cube mentioned in a weak polysaccharide gel, which does not allow
convection, while hardly slowing down diffusion (see Section 5.3.2).
On the other hand, diffusion of a substance into an emulsion droplet
of a few fm in radius would take far less than a second. Diffusion thus
proceeds very fast at very small distances and takes a very long time at long
distances; it cannot be expressed as a linear rate, in m?s^1. If a substance
has to diffuse into a lump of material, say salt in a loaf of cheese by
immersing it in brine, similar rules hold. In the beginning of the process, the
quantity of salt taken up is proportional to the square root of time,
according to Eq. (5.19): if it takes one day to obtain a total salt content of
1 %in the cheese, it will take four days to obtain 2%.
FIGURE5.13 The concentrationðcÞas a function of the distanceðxÞas a solute
diffuses from a liquidainto a materialb, given for two times after diffusion started,
wheret 2 & 66 t 1. The amount ofais considered infinite and the liquidaremains
ideally mixed.