We explain what fluids are, how they are classified and some examples. Also, what are its general characteristics and physical properties.
What are fluids?
Fluid is called matter composed of molecules weakly attracted to each other, so that it cannot maintain a certain 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, which is why solids have their own shape.
In principle, Both gases and liquids are fluidssince none of them retain their specific form. But there are differences between them, since gases have less attraction between their particles than liquids, which allows them to be compressed, something that can hardly be done with liquids.
See also: Separation of mixtures
Why do fluids flow?
Fluids flow because the force holding its particles together is strong enough to keep them together, but not to maintain a certain rigidity or maintain a shape memory (ability to recover its original shape after applying some deforming force).
Then, since fluid particles must be kept together but cannot resist change, the action of some continuous force on them (such as gravity) causes them to continually deform until they move from place to place, and therefore can 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 (a measure of resistance to deformation). In turn, the viscosity of a liquid fluid decreases when the temperature increases.
Examples of fluids
Some examples of fluids are:
- Water
- Oil
- Air
- Alcohol
- Volcanic magma (lava)
- Ketchup
- Paint
- Noble gases neon (Ne), xenon (Xe), krypton (Kr), helium (He)
- Blood
- Wet mixtures of water with flour or water with cement
How are fluids classified?
Fluids can be of three types:
- Newtonian fluids. These are those that have constant viscosity, that is, it does not vary no matter how much force is applied to the fluid. To study them, the laws of Newtonian mechanics established by Isaac Newton can be used. Water and oil behave like Newtonian fluids.
- Superfluids. Also called “perfect fluids”, they are characterized by completely lacking viscosity, that is, they can flow with the slightest applied force without offering resistance, that is, without friction. Helium behaves like a superfluid at very low temperatures.
- Non-Newtonian fluids. They are an intermediate type between fluid and solid, depending on their temperature and shear stress conditions. They will not have a single viscosity, but will depend on the forces exerted on it or changes in temperature: if they are subjected to a sudden force, they will behave like a solid (offering resistance); while if they are left at rest they will flow like a more or less dense liquid.
Physical properties of fluids
Some of the physical properties of fluids are the following:
- Goo. It is the resistance offered by fluids to deformations, and which tends to prevent fluidity. For example, a substance such as tar is extremely viscous and will flow much more slowly and with difficulty than a low viscosity substance such as alcohol or water.
- Density. It is the measure of the amount of matter in a given volume, it is usually represented in kg/m3. Fluids have greater or lesser density, according to the number of particles in the same volume of fluid, although the interactions between these particles also influence the density.
- Volume. It is the three-dimensional space that the fluid occupies in a given region, considering length, height and width. Liquids have a certain volume and take the shape of the container in which they are contained, while gases have neither the volume nor the shape of the container that contains them.
- PressureIt is the force that the mass of a fluid exerts on the bodies that are submerged in it: an object that falls to the bottom of a lake will have the weight of the entire volume of water on it, which means that the object is subjected to greater pressure than when it is on the surface. On the seabed, the pressure is many times greater than that of the Earth’s atmosphere, for example.
- Surface tension. It is the energy needed to increase the surface area of a liquid fluid per unit area, that is, the fluid resists increasing its surface area. This property is what allows some insects to walk on water.
- CapillarityThis intermolecular cohesion force of fluids allows them to rise through a capillary tube against gravity, since the attraction between their particles is much greater than the attraction of their particles to the material of the tube. This is due in part to surface tension.
What is Pascal’s Law?
Pascal’s Law is a principle discovered by Blaise Pascal in the 17th centurywhich states that a change in pressure 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 Pascal’s Principle and It is extremely useful in hydraulicswhich uses fluids as a mechanical tool to achieve movement.
What is surface tension?
Surface tension is a unique property of liquids, which allows to resist the penetration of its surface by a light objectkeeping it out of the liquid completely, as occurs with insects that can move or remain on water.
This is because The liquid presents a resistance to increasing its surface areathat is, the molecules of the liquid are attracted enough to exert some resistance to displacement.
What is thrust?
When an object or body is under a fluid, for example, submerged in water, Its weight is a force that, by gravity, pulls it downwards. and overcomes the pressure that the fluid exerts on it at all its submerged points and is measured against a similar force exerted by the column of fluid under the body, known as thrust.
If an object thrown into water sinks, it is because its weight overcomes the thrust. with which the liquid counteracts its mass; while if the object remains floating, it is because the thrust is equal to or greater than its own weight.
This is the reason why It costs less to lift objects underwater that from the surface: our force must be added to the push of the fluid out of the water.
What differentiates liquid fluids from gaseous ones?
Liquid and gaseous fluids are not the same. Liquids have their own volume and are incompressible.unlike gaseous gases, which lack volume and are compressible (in fact, this is how liquefied gases are made: they are compressed until they are forced to change into a liquid).
By varying the temperature and pressure, a fluid can be forced to change between these two states.
What is hydraulics?
The hydraulics are the branch of physics that studies the behavior of liquids based on their specific properties, and subjects them to forces and conditions that allow their behavior to be predicted and used consciously to obtain a desired result.
What is fluid mechanics?
Unlike hydraulics, this branch of physics interested in all fluids, not just liquids, as well as their interaction with the environment that limits them and with the shear forces to which they are subjected. Study the movement of fluids and the forces that generate it.
It may help you: Laws of thermodynamics