Viscosity is the resistance that some liquid substances have when it comes to flowing and deforming, thus being one of the main characteristics of liquids. In this sense, it can be determined that the greater the resistance a liquid has to proceed to its fluidity and deformation, the greater its viscosity, or the more viscous it is.
A liquid will have more or less viscosity depending on the resistance that its particles or molecules have at the moment of deforming or proceeding to separate. The more adherent these molecules are, the more viscous the liquid will be.
Taking all this into account, it can be determined that the more viscosity a liquid will have, so that the stronger the intermolecular forces of attraction, the more viscous it will be.
There are many examples of liquids that are an example of viscosity, such as vehicle lubricants, honey or even shampoo, which, as you can see, do not move very easily and it is not easy to spill.
This quality can, in any case, vary when the liquid is exposed to a high temperature. This is because when heating the viscosity is affected and decreases, which allows a faster displacement of the liquid. You can check it by heating the honey.
On the other hand, liquids that do not have viscosity are called ideal fluids, because they do not present any resistance, that is, they are not viscous.
Water viscosity
- Water is one of the most important liquids in life. In addition, it is a liquid with a very low viscosity compared to other liquids such as oil, which allows blood to flow properly through the bloodstream and the rest of the organs of the human and animal body. Similarly, plants and other living things also feed on water.
- In the event that the water had a higher (or lower) viscosity, this would affect both life and its development. Also the molecular structure would be different.
Viscosity unit
- The unit of measurement for viscosity is the Poise (P), a name that refers to the French physiologist Jean-Louis-Marie Poiseuille, although the centipoise (cP) is also frequently used.
- In this way it can be known that 1 Poise is equal to 100 centipoise and, at the same time, equal to 1 g/(cm.s). Also, 1 centipoise is equal to one minipascal of a second.
- The unit used for the dynamic viscosity is called Pascal-second, while the kinematic viscosity [nu] is known through the dynamic viscosity by the density of the substance and is determined in the unit Stoke or Centistoke (cm2/sec). In this way 1 Stoke is equal to 100 centistokes and equal to 1 cm2/s.
Dynamic and kinematic viscosity
- In the previous section we have talked about the units of measurement of dynamic viscosity and kinematic viscosity, but now it is time to talk in depth about each one of them.
- You should know that both dynamic viscosity and kinematic viscosity are values that are responsible for determining the movement of a fluid or liquid according to specific conditions.
- In this way, in fluid mechanics or hydraulics, kinematic and dynamic viscosity are two concepts that are not essential to be able to make a relationship between those forces responsible for generating movement and velocity in a fluid. Therefore, it is key to know the way in which these fluids move in order to know the way in which these mechanisms that are activated by liquids work.
- In order to better understand these terms, it is important to bear in mind that the viscosity of a fluid is determined by the level of cohesion between its molecules. Liquid fluids have a stronger cohesion than a gas but weaker than a solid. This makes them have that characteristic fluidity. In this sense, it must also be assessed that the less interaction there is between its molecules, there is less friction and less viscosity.
Dynamic viscosity
- Dynamic viscosity, which is also known as absolute viscosity, is the internal resistance that the molecules of a fluid have when they are in motion, and which determines the forces that are in charge of moving it and also of deform it.
- This behavior in liquids was observed by Isaac Newton when placing them between two plates that were in parallel. A static base plate and a top with constant movement. Thanks to this he ended up creating Newton’s Law of viscosity:
- In it, the tangential force is equal to the dynamic viscosity multiplied by the value resulting from the multiplication of the surface by the speed of one plate with respect to the other divided by the thickness of the liquid sheet.
- In this way it was revealed that liquids really slide in sheets or layers, which means that their speed is really zero at the contact surface. However, it increases as it returns creating a tangent called the tangential force. In this case, the dynamic viscosity, as we have already mentioned, uses the Poise (P) as a unit of measurement.
Kinematic viscosity
- On the other hand, the kinematic viscosity is in charge of relating the dynamic viscosity with the density of the liquid. Knowing the value of the dynamic viscosity it is possible to calculate the kinematics. To do this, use the following formula:
- Kinematic viscosity = dynamic viscosity / density
- In this measure, the viscosity becomes the resistance of a fluid against slipping that it may have, while the density is the weight (mass/volume) divided by gravity. A clear example is seen in motor oil, which is viscous and slowly enters the tube, but it will be less dense than water even if it floats on it. In this situation the water has less viscosity but is more dense than the oil.
- For kinematic viscosity a specific unit is used, which is the Stoke (St).
- It should be noted that both kinematic and dynamic viscosity depend on the temperature and nature of the liquid in question. In this way, the higher the temperature, the lower the viscosity of the liquid. This is because the cohesion of the molecules becomes weaker.
Examples of viscosity
Some well-known examples of viscous fluids are:
- The hair gel or shampoo. They are very viscous and it is because the molecules have resistance against the flow of their movement. When shaken they lose viscosity and in the case of not doing so they can become solid.
- The oil. It is a liquid substance that is obtained after processing some seeds, in the case of edible. The fuel is obtained after refining oil. Both are very viscous, especially the fuel oil used for vehicles.
- Syrups: If you take or have taken syrup and remember it, surely you also remember some difficulty in taking it due to its viscosity. These are because of their chemical composition, but also because they are made of sugar, which contributes to it.