What is Viscosity?
Viscosity Definition: We can define viscosity as the internal friction between layers of a liquid or gas in motion. It is the property of a liquid that opposes the relative motion between different layers. It is the name given to internal friction that exists between the layers of the molecules of fluids (liquid or gas) in contact with surfaces.
For a proper explanation, let us pour water or kerosene into a bowl. Clean the bowl and pour honey into the same bowl. You will observe that it’s very easy to pour water into the bowl than to pour honey. The difference is because of the thickness of the honey which makes it very hard to move from one container to another.
When you drop a little stone into a container of water, it gets to the bottom of the container very easily and faster. Now, drop the same size of stone into a container of honey with the same volume of water. You will realize that the stone will get to the bottom of the container very slowly because of the thickness of the honey.
Therefore, the differences are due to the internal friction between the layers of the fluids. The honey is thicker and encourages more internal friction between its layers. We need to remember that friction is a force that resists motion. Hence, honey tends to resist the motion of objects between its layers due to its thickness. The thicker a liquid is, the higher its viscosity.
We consider liquids that pour slowly to be more viscous than those which pour faster. Hence very cold thick palm oil is more viscous than very cold water. Therefore, we can say that viscosity is friction in liquids.
Viscosity Definition: What is the SI Unit of Viscosity and its Formula
The formula for calculating viscosity is:
Coefficient of viscosity, η = Force (F) / [Area (A) x Velocity gradient (V)]
Which implies that
η = F / (AV)
When layers of water flow slowly and steadily through a pipe, it will be observed that each layer flows at different velocities. if:
- The velocities of adjacent layers A and B are V1 and V2 respectively
- The distance between them is h
- Therefore, the force exerted between the two layers will be directly proportional to the velocity gradient and the surface area A
F ∝ ( V1 – V2)A / h
Which will later become
F = η( V1 – V2)A / h
η = Constant of proportionality called the coefficient of viscosity
Another formula to apply is
Viscosity = shear stress / shear strain
We measure viscosity in Newton seconds per meter square (Ns/m2) or in Pascal seconds (Pas), and it’s a vector quantity. The derived standard international (SI) unit of dynamic viscosity is poiseiulle (PI).
Dimension of Viscosity
We can derive the dimension of viscosity by applying the formula
η = F / (AV)
Where Force, F = Newtons = kgm/s2 = MLT-2
Area, A = m2 = L2
Velocity, V = m/s = LT-1
Hence, by using η = F / (AV) we will have
η = F / (AV) = MLT-2 / (L2 x LT-1) = ML-1T-1
Therefore, we have dimension as ML-1T-1.
Types of Viscosity
We have two types of viscosity: Dynamic and kinematic
- Dynamic Viscosity: This is the resistance to the flow of one layer of fluid in relation to the other. This is a force that is required by the fluid to defeat the internal molecular friction so that it can encourage the flow of the fluid. Hence, fluids move in the opposite direction at different speeds
- Kinematic Viscosity: This is the measure of the flow of the fluid’s internal resistance as a result of the influence of the force of gravity.
Viscosity of Water
Water viscosity refers to the fluid’s resistance to flow. This is caused by the frictional forces between the molecules of the liquid. It is a crucial characteristic of water. Therefore, we can say that it is relatively low when we compare it to other liquids. Therefore, liquids like honey, have higher viscosity values. This is because the molecules of water are relatively small. They also have weak intermolecular forces, which will make it easy for them to move past each other. At 200C, the viscosity of water is 0.01 Pascal second (Pas).
The viscosity of water can vary depending on several factors. These factors are temperature, pressure, and the presence of dissolved substances. Generally, as the temperature of water increases, its viscosity decreases, and the flow rate increases. Similarly, as the pressure increases, the viscosity of water also increases.
Furthermore, the presence of dissolved substances in water can also affect its thickness. For example, adding salt to water increases its viscosity, making it more resistant to flow. This is because the dissolved salt ions create more frictional forces between the water molecules. Thus, making it harder for them to move past each other.
What is Viscosity Index
We can define the viscosity index as the measure of a fluid’s change in viscosity with variations in temperature.
Viscosity Definition: Viscometer
Simply put, a viscometer is a device that can measure the viscosity of liquids. We can use a viscometer to measure the thickness of liquids in the medical field like syrups. We can also apply a viscometer to analyze the flow of milk and car lubricants. Additionally, we can use a viscometer to understand the thickness of ink during its production.
Classification of Fluids According to Their Viscous Properties
Classification of fluids is based on high and low thickness depending on the frictional force:
- When frictional force is low: Examples of low viscous fluids are water, kerosene, petrol, and ethanol.
- When frictional force is large: Examples of highly viscous fluids are glue, syrup, grease, honey, blood, and glycerine.
Viscosity Definition: Effects
- It is responsible for the different rates of flow of fluids.
- It also affects the motion of the bodies in fluids.
Viscosity Definition: Application
- The viscous liquid is used as a lubricant
- The knowledge of viscous drag is used in the design of ships and aircraft.
- Particles of suspension in liquid fall with a steady terminal speed and this can be used to estimate the enlarged size of the particles.
- The effect of the viscosity of air on the bob of a swinging pendulum
Similarities Between Viscosity and Friction
- Both forces oppose relative motion between surfaces
- They all depend on the nature of the material in contact
Differences Between Viscosity and Friction
| Friction | Viscosity |
| Does not depend on areas of surfaces in contact | Depends on the areas of surfaces in contact |
| Depend on normal reaction | Does not Depend on normal reaction |
| Does not depend on the relative velocities between two layers | Depends on the relative velocities between two layers of fluid. |
| Occurs in solids. | Occurs in fluids (liquids and gases) |
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