Energy required to raise a liquid inside a capillary tube:
When a capillary tube is partly dipped inside a liquid which wets the tube, then the liquid inside the tube rises up over the liquid surface outside the tube and its potential energy increases. Now the question is from where the liquid gets this increased potential energy.
When a capillary tube is immersed in a liquid, there are three surfaces of separation \( \cdots \)
- an air-liquid surface
- an air-glass surface
- a glass-liquid surface
Each has its own surface tension, different from others and equal to its free surface energy per unit area.
As the liquid rises inside the capillary tube, the glass-liquid surface increases and the glass-sir surface decreases by the same amount. Since the surface of the liquid inside the capillary tube is concave instead of place, there is an increase in the liquid-air surface. So the surface energy of the glass-air surface decreases and the surface energy of the glass-liquid surface and liquid-air surface increases.
So there is a decrease in the total energy of the system and this energy is responsible for raising the liquid against gravity.
On the other hand, in the case of a liquid which does not wet the wall of the glass tube, the liquid column inside the capillary tube gets depressed below the level outside the capillary tube. So the glass-liquid surface decreases and the air-glass surface increases by the same amount. As a result, the surface energy of the whole system increases. This energy is derived from the depression of the liquid inside the capillary tube, whose gravitational potential energy is thus decreased by an equal amount.