 Bond number

The Bond number compares hydrostatic stress over a length $\ell$ with capillary stress. For a gas bubble of radius $a$ and negligible density, $\mathrm{Bo}=\rho g{a}^2/\gamma$. Small Bond number is the usual sufficient criterion for capillarity to resist gravity-scale deformations. For the clean-bubble terminal speed, the capillary number is $\mathrm{Ca}=\mu U/\gamma =\mathrm{Bo}/3$.