Capillary action

In narrow tubes (which we call capillaries), we can observe that the level of a liquid that wets the surface will rise above the external liquid level, whilst the level of a liquid that does not wet the surface stays below the external level. This effect is called capillary action.

To understand this phenomenon, consider that for a liquid that wets the surface, adhesion of the liquid to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upwards. Surface tension acts to hold the surface intact, so instead of just the edges moving upwards, the whole liquid surface is dragged upwards. This upward force can raise a liquid column whose weight is equal to its magnitude. The height of such a liquid column will be \[h = \frac{2 \gamma \cos\theta}{\varrho r g},\] where \(\gamma\) is the surface tension of the liquid, \(\theta\) is the angle between the meniscus and the walls of the tube and \(r\) denotes the radius of the tube. This formula is called Jurin's law.