7 junio, 2024

Fluids: characteristics, properties, types, examples

The fluids They are continuous media whose molecules are not as bound as in solids, and therefore have greater mobility. Both liquids and gases are fluids and some, such as air and water, are of vital importance, as they are necessary to sustain life.

Examples of fluids are water, superfluid helium, or blood plasma. There are materials that appear to be solid, but nevertheless exhibit the characteristics of fluids, for example tar. By placing a brick on top of a large piece of tar, it is observed that it sinks little by little until it reaches the bottom.

Some plastics also appear to be solid, but are actually highly viscous fluids capable of flowing extremely slowly.


fluid characteristics

Fluids are mainly characterized by:

-Have a greater separation between its molecules compared to solids. In the case of liquids, the molecules still maintain some cohesion, while in gases they interact much less.

-Flow or drain, when shear forces act on them. Fluids do not resist stresses, therefore they deform continuously and permanently when one is applied to them.

-Adapt to the shape of the container that contains them and if they are gases, they immediately expand to cover the entire volume of the container. Furthermore, if they can, the molecules will quickly escape the container.

-Gases are easily compressible, that is, their volume can be easily changed. On the other hand, to change the volume of a liquid requires more effort, which is why they are considered incompressible over a wide range of pressures and temperatures.

-Liquids have a flat free surface when the pressure acting on them is constant. At atmospheric pressure, for example, the surface of a lake without waves is flat.

fluid properties

The macroscopic behavior of a fluid is described by several concepts, the main ones being: density, specific weight, relative density, pressure, compressibility modulus, and viscosity. Let’s see what each one consists of briefly.


In a continuous medium such as a fluid, it is not easy to keep track of a single particle or molecule, so instead of working with the mass of one, it is preferred to work with density, a characteristic that concerns the fluid as a whole.

Density is defined as the ratio of mass to volume. Denoting the density with the Greek letter ρ, the mass m and the volume V:

ρ = m/V

When the density varies from one point to another in the fluid, the expression is used:

ρ = dm/dV

In the International System of units, density is measured in kg/m3.

The density of any substance in general is not constant. All expand when heated, except water, which expands when frozen.

However, in liquids the density remains almost constant over a wide range of pressures and temperatures, although gases do experience variations more easily, since they are more compressible.

Specific weight

The specific weight is defined as the quotient between the magnitude of the weight and the volume. Therefore it is related to density, since the magnitude of the weight is mg. Denoting the specific weight with the Greek letter γ, we have:

γ = mg/V

The unit of specific weight in the International System of Units is the newton/m3 and in terms of density, the specific weight can be expressed as follows:

γ = ρg

Relative density

Water and air are the most important fluids for life, which is why they serve as a standard of comparison for the others.

In liquids, relative density is defined as the ratio between the mass of a portion of fluid and the mass of an equal volume of water (distilled) at 4ºC and 1 atmosphere of pressure.

In practice, it is calculated by making the quotient between the density of the fluid and that of the water under said conditions (1 g/cm3 or 1000 kg/m3), therefore the relative density is a dimensionless quantity.

It is denoted as ρr or sg for the acronym in English of specific gravitywhich translates as specific gravity, another name for relative density:

sg = ρfluid / ρwater

For example, a substance with sg = 2.5 is 2.5 times heavier than water.

In gases, the relative density is defined in the same way, but instead of using water as a reference, the density of air equal to 1.225 kg/m3 at 1 atmosphere of pressure and 15 ºC is used.


A fluid consists of innumerable particles in constant motion, capable of exerting force on a surface, for example that of the container that contains them. The average pressure P that the fluid exerts on any flat surface of area A is defined through the quotient:

P = F┴/A

Where F┴ is the perpendicular component of the force, therefore the pressure is a scalar quantity.

If the force is not constant, or the surface is not flat, then the pressure is defined by:

p = dF/dA

The SI unit of pressure is the newton/m2, called the pascal and abbreviated Pa, after the French physicist Blaise Pascal.

