Fractal aggregates in geophysics

The combination of small particles to form larger structures is important in a broad range of natural processes, including the flocculation of organic and mineral particles in rivers, estuaries, and the open sea, the growth of smoke particles in the atmosphere, and accretion in the primordial solar nebula. Very often, these structures (cluster, flocs, and aggregates) have a low‐density random structure that can be described quite well in terms of the concepts of fractal geometry. The structure, growth kinetics, and properties of these “fractal aggregates” have been of considerable interest in statistical physics for the past decade. Other processes such as fluid‐fluid displacement in a porous medium, the dissolution of porous materials, dielectric breakdown, and random dendritic growth also lead to the growth of random fractals and can be understood in terms of aggregation models such as diffusion‐limited aggregation. The characterization of fractal aggregates produced by computer simulations, laboratory experiments, and natural processes is described, and some of the implications of the aggregates' fractal structure are discussed. The role played by fractal aggregates in geophysical processes such as sedimentation, air and water pollution, the evolution of river networks, and multiphase flow in porous materials is also discussed.