# Capillary Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Capillary number, abbreviated as Ca, a dimensionless number, representing the relative effect of viscous forces against the surface tension between a liquid/gas or liquid/liquid interface.  It is the ratio of viscous forces to surface tension forces.

## Capillary number formula

 $$\large{ Ca = \frac { \mu \; v } { \sigma } }$$

### Where:

 Units English Metric $$\large{ Ca }$$ = Capillary number $$\large{dimensionless}$$ $$\large{ \mu }$$  (Greek symbol mu) = dynamic viscosity $$\large{\frac{lbf-sec}{ft^2}}$$ $$\large{ Pa-s }$$ $$\large{ \sigma }$$  (Greek symbol sigma) = shear tension $$\large{\frac{lbf}{in^2}}$$ $$\large{Pa}$$ $$\large{ v }$$ = velocity of fluid $$\large{\frac{ft}{sec}}$$ $$\large{\frac{m}{s}}$$