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. And what is viscosity of a fluid? It is the measure of the resistance of a fluid to gradual deformation by shear or tensile stress or in other words viscosity describes a fluid’s resistance to flow.  Simply put, we can say that honey is thicker than water, meaning honey is more viscous than water.

It is measured in terms of 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 = 2ga2(∆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

It 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.

Another thing you should know is that the viscosity of liquids decreases with increase in temperature while for gases, it increases with increase in temperature.

Given below are the different types

  • Dynamic or Absolute
  • Kinematic and
  • Bulk

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


Practise This Question

Water flows at a speed of 6 cm s-1 through a tube of radius 1 cm. Coefficient of viscosity of water at room temperature is 0.01 poise. Calculate the Reynolds number. Is it a steady flow?