In fluid dynamics, the Morton number () is a dimensionless number used together with the Eötvös number to characterize the shape of bubbles or drops moving in a surrounding fluid. The Morton number is defined as
where g is the acceleration of gravity, is the viscosity of the surrounding fluid, the density of the surrounding fluid, the difference in density of the phases, and is the surface tension coefficient. For the case of a bubble with a negligible inner density the Morton number can be simplified to
The Froude number in the above expression is defined as
where V is a reference velocity and d is the equivalent diameter of the drop or bubble.