The capillary number compares viscous stresses with surface tension. Its velocity and viscosity conventions must be specified; a porous-medium displacement can use the Darcy velocity and the viscosity of the displaced fluid.
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The Capillary number (Ca) is a dimensionless number that describes the relative effect of viscous forces to surface tension forces in a fluid. It is a critical parameter in fields such as fluid mechanics, petroleum engineering, and the study of multiphase flows.