In fluid dynamics, the capillary number (Ca) is a dimensionless quantity representing the relative effect of viscous drag forces versus surface tension forces acting across an interface between a liquid and a gas, or between two immiscible liquids.
Alongside the Bond number, commonly denoted
, this term is useful to describe the forces acting on a fluid front in porous or granular media, such as soil.
Being a dimensionless quantity, the capillary number's value does not depend on the system of units.
In the petroleum industry, capillary number is denoted
[4] The capillary number plays a role in the dynamics of capillary flow; in particular, it governs the dynamic contact angle of a flowing droplet at an interface.
[6] Multiphase flows forms when two or more partially or immiscible fluids are brought in contact.
[7] The capillary number in multiphase flow has the same definition as the single flow formulation, the ratio of viscous to surface forces but has the added(?)
effect of the ratio of fluid viscosities: [clarification needed]
[7] Multiphase microflows are characterized by the ratio of viscous to surface forces, the capillary number (Ca), and by the ratio of fluid viscosities:[7]