Viscosity is the property of a fluid by virtue of which it opposes the relative motion between its different layers. It arises because of the transfer of momentum between adjacent layers of the fluid.
The behaviour of viscosity in gases and liquids is different because the mechanisms responsible for momentum transfer are different in the two states.
Viscosity of Gases
In a gas, the molecules are widely separated and intermolecular forces are relatively weak. When adjacent layers of gas move with different velocities, molecules travel between the layers and transfer momentum. This molecular transport produces viscosity.
According to the kinetic theory of gases, the coefficient of viscosity of a dilute gas can be approximately expressed as:
where η is the coefficient of viscosity, ρ is the density of the gas, c̄ is the mean molecular speed, and λ is the mean free path of the molecules.
Viscosity of Liquids
In liquids, molecules are much closer together and intermolecular attractive forces are significant. The resistance to flow is therefore mainly associated with the intermolecular forces that must be overcome when one layer moves relative to another.
Unlike gases, the viscosity of a liquid generally decreases as temperature increases because heating weakens the effective influence of intermolecular attractions and facilitates molecular movement.
Comparison between the Viscosity of Gases and Liquids
| Property | Gases | Liquids |
|---|---|---|
| 1. Molecular separation | Molecules are widely separated. | Molecules are closely packed. |
| 2. Intermolecular forces | Relatively weak. | Relatively strong. |
| 3. Main mechanism of viscosity | Mainly due to molecular transport and transfer of momentum between adjacent layers. | Mainly associated with intermolecular attractions and molecular rearrangement between adjacent layers. |
| 4. Effect of temperature | Viscosity generally increases with increase in temperature. | Viscosity generally decreases with increase in temperature. |
| 5. Effect of pressure | At ordinary pressures, viscosity is only weakly affected by pressure. | Viscosity can increase appreciably with pressure because molecular packing and intermolecular interactions are affected. |
| 6. Magnitude | Generally much lower than that of liquids at comparable conditions. | Generally much higher than that of gases. |
| 7. Density dependence | Gas viscosity is related to molecular momentum transport and, in the dilute-gas regime, is not simply proportional to density. | Liquid viscosity is strongly influenced by molecular packing and intermolecular interactions. |
| 8. Molecular motion | Molecules move freely over comparatively long distances between collisions. | Molecules remain close together and undergo frequent interactions. |
Effect of Temperature on Viscosity
The opposite temperature dependence of gases and liquids is one of the most important differences between their viscosities.
For gases: When temperature increases, molecular speed increases. The faster molecules transfer momentum more effectively between adjacent layers, so the viscosity generally increases.
For liquids: When temperature increases, intermolecular attractions become less effective in resisting molecular movement. Therefore, the viscosity generally decreases.
Read more Effect of Temperature on Viscosity
Conclusion
The viscosity of gases is primarily related to the transfer of momentum by moving molecules between adjacent layers, whereas the viscosity of liquids is strongly influenced by intermolecular attractive forces and molecular rearrangement. Consequently, the viscosity of gases generally increases with temperature, while that of liquids generally decreases with temperature.