Capillarity:

Rise or fall of a liquid in a capillary  tube is caused by surface tension and depends on the relative magnitude of cohesion of the liquid and the adhesion of the liquid to the walls of the containing vessel. Liquids rise in tubes if they wet (adhesion > cohesion) and fall in tubes that do not wet (cohesion > adhesion).

Wetting and contact angle

Fluids wet some solids and do not others.

The figure shows some of the possible wetting behaviors of a drop of liquid
placed on a horizontal, solid surface (the remainder of the surface is covered with air, so two fluids are present).
Figure.(a) represents the case of a liquid which wets a solid surface well, e.g.
water on a very clean copper. The angle    shown is the angle between the edge
of the liquid surface and the solid surface, measured inside the liquid. This angle
is called the contact angle and is a measure of the quality of wetting.
For perfectly wetting, in which the liquid spreads, as a thin film over the surface
of the solid, Ĭ is zero.



Fig.(c) represents the case of no wetting. If there were exactly zero wetting, Ĭ
would be 180o. However, the gravity force on the drop flattens the drop, so that
180o angle is never observed. This might represent water on teflon or mercury on clean glass. We normally say that a liquid wet a surface if Ĭ is less than 90o and does not wet if Ĭ is more than 90o. Values of Ĭ  less than 20o are considered
strong wetting and values of Ĭ greater than 140o are strong non wetting.

Capillarity is important (in fluid measurements) when using tubes smaller than about 10 mm in diameter Capillary rise (or depression) in a tube can be

calculated by making force balances. The forces acting are forces due to surface
tension and gravity. The
force due to surface tension, Fs = Ʌd ı cos( Ĭ), where Ĭ is the wetting angle or contact angle. If tube (made of glass) is clean Ĭ is zero for water and about 140o
for Mercury. This is opposed by the gravity force on the column of fluid, which is
equal to the height of the liquid which is above (or below) the free surface and which equals Fg = (Ʌ/4)d2hg ȡ, where ȡ  is the density of liquid. Equating these forces and  solving for Capillary rise (or depression), we find
h = 4ıcos( Ĭ)/( ȡ gd).