of the liquid is so small that the meniscus phenomenon affects practically the entire surface. The capillary also has another characteristic: if a capillary is immersed in a liquid contained in a container, the level of the liquid in the capillary does not reach the same level as the liquid in the container, as would happen for a larger tube, but undergoes a abnormal rise due to surface tension. If the liquid forms a concave meniscus, the liquid level undergoes an increase (fig. 10.1 A); if the meniscus is convex, the fluid level drops (fig. 10.1 B). L’h is directly proportional to the surface tensionof the liquid and inversely proportional to the densityof the liquid and the radius r of the capillary:

where g = 9.8 m / s 2 is the acceleration of gravity. The phenomenon, called capillarity, is found, for example, in sponges and absorbent papers and takes on particular importance in nature in the ascent of the sap along the stems of plants, which occurs against the force of gravity, and in the peripheral circulation of the blood.


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