Capillarity

The word 'capilla' means hair in Latin, and a tube with a very fine, hair-thin bore is accordingly called a CAPILLARY TUBE. When a glass capillary tube, open at both ends, is dipped vertically into water, the water inside the tube rises ABOVE the level of the water in the surrounding vessel; when the same kind of tube is dipped into mercury instead, the liquid inside the tube is DEPRESSED BELOW the level of the mercury outside. In general, a liquid whose angle of contact with the solid tube is LESS than 90 degrees undergoes CAPILLARY RISE (as water does in glass), while a liquid whose angle of contact is GREATER than 90 degrees undergoes CAPILLARY FALL (as mercury does in glass). This rise or fall of a liquid inside a narrow tube is called CAPILLARITY or CAPILLARY ACTION, and the exact height risen or fallen depends on the diameter of the capillary tube -- a finer tube produces a larger rise or fall (as the next section derives quantitatively). The full range of behaviour, classified by contact angle, is: a contact angle of exactly zero degrees (perfect wetting -- weak cohesive force, strong adhesive force) gives a flat (plane) meniscus with neither rise nor depression; a contact angle less than 90 degrees (still weak cohesion, strong adhesion, but not perfect wetting) gives a concave meniscus and a capillary RISE (water in glass being the standard example); and a contact angle greater than 90 degrees (strong cohesion, weak adhesion, low wetting) gives a convex meniscus and a capillary FALL (mercury in glass being the standard example). PRACTICAL APPLICATIONS OF CAPILLARITY include: oil rising up through the cotton wick of an earthen (clay) oil lamp; sap rising from a plant's roots, up through its …