Abstract
We consider the penetration length l of random walkers diffusing in a medium of perfect or imperfect absorbers of number density rho. We solve this problem on a lattice and in the continuum in all dimensions D, by means of a mean-field renormalization group. For a homogeneous system in D > 2, we find that l is similar to max(xi,rho(-1/2)), where is the absorber density correlation length. The cases of D = 1 and D = 2 are also treated. In the presence of long-range correlations, we estimate the temporal decay of the density of random walkers not yet absorbed. These results are illustrated by exactly solvable toy models, and extensive numerical simulations on directed percolation, where the absorbers are the active sites. Finally, we discuss the implications of our results for diffusion limited aggregation (DLA), and we propose a more effective method to measure l in DLA clusters.
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