fluid properties

Physical

2022

We explain what are the properties of fluids, the primary or thermodynamic and the secondary or specific behavior.

Fluids have different viscosities, depending on the substance.

What are the properties of fluids?

Fluids are continuous material media formed by substances in which there is a weak attraction between their particles. Therefore, they change shape without being produced inside forces that tend to restore their original configuration (as is the case in the solid deformable).

Another important property of fluids is the viscosity, thanks to which they can be classified into:

  • Newtonian or constant viscosity fluids.
  • Non-Newtonian fluids, whose viscosity depends on their temperature and the shear stress applied to them.
  • Perfect or superfluid fluids, which present an apparent absence of viscosity.

Let us remember that only liquids Y gases they are considered fluid. Many times we speak of "ideal fluids" because they are easier to study and, although they do not exist in reality, they are an excellent approximation. Solids lack the elemental property of flow and therefore tend to retain their shape, since the attraction between their particles is much more intense.

Basic characteristics of fluids

Fluids, like air, take the shape of their container.

Fluids have elementary physical characteristics that define and differentiate them from other forms of the matter, such as:

  • Infinite deformability. Their molecules they follow unbounded movements and between all of them there is no equilibrium position.
  • Compressibility. It is possible to compress fluids to a certain degree, that is, make them occupy a volume less than dice. Gases are more compressible than liquids.
  • Viscosity. This is the name given to the internal tension of the fluid that opposes the movement, that is, to the endurance to move offered by a fluid and that is much greater in liquids than in gases.
  • Lack of shape memory. The fluids occupy the shape of the container that contains them, that is, if they are deformed, they do not return to their original configuration, therefore they are totally devoid of elasticity.

Thermodynamic (or primary) properties

The density of a fluid is defined as its mass divided by the volume it occupies.

Also called primary properties, they are those that have to do with the levels of Energy in fluids.

  • Pressure. Measure in pascals in the International system (SI), pressure is the projection of the force that a fluid exerts perpendicular to a unit area. For example: atmospheric pressure or air pressure Water on the ocean floor.
  • Density. It is a scalar quantity that is generally measured in kilograms per cubic meter or grams per cubic centimeter. Measures the amount of matter per given volume of a substanceregardless of size and mass.
  • Temperature. It is related to the amount of internal energy of a thermodynamic system (a body, a fluid, etc.), and it is directly proportional to the Kinetic energy average of its particles. Temperature can be measured by recording heat that the system yields to a thermometer.
  • Enthalpy. Symbolized in physical With the letter H, it is defined as the amount of energy that a given thermodynamic system exchanges with its environment, either by losing or gaining heat through different mechanisms but at constant pressure.
  • Entropy. Symbolized with the letter S, it consists of the degree of disorder of thermodynamic systems in equilibrium and describes the irreversible nature of the processes they undergo. In an isolated system, entropy can never decrease: either it remains constant or it increases.
  • Specific heat. It is the amount of heat that a unit of a substance requires to increase its temperature by one unit. Depending on the units used and the scales for measuring temperatures, the unit for specific heat can be cal / gr.ºC, or J / kg.K, for example. It is represented by the letter c.
  • Specific weight. It is the reason between weight of a quantity of a substance and its volume, measured according to the International System in Newtons per cubic meter (N / m3).
  • Cohesion force. The particles of a substance are held together by various intermolecular (or cohesion) forces, which prevent each one from going away on its own. These forces are stronger in solids, less in liquids, and very weak in gases.
  • Internal energy. It is the sum of the total kinetic energy of the particles that make up a substance, together with the potential energy associated with their interactions.

Specific (or secondary) behavioral properties

Surface tension is what allows insects to walk on water.

These properties, also called secondary, are typical of the physical mode of behavior of fluids:

  • Viscosity. It is a measure of the fluid's resistance to deformations, tensile stresses and movement. The viscosity responds to the fact that the particles of the fluid do not all move at the same speed, which produces collisions between them that delay the movement.
  • Thermal conductivity. Represents the ability to heat transmission of fluids, that is, of transferring the kinetic energy of the particles to other adjacent particles with which it is in contact.
  • Surface tension. It is the amount of energy necessary to increase the surface of a liquid per unit area, but it can be understood as the resistance that fluids, especially liquids, present when increasing their surface. This is what allows some insects to "walk" on water.
  • Compressibility. It is the extent to which the volume of a fluid can be decreased by subjecting it to a Pressure or compression.
  • Capillarity. Linked to the surface tension of fluids (and, therefore, their cohesion), it is the ability of a fluid to go up or down a capillary tube, that is, how much a liquid “wets”. This can be easily seen when we dip the tip of a dry napkin into a liquid and observe how far up the liquid stain spreads on the paper against the force of gravity.
  • Diffusion coefficient. It is the ease with which a specific solute moves in a given solvent, depending on the size of the solute, the viscosity of the solvent, the temperature of the mixture and the nature of the substances.
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