Capillary action (or capillarity) describes the ability of a liquid to flow against gravity in a narrow space such as a thin tube.

This spontaneous rising of a liquid is the outcome of two opposing forces:

Cohesion – the attractive forces between similar molecules or atoms, in our case the molecules or atoms of the liquid. Water, for example, is characterized by high cohesion since each water molecule can form four hydrogen bonds with neighboring molecules.

Adhesion – the attractive forces between dissimilar molecules or atoms, in our case the contact area between the particles of the liquid and the particles forming the tube.

The capillarity of the liquid is said to be high when adhesion is greater than cohesion, and vice versa. Hence, knowledge of the liquid is not sufficient to determine when capillary action will occur, since we must also know the chemical composition of the tube. These two, together with the contact area (the tube's diameter), comprise the key variables. For example, water in a thin glass tube has strong adhesive forces due to the hydrogen bonds that form between the water molecules and the oxygen atoms in the tube wall (glass = silica = SiO2). In contrast, mercury is characterized by stronger cohesion, and hence its capillarity is much lower.

The height (h) of a liquid inside a tube is given by the formula

So what's going on here?

In case the forces of adhesion are greater than those of cohesion and gravity (when it exists), the molecules of the liquid cling to the wall of the tube. We will observe that the upper surface of the liquid becomes concave (the height of the liquid at the contact area is higher than its height at the center of the tube). The cohesive forces between the molecules of the liquid are "attempting" to reduce the surface tension (i.e. to flatten the upper surface of the liquid and thus prevent the increased surface area in the concave state). In doing so, the molecules keep climbing up until a steady state between cohesion and adhesion is achieved (with or without the gravity component).

This also explains why this phenomenon occurs exclusively in thin tubes (also in the absence of gravity). In wider vessels, only a small fraction of the liquid comes into contact with the vessel walls, and so adhesive forces are negligible and there is hardly any rising of the liquid.

Many everyday phenomena are a result of capillary action, including:

(1) A kerosene lamp or a candle "sucking up" oil or liquid wax, respectively.
(2) Water climbing up the microscopic fibers of paper towels.
(3) Located at the inner ends of each eye, the lacrimal ducts drain our tears using 
      capillary action.
(4) In chromatography, a method for separating solutes, different solutes climb up the
      surface of a stationary phase at different rates, resulting in separation (see picture of
      thin layer chromatography below).


And finally an interesting piece of trivia:

Did you know that Albert Einstein's first ever published scientific article deals with capillary action? Published in German in 1901, it was entitled Folgerungen aus den Capillaritätserscheinungen ('conclusions drawn from the phenomena of capillarity').

Adopted from Wikipedia

Dr. Avi Saig
Department of Neurobiology and the Davidson Institute
Weizmann Institute of Science
A note to the reader

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