Viscosity: Definition, Formula, And Unit

Fluids, when flowing, offer specific resistance or hindrance. This resistance is termed viscosity. It arises due to the relative motions between two layers of that respective fluid. It can also be thought of as a fluid’s resistance to objects or a measure of the fluid’s thickness when it passes through it. It arises due to the internal frictions between the two layers of the fluid when it flows.   

A fluid with an excellent viscosity can resist motion, while a fluid with a low viscosity can easily pass or seep through objects. A highly viscous fluid has greater intermolecular forces that give it a lot of internal friction for the layers to slide past one another. On the contrary, low viscous fluid has fewer intermolecular forces, giving less internal friction for layers to slide past one another. 

Definition and Unit

It can be defined as a measure of a fluid’s resistance when it flows. The resistance arises due to the internal friction between the fluid layers. A high viscous fluid resists motion, while a low viscous fluid doesn’t. 

Poiseuille (PI)is the SI unit. Other units include Pascal second (Pa-s) and newton-second per metre square (N s m⁻²). The dimensional formula is [M L⁻¹T⁻¹].

Types

There are mainly two types – 

1. Kinematic 
2. Absolute or Dynamic

Newtonian and non-Newtonian fluids

Even though it is not nature’s fundamental law, Newton’s laws describe and define the phenomenon and its related concepts. Liquids like water and gases are considered Newtonian fluids because they are independent of strain rate. 

However, some fluids do not abide by this principle. Those fluids are termed non-Newtonian fluids.  

• Shear thickening liquids or dilatant – as shear strain rate increases, viscosity increases.
• Shear-thinning liquids – as the shear strain rate decreases, it decreases.
• Rheopectic liquids – as agitation and shaking increase, it increases.
• Thixotropic liquids – as agitation and shaking decrease, it increases.
• Bingham plastics – at low stress, they behave as solids, but they behave as thick viscous fluids at high stress.

For Newtonian fluids, even temperature and pressure play an instrumental role. 

Factors that affect the viscosity of the fluid

• Suppose there is an increase in temperature and the viscosity of a fluid decreases. Similarly, if the temperature decreases, it increases. 
• If there is an increase in temperature, the viscosity of a gas increases and vice versa.
• Even if the amount of matter remains constant or changes, it does not change. Hence, it is an intensive property.
• If there is an increase in pressure, it changes, i.e., it increases. If it changes, we call it non-Newtonian fluid like toothpaste on applying pressure. If it does not change with exerting pressure, we describe it as a Newtonian fluid like water.
• There are other factors as well. For example, a magnetorheological fluid behaves thicker and more viscous when subjected to a strong magnetic field. 

Measurement 

The elementary and fundamental way to measure is to make a metal sphere or ball drop through a fluid column. Whilst falling, mark the ball’s timing to pass through the fluid column. The faster the ball falls, the less viscous the fluid is, while if it falls fast, the liquid is said to be more viscous.

However, a more accurate demonstration is given with a  u-tube viscometer. The instrument is also known as an Ostwald or glass capillary viscometer; the instrument consists of a capillary tube and two reservoir bulbs. The time taken for the fluid to pass through both the bulbs is noted. The time taken by the fluid to cross the bulbs is directly proportional to kinematic viscosity. 

Most industries are given a special conversion factor. The time taken by the fluid to cross the two bulbs is measured and multiplied by the conversion factor. Upon doing so, the kinematic viscosity is attained, which is much more accurate. 

Conclusion

Viscosity is the measurement of a fluid’s resistance when it is allowed to flow through a medium. Depending upon the intermolecular forces of attraction, the internal forces between the fluid layers allow themselves either to slide past one another or not. If they easily cross each other, the fluid is not viscous, and their degree to cross each other determines how viscous the fluid is. It is subdivided into kinematic and dynamic. Several factors determine how viscous any fluid is, like temperature, pressure and even the magnetic field. A viscometer can measure this phenomenon.