Viscosity

What happens when you pour 500 ml of water on someone’s head (apart from them getting angry). It runs through their hair and then flows over their face. But what happens when you pour 500 ml of honey on someone’s head? It takes its own sweet time in running its course through that person’s head, doesn’t it? Why the difference? This is because of a property of fluids called viscosity. Here, in the article let us learn about viscosity.

What is Viscosity?

Viscosity is defined as the measure of the resistance of a fluid to gradual deformation by shear or tensile stress. In other words, viscosity describes a fluid’s resistance to flow. Simply put, we can say that honey is thicker than water, in turn, honey is more viscous than water.

Viscosity Formula

Viscosity is measured in terms of a ratio of shearing stress to the velocity gradient in a fluid. If a sphere is dropped into a fluid, the viscosity can be determined using the following formula:

\(η=\frac{2ga^2(∆ρ)}{9v}\)

where,

ρ is the density difference between fluid and sphere tested

a is the radius of the sphere

g is the acceleration due to gravity

v is the velocity of the sphere

where,

v = distance travelled by sphere/ time it takes to travel that distance

Viscosity is measured in Pascal seconds (Pa s). As you can see in this equation, if the speed of the sphere is less, the viscosity will be more. The more viscous a fluid is, the more resistance it offers to any object moving inside it. Although all liquids have a certain value of viscosity, the for liquids is generally considered as high or low, keeping the viscosity of water as a benchmark. For example viscosity of water is 0.001 Pa s, that of air is 0.000019 Pa s and that of motor oil is 1… so you can pretty much do the math here. And also, the viscosity of liquids decreases as the temperature increases, while for gases, it increases with the increase in temperature.

Types of Viscosity

As we know, the viscosity is the measure of the friction of fluids. There are two ways to measure a fluid’s viscosity as follows:

  • Dynamic Viscosity (absolute viscosity)
  • Kinematic Viscosity

Many are confused between the two viscosity measures and consider them to be one and the same. In reality, they have significant differences between them. For a few applications, kinematic viscosity is more useful than absolute or dynamic viscosity. The unit of kinematic viscosity is stokes, named after the British physicist, Sir George Gabriel Stokes. A stoke is defined as one centimetre squared per second.

Learn more about the concept and its various applications in fluid mechanics with our expert faculty, here at BYJU’S.

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Practise This Question

An air bubble of diameter 2 mm rises steadily through a solution of density 1750 kg m-3 at the rate of 0.35 cm s-1. Calculate the coefficient of viscosity of the solution. The density of air is negligible.