Capillary Action

A phenomenon whereby the narrower the tube the higher the liquid will climb above it's normal bulk level in the container.

Liquid rises in a capillary to an equilibrium position determined by the balance of the surface tension force and the gravity force acting on the column of elevated fluid. Note that the rise height will increase as the radius of the tube is made smaller. For nonwetting liquids, such as mercury, the contact angle is greater than π/2, and the height is negative, i.e. the fluid moves downward in the capillary tube rather than upward.

Capillary action is essential in carrying substances and nutrients from one place to another in plants and animals.

h = elevation of the liquid [m]
T = surface tension [Nm-1]
θ = angle of contact of the liquid with the capillary tube [radians]
ρ = density of the liquid [kgm-3]
g= standard acceleration due to gravity [ms-2]
r = radius of capillary tube [m]

Capillary action can be important when liquids infiltrate into porous materials. For example, the upward flow of liquid by capillary action is essential to the maintenance of a candle flame. Radiant heat from the candle flame melts the wax at the base of the candle wick. The melted wax is drawn upward in the wick by capillary action, evapourating as it rises because of heat transfer from the flame surrounding the wick. The vapour then diffuses into the flame, burning by mixing with the surrounding air and feeding back some heat to maintain the process of melting and evapouration.

See also: Meniscus, Wicking.

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Subjects: Physics