What is viscosity?

We explain what viscosity is and the types that exist. In addition, how is the viscosity of water and some examples of this property.

1. What is viscosity?

When we talk about viscosity we mean  a property of fluids equivalent to the concept of  thickness , that is, the resistance that certain substances have to flow, to suffer gradual deformations due to shear stresses or tensile stresses.

All fluids have viscosity (except ideal or superfluid fluids), due to collisions between their particles that move at different speeds. Thus, when the fluid is forced to move, said particles generate frictional resistance, retarding or preventing displacement.

This is because the liquid is composed of several layers of matter, which tend to hold together with each other even in the presence of external forces. For that reason viscous liquids do not generate splashes .

So a fluid with a very high viscositye will be very close to being a solid , since its particles are attracted with a force that prevents the movement of the upper layers. There are two types of viscositye: dynamics and kinematics.

The viscositye also depends on the nature of the fluid, and can be measured using a viscometer or rheometer. Liquids or gases can be used for this, and it is usually represented by the Greek letter  μ  for dynamics and by the letter v in the case of kinematics.

2. Types of viscosity

As said before, there are two types of viscositye: 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 velocitye gradient (particle movement speed) and shear stress. It is measured, according to the International System (SI) in pascal-seconds. This also depends on the temperature: the higher the temperature, the lower the viscosity.
• Kinematic viscosity (v) . In a constant temperature fluid, the kinematic viscosity will be calculated by dividing the dynamics by the density of the fluid, and expressing the result in square meters over a second.
• Extensional viscosity . It is the viscositye of a conventional fluid against tensile forces, representing the relationship between stress and strain rate.
• Apparent viscosity . Resulting from the division of the shear stress between the speed of deformation of the fluid, when its behavior is non-linear. This property varies according to the velocitye gradient of matter.

3. Water viscosity

The viscositye of water at a temperature of 20 ° C is 1 x 10-3 Ns / m2. However, if it is at about 90 ° C, that is, close to boiling, its viscositye varies and decreases to 0.32 x 10-3 Ns / m2.

4. Examples of viscosity

Some examples of viscosity are as follows:

• Glycerin at 20 ° C: 1.5 Ns / m2
• Engine oil at 20 ° C: 0.03 Ns / m2
• Gasoline at 20 ° C: 2.9 x 10-4 Ns / m2
• Human blood at 37 ° C: 4.0 x 10-3 Ns / m2
• Air at 20 ° C: 1.8 x 10-5 Ns / m2
• Carbon dioxide at 20 ° C: 1.5 x 10-5 Ns / m2