The word viscosity comes from the Latin word “viscum”, which refers to the gum derived from the mistletoe berries. Viscosity is described as the state of being too thick, sticky, or semi-fluid. In general, whenever we say that a liquid is viscous, we mean that the liquid is very thick. However, in physics, the definition is much more proper as it goes, “viscosity of a fluid is the measure of its resistance to deformation at a given rate”. One such example of viscosity is oil being thicker than water. 

Viscosity of fluids

The viscosity of fluids is the measure of their resistance to deformation at a certain rate. Viscosity gives a quantity to the internal frictional force between layers of fluid that is in motion. Whenever a viscous liquid is flown through a tube, it seems that it flows more quickly near the axis of the tube as compared to the walls of the tube. 

Generally, the viscosity of the liquid is dependent on some states of the liquid. These states include the temperature of the fluid, pressure, and rate of deformation. A fluid with zero viscosity is observed at a very low temperature. These fluids are called superfluids. A fluid with zero viscosity is often called an ideal fluid. 

Viscosity is represented with the help of the expression,

Viscosity = shear stress/ shear strain rate

Units

The S.I unit of dynamic viscosity is newton-second per square metre(N.s/m²), which is also frequently expressed in the forms of pascal-second, Kg/ms, and Poiseuille.

The S.I unit of kinematic viscosity is metre square per second, written as m²/s. The CGS unit of kinematic viscosity is the stokes.

Types of Viscosity

There are five types of viscosity:

• Dynamic Viscosity- The measurement of the fluid’s internal resistance to flow when force is applied.
• Kinematic Viscosity- It refers to the ratio of dynamic viscosity to the density of the fluid. It is the measurement of the viscosity of the fluid in motion.
• Steady Shear Viscosity- It refers to the relationship between viscosity and shear rate. It is the shear stress divided by the shear rate.
• Relative Viscosity- It refers to the ratio of the viscosity of a solution to the viscosity of the solvent used. 
• Extensional Viscosity- Extensional viscosity is the viscosity coefficient when the applied stress is extensional stress. 

Applications of Viscosity

There are endless applications of viscosity in daily life. Here are some of the most noticeable applications of viscosity. 

1. The oil used for greasing the heavy machinery parts must have a higher viscous coefficient. To select the most suitable oil for machinery, we must know about the viscosity coefficient. The oils with a lower viscous coefficient can be used for car engines. Hence having an idea about the viscosity coefficient is crucial for deciding the purpose of the oil. 
2. Highly viscous liquids are used in some machinery parts, such as brake oil. It is used for slowing down the machine as in hydraulic brakes.
3. It is due to the viscosity of the blood and other bodily fluids that proper circulation of nutrients is possible in the human body.
4. Viscosity is also crucial when determining and buying suitable cooking oil. The viscosity of different cooking oils varies; hence, viscosity is an important parameter.
5. Being aware of the viscosity of ink for printing is crucial for estimating the quality of the ink. An optimum viscosity of the ink must be ensured for the ink to transfer to the medium.
6. The adhesive industry is another such industry dependent on viscosity. Viscosity helps us differentiate between the type of adhesive. 
7. The viscosity of bitumen has a great role in determining the strength of the road. Depending on the location, bitumen of different viscosity is chosen.

Why Does Water Flow Faster Than Honey?

Water flowing faster than honey is a good old example of viscosity. 

One must have noticed that when a bottle of honey is almost empty, the little quantity of honey left at the bottom takes a very long time to reach the mouth of the bottle. Here honey shows its viscosity property. 

As we have read earlier, viscosity is the property of a fluid to show resistance between different layers of the fluid whenever there is a relative motion between the layers of the fluid.

We can simply understand viscosity as “thickness” in layman’s terms. We have noticed that honey is thicker than water which means that honey is more viscous than water. 

Honey is more viscous due to the higher intermolecular forces between the individual particles. Hence, the particles are more strongly bound in honey than in water. Honey has a viscosity of 10000 cps, and water has a viscosity of 1.0020 cps. Hence, the answer to the above question is viscosity, which is the internal property of honey.

Fluids with Higher Viscosity than Water

Apart from honey, there are many viscous fluids found in nature. Here is a list of fluids that show higher viscosity than water. 

• Engine oil
• Brake oil
• Lubricant
• Edible oil
• Shampoo
• Liquid soap
• Paints
• Medicinal syrup
• Blood

Conclusion

Viscosity can be understood as thickness in layman’s terms. There are many applications of viscosity in daily life as it is used to create lubrication in heavy machinery, for hydraulic brakes, etc. Honey is a viscous liquid. The higher intermolecular force of attraction between the honey particles leads to the higher viscosity of honey than water. This leads honey to flow slower than water.