The Capillary number ($Ca$), 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,

• $Ca$ is the Capillary number,
• $\mu$ is the dynamic viscosity of the fluid,
• $v$ is the characteristic velocity of the fluid,
• $\gamma$ is the surface tension of the fluid.