What is viscosity?
When we talk about viscosity we are referring to a property of fluids equivalent to the concept of thickness , that is, the resistance that certain substances have to flow and to undergo gradual deformations as a result of shear stresses or tensile stresses.
All fluids have viscosity due to collisions between their particles, which move at different speeds. Thus, when the fluid is forced to move, these particles generate frictional resistance, slowing or preventing movement. The only fluids that do not have viscosity are ideal fluids or superfluids, which are fluids in which friction is zero, that is, they can flow endlessly.
Liquids are made up of several layers of matter, which tend to stay together even in the presence of external forces. For this reason, viscous liquids do not generate splashes.
Therefore, a fluid with a very high viscosity will be very close to being a solid, since its particles are attracted with such a force that they prevent the movement of the upper layers. Viscosity also depends on the nature of the fluid, and can be measured using a viscometer or a rheometer.
There are several types of viscosity: dynamic, which is represented by the letter 𝛍 , and kinematic, which is represented by the letter 𝛎 . On the other hand, we can also talk about extensional and apparent viscosity.
There are two types of viscosity: dynamic and kinematic. To this can be added the extensional and the apparent.
- Dynamic viscosity ( μ ). Also called absolute viscosity, it is understood as the relationship between the velocity gradient (speed of movement of the particles) and the shear stress. It is measured according to the International System (SI) in pascals-seconds. This also depends on the temperature: the higher the temperature, the lower the viscosity.
- Kinematic viscosity (v). In a fluid at constant temperature, the kinematic viscosity will be calculated by dividing the dynamics by the density of the fluid, and expressing the result in square meters per second.
- Extensional viscosity. It is the viscosity that a conventional fluid presents against traction forces, representing the relationship between stress and deformation speed.
- Apparent viscosity. It is the result of dividing the shear stress (for example, when we put a knife in mayonnaise) by the deformation speed of the fluid. This property varies depending on the velocity gradient of the matter.
The viscosity of water at a temperature of about 20 °C is 1×10 -3 (N s)/m 2 . However, if it is at about 90 °C, that is, close to boiling, its viscosity varies and decreases to 0.32×10 -3 (N s)/m 2 .
Examples of viscosity of some compounds
The viscosity of some compounds is the following:
- Glycerin at 20° C: 1.5 (N s)/m 2
- Engine oil at 20° C: 0.03 (N s)/m 2
- Gasoline at 20° C: 2.9×10 -4 (N s)/m 2
- Human blood at 37° C: 4.0×10 -3 (N s)/m 2
- Air at 20 °C: 1.8×10 -5 (N s)/m 2
- Carbon dioxide at 20° C: 1.5×10 -5 (N s)/m 2
- Physics for Scientists and Engineers.
- University Physics. Sears, Z & Y.