Scaling range and cutoffs in empirical fractals

Fractal structures appear in a vast range of physical systems. A literature survey including all experimental papers on fractals which appeared in the six Physical Review journals (A–E and Letters) during the 1990s shows that experimental reports of fractal behavior are typically based on a scaling range $\Delta$ that spans only 0.5–2 decades. This range is limited by upper and lower cutoffs either because further data are not accessible or due to crossover bends. Focusing on spatial fractals, a classification is proposed into (a) aggregation, (b) porous media, (c) surfaces and fronts, (d) fracture, and (e) critical phenomena. Most of these systems [except for class (e)] involve processes far from thermal equilibrium. The fact that for self-similar fractals [in contrast to the self-affine fractals of class (c)] there are hardly any exceptions to the finding of $\Delta <~2$ decades, raises the possibility that the cutoffs are due to intrinsic properties of the measured systems rather than the specific experimental conditions and apparatus. To examine the origin of the limited range we focus on a class of aggregation systems. In these systems a molecular beam is deposited on a surface, giving rise to nucleation and growth of diffusion-limited-aggregation-like clusters. Scaling arguments are used to show that the required duration of the deposition experiment increases exponentially with $\Delta$. Furthermore, using realistic parameters for surfaces such as Al(111) it is shown that these considerations limit the range of fractal behavior to less than two decades in agreement with the experimental findings. It is conjectured that related kinetic mechanisms that limit the scaling range are common in other nonequilibrium processes that generate spatial fractals.