MECHANISM OF THE TRANSITION FROM FRACTAL TO DENDRITIC GROWTH OF SURFACE AGGREGATES

THE similarity of many patterns formed in non-equilibrium growth proce sses in physics, chemistry and biology is conspicuous, and many attemp ts have been made to discover common mechanisms underlying their forma tion(1). A central question is what causes some patterns to be dendrit ic (symmetrically branched, like snowflakes) and others fractal (rando mly ramified). In general, the transition from fractal to dendritic gr owth is regarded as a manifestation of the predominance of anisotropy over random noise in the growth process. In electrochemical deposition , this transition is observed as the growth speed is varied(2,3). Here we report a crossover from fractal to dendritic growth in two dimensi ons on the microscopic scale. We use the scanning tunnelling microscop e to study diffusion-limited aggregation of silver atoms on a Pt(111) surface. The transition occurs as the deposition flux is increased, an d our observations suggest that the increasing importance of anisotrop y of edge diffusion at higher flux is responsible for this crossover. We anticipate that a similar phenomenon may operate in three-dimension al crystal growth.