However, in practice many other units are used, either for historical, geographical reasons or also according to the field of study. The units of the British system or imperial system are used very frequently in English-speaking countries. For the pressure in this system we use the psi or pound-force/inch2.


When a portion of fluid is subjected to volume stress, it decreases to some extent. This decrease is proportional to the effort made, being the constant of proportionality the compressibility modulus Or simply compressibility.

If B is the modulus of compressibility, ΔP the change in pressure, and ΔV/V the unit change in volume, then mathematically:

B = ΔP / (ΔV/V)

The unit change of volume is dimensionless, since it is the quotient between two volumes. Thus, compressibility has the same units as pressure.

As stated at the beginning, gases are easily compressible fluids, whereas liquids are not, therefore they have compressibility moduli comparable to those of solids.


A moving fluid can be modeled by thin layers moving relative to each other. Viscosity is the friction between them.

To give movement to the fluid, a shear stress (not very large) is applied to a section, the friction between layers prevents the disturbance from reaching the deeper layers.

In this model, if the force is applied to the surface of the fluid, the velocity decreases linearly in the lower layers until it annuls at the bottom, where the fluid is in contact with the surface at rest of the container that contains it.

Mathematically it is expressed by saying that the magnitude of the shear stress τ is proportional to the variation of speed with depth, which is denoted as Δv/ Δy. The constant of proportionality is the dynamic viscosity μ of the fluid:

τ = μ (Δv/ Δy)

This expression is known as Newton’s law of viscosity, and fluids that follow it (some do not follow this model) are called Newtonian fluids.

In the International System the units of dynamic viscosity are Pa. s, but the poiseabbreviated P, which is equal to 0.1 Pa.s.

Classification: types of fluids

Fluids are classified according to various criteria, the presence or absence of friction is one of them:

ideal fluids

Its density is constant, it is incompressible and its viscosity is zero. It is also irrotational, that is, eddies do not form inside it. And finally it is stationary, which means that all fluid particles that pass through a certain point have the same speed.

real fluids

In the layers of real fluids there is friction and therefore viscosity, they can also be compressible, although as we have said, liquids are incompressible in a wide range of pressures and temperatures.

Another criterion establishes that fluids can be Newtonian or non-Newtonian, depending on the viscosity model they follow:

Newtonian fluids

They obey Newton’s law of viscosity:

τ = μ (Δv/ Δy)

non-newtonian fluids

They do not obey Newton’s law of viscosity, so their behavior is more complex. They are classified in turn into fluids with viscosity independent of time and those with viscosity time dependenteven more complex.

examples of fluids


Water is a Newtonian fluid, although under certain conditions the ideal fluid model describes its behavior very well.

Blood plasma

It is a good example of a time-independent non-Newtonian fluid, specifically pseudoplastic fluids, in which the viscosity increases a lot with the applied shear stress, but then, as the velocity gradient increases, it stops increasing progressively.


The only liquid metal at room temperature is also a Newtonian fluid.


It takes a lot of shear stress to get these kinds of fluids to start flowing. Then the viscosity remains constant. This type of fluid is called bingham fluid. Toothpaste and some paints also fall into this category.


It is a fluid that is used to pave roads and as a waterproofing agent. It has the behavior of a Bingham fluid.

superfluid helium

It is totally devoid of viscosity, but at temperatures close to absolute zero.


Cimbala, C. 2006. Fluid Mechanics, Fundamentals and Applications. Mc. Graw Hill.
Measurement of the viscosity of a liquid. Retrieved from: sc.ehu.es.
Mott, R. 2006. Fluid Mechanics. 4th. Edition. Pearson Education.
Wikipedia. superfluidity Recovered from: es.wikipedia.org. Zapata, F. Fluids: density, specific weight and specific gravity. Retrieved from: francesphysics.blogspot.com.

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