The viscosity of any liquid is measured in terms of the coefficient of viscosity and its related terms. It is always constant for a liquid and depends only on its nature. In terms of temperature and pressure, it varies to a great extent. It is different in both liquids and gases. There is a definitive formula by which we can calculate the viscosity of a given liquid, and it has its units assigned to itself as well. In other words, the coefficient of viscosity has been defined as the frictional force required for maintaining a steady 1 cm/s velocity difference between the two parallel layers of the fluid or a given liquid.

What Is Viscosity?

Viscosity is the degree to which a fluid resists itself or tries to resist when it flows through any medium. This resistance offered by the fluid is termed viscosity. It is nothing but the internal force of friction that is occurring between the two layers of the fluid. The more the intermolecular attraction, the stronger the internal forces of friction and the greater the viscosity. For example, honey, oil, and glycerine solution are said to be thick, viscous fluids. On the other hand, fluids that have lesser intermolecular attractions will have lesser internal frictional forces to act between the two layers, hence having poor viscosity. Classic examples include water, petrol, and alcohol. 

Measuring Viscosity

Viscosity is calculated based on the coefficient of viscosity. It depends on the nature of the liquid. It is estimated by using Poiseuille’s method, where a liquid is subjected to flow through a tube at different pressure levels. 

It is different in liquids and gases. Several factors are responsible for determining it, both in liquids and in gases. For example, if the temperature increases, the viscosity of a liquid decreases; thereby, its coefficient also decreases, and vice versa. But this is not the case with gases. As temperatures increase, the viscosity of the gases increases; hence their coefficient also increases. 

The most simple formula to calculate viscosity is:

Viscosity=shear stress rate/shear strain rate

The formula of the coefficient of viscosity is:

f α A(δv/δx)

f= ηA(δv/δx)

• η = coefficient of viscosity
• A= area of cross-section
• (δv/δx)=velocity gradient
• f= tangential force

Units

Every liquid has its viscosity, and to measure this attribute, we use the coefficient of viscosity. It is the tangential force denoted by (F) of a cross-sectional area (A), which is required for maintaining a velocity gradient (v/x) between the two layers. The SI units are Pascal second (Pa-s) and newton-second per metre square (N s m⁻²). The CGS units include dyne-sec cm⁻², which is equal to 1 Poise.

1 Poise=0.1 Pa

The MKS unit is kg m⁻¹ s⁻¹. The dimensional formula of the coefficient of viscosity is [M L⁻¹T⁻¹]. 

Factors Affecting The Viscosity Of The Fluid

Below are the factors that affect the viscosity of the fluid:

• If there is an increase in temperature, the viscosity of a fluid decreases. Similarly, if the temperature decreases, the viscosity 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. On applying pressure, if the viscosity changes, then we call it the non-Newtonian fluid (for example, toothpaste). If it does not change when exerting pressure, we describe it as the Newtonian fluid (example, water).
• There are other factors as well. For example, a magnetorheological fluid behaves thicker and more viscous when it is subjected to a strong magnetic field.  

Examples

1. In day-to-day life, for washing utensils and cleaning, we prefer liquids that are more viscous, like handwashes and dishwashers.
2. If we drop oil from a surface, it sticks to the surface like a droplet and does not fall like water. This is because of the viscous force that is present in it. 
3. Honey is said to have a very high viscosity coefficient. Because of its viscous nature, it is often difficult to extract. Heat makes honey less viscous and lowers its coefficient. 
4. Highly viscous engine oils help in maintaining the oil pressure and film strengths at high temperatures, whereas poorly viscous oils aid in reducing friction and help the engines start quickly in cold weather. 
5. Viscosity and its coefficient play a pivotal role in brake fluids so that they can work in a variety of temperatures. In cold weather countries, the viscosity of the brake fluid is kept low so that the movement of the fluid is efficient and does not give a hard winter to someone.
6. In industries, the ball bearings and hydraulics have lubricants that ought to have a fixed coefficient of viscosity. Too much viscosity will result in the machine performing more strongly, reducing efficiency and increasing power consumption. Too little viscosity will make the machine frictionless, creating wear and tear. 

Conclusion

Viscosity is the measurement of a fluid’s resistance when it is allowed to flow through a medium. It is measured based on the viscosity coefficient. It is constant for all liquids and only depends on the nature of that specific liquid. The coefficient is defined as the tangential force that is required to maintain a uniform velocity gradient of 1 cm/s in any cross-sectional area. Viscosity is mainly of two types, kinematic (on account of gravity) and dynamic (on account of external force). Several factors determine how viscous a fluid is, such as temperature, pressure, and magnetic field.