Question

The mechanism of momentum transfer in liquids involves the continual action of intermolecular forces rather than the free flight mechanism in gases. Qualitatively explain the different behavior of η with T on this basis.

Answer 1

1. The flowing fluid's behaviour is largely dependent on the influence of the forces acting on the fluid such as liquid jets, solid particles, solid boundaries.

2.The momentum of the fluid varies according to the position, the fluid layer in contact with the boundary has zero momentum, and as the distance increases from the solid boundary, the momentum of the liquid increases.

3. The same case may not be applied in the case of gases, as the relation between viscosity ( η ) and temperature ( T ) is different when compared with liquids.

4. In gases, the regions where the velocity is very much low transfers the momentum by molecules to a large distances, the viscosity however depends on the average momentum of the molecules as (MT)^(1/2).

5. Viscosity in the gases also depends on the mean free path, which reduces with the increase in the molecular size as (MT)^1/2 / d² where d is the molecular diameter.

6. However the viscosity of a gas is independent of pressure in the conditions where ideal gas laws apply.

7. In case of liquids the viscosity is much higher in comparission of the gases at the given conditions of temperature and density. The liquid viscosity increases with the molecular weight and decreases with the increase in temperature.

8. Viscosity in case of liquids can be representated as non linear function of temperature as

where η is the viscosity at temperature T.