# Laplace formula

The Laplace formula is a result in fluid mechanics that gives the surface pressure between two continuous media. The surface of separation is really a transitional layer, but it is so thin that in classical mechanics it can be considered a dividing surface between the two media.

## Definition

Consider two continuous media (top and bottom) whose surface of separation undergoes a small displacement. If ${\displaystyle r_{t}}$ and ${\displaystyle r_{b}}$ are the principal radii of curvature of the surface in some sample region, then the difference of pressures is

${\displaystyle \Delta P=\alpha \left({\frac {1}{r_{t}}}+{\frac {1}{r_{b}}}\right)}$

where ${\displaystyle \alpha }$ is the surface tension coefficient.