# Capillary number

In fluid dynamics, the capillary number (Ca) represents 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. For example, an air bubble in a liquid flow tends to be deformed by the friction of the liquid flow due to viscosity effects, but the surface tension forces tend to minimize the surface. The capillary number is defined as:[1][2]

${\displaystyle \mathrm {Ca} ={\frac {\mu V}{\sigma }}}$

where µ is the dynamic viscosity of the liquid, V is a characteristic velocity and ${\displaystyle \sigma }$ is the surface tension or interfacial tension between the two fluid phases.

The capillary number is a dimensionless quantity, hence its value does not depend on the system of units. In the petroleum industry, capillary number is denoted ${\displaystyle N_{c}}$ instead of ${\displaystyle \mathrm {Ca} }$.[3]

For low capillary numbers (a rule of thumb says less than 10−5), flow in porous media is dominated by capillary forces[4] whereas for high capillary number the capillary forces are negligible compared to the viscous forces. Flow through the pores in an oil field reservoir have capillary number on the order of 10−6, whereas flow of oil through an oil well drill pipe has a capillary number on the order of 1.[3]

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.[5]