Fipy/Surface Adsorption Equation

Intro

The Fipy documentation covers the level set method as applied to a surfactant equation:

http://www.ctcms.nist.gov/fipy/fipy/generated/models.levelSet.surfactant.html#fipy.models.levelSet.surfactant.surfactantEquation.SurfactantEquation

This is basically an ordinary differential equation consisting of the following:

$ \frac{d \theta}{dt} = k (1 - \theta) + S $