Viscosity

Viscosity-Understanding the science of thickness of liquids.

Have you ever wondered why honey is thicker than water? Or why do different liquids have different consistency? 

For example, just pour some water and honey into different tumblers simultaneously. Did you notice any difference in the time taken by each liquid to reach the respective tumblers? Did honey take more time to reach the tumbler than water or vice versa? 

This is because of an internal resistance between the molecules that prevent a liquid from moving. This thickness or friction of liquids is referred to as the viscosity of a liquid. So, we can say that honey is more viscous than water. Now, let’s see the topic in detail.

Viscosity explained

As you have seen, more viscous fluid like honey tends to flow slowly, and less viscous fluid, for example, water, flows freely. Viscosity can be simply explained as the resistance caused due to friction between adjacent layers of a liquid that are in relative motion. So, the definition of viscosity would be a fluid’s resistance to flow. The SI unit of Viscosity is Pascal-second(Pas) and the CGS unit is Poise(P).

They are related as 

1 Pa.s=10 poise

Factors affecting viscosity

Pressure and temperature are the major factors that affect viscosity. On increasing pressure, the viscosity of liquid molecules increases due to an increase in the resistance to the flow caused by increased interaction between the molecules whereas, the viscosity of gas molecules decreases with increasing pressure. 

The viscosity of a liquid decreases with increasing temperature, and for gases, it increases with increasing temperature because upon heating, liquids flow more freely, and gases flow slowly, but the amount of matter is the same in both liquids and gases.

Viscosity Types

Viscosity is a measure of fluid’s resistance to its flow. There are two ways to measure the fluid’s viscosity as follows:

• Dynamic Viscosity (Absolute Viscosity)
• Kinematic Viscosity

One way is to measure fluid’s resistance to flow when an external force is applied. This is known as Dynamic Viscosity. And the other way is to measure the resistive flow of a fluid under the effect of gravity. We call this measure of fluid viscosity as kinematic viscosity.

We usually deal with dynamic viscosity in real life.

Newtonian and Non-Newtonian fluids

Newtonian fluids are fluids that follow Newton’s law of viscosity, and non-Newtonian fluids do not follow Newton’s law of viscosity.

What is the law of viscosity?

It states that the shear stress is directly proportional to the rate of shear strain. Rate of shear strain in other words implies velocity gradient. 

Shear stress – the force induced to cause slippage between adjacent layers.

Shear strain – the ratio of displacement to an object’s original dimension due to stress. It is denoted by displacement along a layer divided by the height from the base.

Viscosity of water

The viscosity of water at room temperature (250C) is 0.89m Pa.s (milli Pascal seconds). As mentioned earlier, the temperature directly influences the viscosity of a fluid, and hence at different temperatures, liquids show different values for viscosity. As temperature increases, viscosity decreases and vice versa.

Viscosity of air

The viscosity of air is highly dependent on temperature. The viscosity of air at 25 degrees Celsius is 18.5 μPa s (micro Pascal seconds), which is roughly 50 times smaller than the viscosity of water at the same temperature.

How to measure viscosity?

There are various methods to measure viscosity.

• Capillary viscometer

This is one of the earliest methods to measure viscosity. Viscosity was calculated by measuring the time taken by a volume of a liquid to pass through a capillary tube. This method was used way back in the 20th century.

• Zahn cup

It is a small container with a handle and a hole at the bottom. The time taken by the liquid to empty the cup is correlated with viscosity.

• Falling sphere viscometer

In this method, a sphere of known density is dropped into a liquid, and the time taken by the sphere to reach a specific point is calculated.

• Vibrational viscometer

It measures the damping of an oscillating electromechanical resonator immersed in a fluid.

• Rotational viscometer

It measures the torque required to turn an object which is a function of a liquid’s viscosity. This method is widely used in quality control and production laboratories.

Applications

• Motor industry

In the motor industry, it is important for engineers to consider the viscosity of fluid for designing a pipeline for its transport.

Industrial lubricants are designed in such a way that the fluid may protect the other mechanical parts from friction.

• Food industry

It helps to maximise production and ensures cost-effectiveness. It also helps understand the rate at which a liquid flows through a pipe.

• Dentistry

The viscosity of dental materials is measured as impression materials like mould undergo liquid to solid transformation inside the mouth.

• Construction field

Bitumen is used for road construction considering the viscosity properties.

Conclusion

Now you are pretty much aware of what viscosity is and what makes honey more viscous than water. Understanding the viscosity of a fluid is significant, as it has numerous applications in our daily life. Understanding even a single physical property like the viscosity of water and other substances can bring about tremendous changes in our daily lives.