Related Resources: fluid flow

Capillary Action Review and Equations

Capillary Action is due to both the cohesive forces between liquid molecules and adhesive forces of liquid molecules. It shows up as the difference in liquid surface elevations between the inside and outside of a small tube that has one end submerged in the liquid (Fig. 1).

Capillarity is commonly expressed as the height of this rise. In equation form,

h = ( 2 · σ · cosθ ) / [ ( w1 - w2 ) r ]

Where:

h = capillary rise, ft (m),
σ = surface tension, lb/ft (N/m),
w1 and w2 = specific weights of fluids below and above meniscus, respectively, ft (m),
θ = angle of contact,
r = radius of capillary tube, ft (m)

Capillarity, like surface tension, decreases with increasing temperature. Its temperature variation, however, is small and insignificant in most problems.

Capillary action raises water in a small-diameter tube
Figure 1
Capillary action raises water in a small-diameter tube. Meniscus, or liquid surface, is concave upward.

Reference:

Civil Engineering Formulas SEcond Edition, Tyler G. Hicks, P.E.

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