Diffusion boundary layer

The diffusion boundary layer is a thin (usually on the scale of micrometers) layer of fluid which is not mixed with the main body of the fluid (or cellular arrangement in the case of organisms), by virtue of its proximity to the fluid's edge. In the case of flora the diffusion boundary layer is often associated with the leaf surface, where stomatal gas exchange occurs. In an estuary this layer is immobile due to friction with the sediment.