Viscosity

 

VISCOSITY

What is viscosity - Liquids exhibit the resistance to flow known as viscosity.

The resistance experienced by one layer in moving past another layer is called the viscosity which consequently can be regarded as a type of internal friction.

Coefficient of viscosity is represented by " " (pronounced as the Eta) and is defined as force in dynes per unit area (sq. cm.) required to cause two parallel layer of liquid one centimetre apart, to slide past each other with a velocity of one centimetre per second.

Unit of viscosity under such conditions is poise.

The viscosity of a liquid is usually measured by observing the rate of flow of liquid through some form of capillary tube.

The flow of homogeneous liquid through a capillary tube is governed by Poiseuille equation.

 

 

In simple format, we can write it as,

 

Where

   = coefficient of viscosity

p = driving force (pressure difference between the ends of the tube)

r = radius of capillary tube, in cm

v = volume (in cubic cm) of the liquid passing through a tube of length  ” (in cm) in time “ t “ (in second).

 

Although the values of all the variables in this equation can be determined experimentally and absolute viscosity can be calculated, the procedure is tedious one.

So usually viscosity is determined relative to a reference liquid of known value of .

In the Ostwald viscometer for the two liquids time is noted when same volume (contained between two fixed marks) is allowed to flow through the same capillary tube.

Naturally for the two liquids (one of which is the reference liquid) v,  and r are constant.

p the driving force will be equal to hydrostatic pressure on the liquid.