Capillary length

In fluid mechanics, capillary length is a characteristic length scale for an interface between two fluids which is subject both to gravitational acceleration and to a surface force due to surface tension in the interface.

The capillary length is defined as:

where g is the gravitational acceleration and ρ is the density of the fluid, and γ is the surface tension of the fluid-fluid interface.

A capillary surface that has a characteristic length smaller than the capillary length can be considered a low Bond number surface. A sessile drop whose largest dimension is smaller than the capillary length, for example, will take the shape of spherical cap, which is the solution to the Young-Laplace equation with gravity completely absent.