A dimensionless number, Bo, used to characterize the shape of drops of liquid and relates the interactions of gravitational and surface tension forces, σ, in a capillary of diameter d:
where ρ is the density of the droplet and ρ′ is the density of the surrounding fluid. It is also known as the Eötvös number (Eo) and named after Hungarian physicist Loránd Eötvös (1848–1919).