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 is the gravitational acceleration and is the density of the fluid, and is the surface tension of the fluid-fluid interface.
- For clean water at standard temperature and pressure, the capillary length is around 2 mm.
- For a soap bubble, the capillary length is around 4 meters.
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.
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