# 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:[1]

${\displaystyle \lambda _{c}={\sqrt {\frac {\gamma }{\rho g}}}}$,

where ${\displaystyle g}$ is the gravitational acceleration and ${\displaystyle \rho }$ is the density of the fluid, and ${\displaystyle \gamma }$ is the surface tension of the fluid-fluid interface.

• For clean water and air at standard temperature and pressure, the capillary length is around 2.7 mm.
• For a soap bubble in air, the capillary length is around 4 meters (13 feet).

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.