The Capillary number (), also known as the Ca number, is a dimensionless quantity used in fluid mechanics to characterize the relative importance of viscous forces to surface tension forces in a fluid flow. It indicates whether viscous forces dominate over surface tension forces or vice versa.
Where,
- is the Capillary number,
- is the dynamic viscosity of the fluid,
- is the characteristic velocity of the fluid,
- is the surface tension of the fluid.