We explain what viscosity is and the types that exist. Also, how is the viscosity of water and some examples of this property.
What is viscosity?
When we talk about viscosity we mean a fluid property equivalent to the concept of thickness, that is, to the endurance that have certain substances 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, said particles generate resistance of friction, delaying or preventing displacement. The only fluids that have no viscosity are ideal or superfluid fluids, which are fluids in which there is zero friction, that is, they can flow endlessly.
Liquids are made up of several layers of matter, which tend to stick 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 attract each other with such a force that they prevent the movement of the upper layers. The 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: the dynamic, which is represented by the letter 𝛍, and the kinematics, which is represented by the letter 𝛎. On the other hand, one can also speak of extensional and apparent viscosity.
Viscosity types
There are two types of viscosity: dynamics and kinematics. 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 (velocity of movement of the particles) and the shear stress. It is measured according to the International system (SI) in pascal-seconds. It 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 meters squared over second.
- Extensional viscosity. It is the viscosity of a conventional fluid compared to forces of traction, representing the relationship between stress and strain rate.
- Apparent viscosity. It is the result of dividing the shear stress (for example, when we put a knife in mayonnaise) by the deformation rate of the fluid. This property varies according to the velocity gradient of matter.
Water viscosity
The viscosity of the Water at a temperature of about 20 ° C it is 1 × 10-3 (N s) / m2. However, if it is around 90 ° C, that is, close to boiling, its viscosity varies and decreases to 0.32 × 10-3 (N s) / m2.
Examples of viscosity of some compounds
The viscosity of some compounds is as follows:
- Glycerin at 20 ° C: 1.5 (N s) / m2
- Motor oil at 20 ° C: 0.03 (N s) / m2
- Gasoline at 20 ° C: 2.9 × 10-4 (N s) / m2
- Human blood at 37 ° C: 4.0 × 10-3 (N s) / m2
- Air at 20 ° C: 1.8 × 10-5 (N s) / m2
- Carbon dioxide at 20 ° C: 1.5 × 10-5 (N s) / m2