Viscosity

It is well known that all liquids do not flow with the same speed. Some liquids like water, alcohol, ether, etc. flow very rapidly while some one like glycerine, honey, castor oil, etc. flow slowly. This indicates that every liquid has some internal resistance to flow. This internal resistance to flow possessed by a liquid is called its viscosity. The liquids which flow rapidly have low internal resistance and hence are said to be low viscous, i.e. their viscosity is less. On the other hand, the liquids which flow slowly have high internal resistance and hence are said to be more viscous, i.e. their viscosity is high. This force of friction which one part of the liquid offers to another part of the liquid is called Viscosity.

Coefficient of viscosity may be defined as the force per unit area required to maintain unit difference of velocity between two parallel layers in the liquid, one unit apart. In C.G.S. units, it is expressed in dynes per square centimeter (dynes cm^-2). This unit is called a poise after the name of Poiseulle who pioneered the study of viscosity. Low values of viscosity are expressed in centipoise (10 ^-2 poise) and millipoise(10 ^-3 poise). In S.I. units, viscosity is expressed in Nm^-2 s or Pas (pascal second).

1 Pas = 1 Nm^-2 s = (105 dyn) (10⁺² cm)^-2 s = 10 dyn cm^-2 s = 10 poise

The viscosity of a liquid generally decreases with rise in temperature. With increase of temperature, the kinetic energy of the molecules of the liquid increases and hence the liquid starts flowing faster, i.e. the viscosity decreases. The decrease in viscosity is found to be about 3% per degree rise of temperature.

Viscosity in everyday life

• Lubricating oils are graded according to their viscosity. A good quality or ‘all – weather’ lubricating is one whose viscosity and hence lubricating property does not change much with increase of temperature. Such oils are obtained by adding long chain coiling polymers to the oil. As temperature rises, the polymer particles tend to uncoil and thus increase the viscosity of oil thereby compensating for the decrease of viscosity of the oil with rise of temperature.

• The condition of high blood pressure and thus strain on heart may also be explained on the property of viscosity. In arteriosclerosis (hardening of arteries), arterial walls contract resulting in decrease of diameter of capillaries.

Narrow capillaries offer resistance to the flow of blood due to viscosity with the result greater force is needed to make blood flow through capillaries. This results in a condition of high blood pressure and strain on the heart.

• The increased blood circulation required during fever is supplied by its temperature dependence properly (recall that rise of every degree centigrade temperature decreases viscosity of blood by about 3%). Thus lowering of viscosity results in a more rapid flow of blood without any extra strain on the heart.

• In case of asphyxia, concentration of CO in blood increases resulting in swelling of corpuscles which then increases viscosity of blood.