Laplacian Needle Growth

Abstract
We study the scaling properties of a forest of one-dimensional needles that grow from a d-dimensional substrate by the aggregation of individual random walkers. Using opacity arguments we establish the existence of an upper critical dimension dc such that for ddc the decay of the needle density ρ(h) as a function of the height h above the substrate is correctly described by a continuum mean-field theory. Below dc the decay of the density profile can be inferred from the competition between two needles. Scaling arguments in combination with a conformal mapping calculation indicate that ρ(h) ~ ln h/h in d = 1, in agreement with extensive simulations.

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