Eötvös number

Describing the ratio of gravitational to capillary forces, the Eötvös or Bond number is given by the equation:[7]

$${\displaystyle \mathrm {Eo} =\mathrm {Bo} ={\frac {\Delta \rho \,g\,L^{2}}{\gamma }}.}$$

• ${\displaystyle \Delta \rho }$: difference in density of the two phases, (SI units: kg/m³)
• g: gravitational acceleration, (SI units : m/s²)
• L: characteristic length, (SI units : m) (for example the radii of curvature for a drop)
• ${\displaystyle \gamma }$: surface tension, (SI units : N/m)

The Bond number can also be written as

$${\displaystyle \mathrm {Bo} =\left({\frac {L}{\lambda _{\rm {c}}}}\right)^{2},}$$

where ${\textstyle \lambda _{\rm {c}}={\sqrt {{\gamma }/{\rho g}}}}$ is the capillary length.

A high value of the Eötvös or Bond number indicates that the system is relatively unaffected by surface tension effects; a low value (typically less than one) indicates that surface tension dominates.[7] Intermediate numbers indicate a non-trivial balance between the two effects. It may be derived in a number of ways, such as scaling the pressure of a drop of liquid on a solid surface. It is usually important, however, to find the right length scale specific to a problem by doing a ground-up scale analysis. Other similar dimensionless numbers are:

$${\displaystyle \mathrm {Bo} =\mathrm {Eo} =2\,\mathrm {Go} ^{2}=2\,\mathrm {De} ^{2}}$$

where Go and De are the Goucher and Deryagin numbers, which are identical: the Goucher number arises in wire coating problems and hence uses a radius as a typical length scale while the Deryagin number arises in plate film thickness problems and hence uses a Cartesian length.

In order to consider all three of the forces that act on a moving fluid front in the presence of a gas (or other fluid) phase, namely viscous, capillary and gravitational forces, the generalized Bond number, which is denoted commonly as Bo^*, can be used.[1] This is defined as:

$${\displaystyle \mathrm {Bo^{*}} =\mathrm {Bo} -\mathrm {Ca} .}$$