Broadbent and Hammersley (1957) discussed the general situation of a fluid spreading randomly through a medium, where the abstract terms ‘fluid’ and ‘medium’ could be interpreted according to context.1 The randomness may be of two quite different types. In the familiar diffusion processes the randomness is the random walks of the fluid particles — an example is the irregular thermal motion of molecules in a liquid. The other case in which the randomness is frozen into the medium itself, was christened a percolation process by Hammersley, since it behaves like coffee in a percolator.
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