When two mixed materials separate, like water and oil, phases or domains are formed and grow as the materials move and diffuse into them. If the movement of the particles controls the rate of growth of the domains, then patterns similar to the image here are seen. These are called diffusion limited aggregates, commonly termed fractals. An entire branch of mathematics is based on these types of structures. Using fluorescent particles, these fractal-type structures can be easily observed in the microscope. The formation here looks like the branches of a tree. Whether you look at an entire tree, or branches going out from the trunk, or branches going out from other branches, the pattern of the branch formation looks the same. In the same way, these structures look the same even on different length scales.