Nucleation-limited aggregation of crystallites in fractal growth

Abstract We report in this article a crystallization behavior of NH 4 Cl in a gel system, which leads to a new type of fractal aggregate. In situ optical microscopy and atomic force microscopy are used to study the growth process and the aggregate morphology. Microscopically the crystallites in the aggregate branches have remarkable correlations with their crystallographic orientations, which introduces a strong effective anisotropy to the fractal growth. Macroscopically, the fractal pattern has a scaling property different from the fractals generated by diffusion-limited aggregation and that generated by homogeneous nucleation-limited aggregation. It seems that the fractal patterns do not arise exclusively in the far-from-equilibrium situations; nor do they need the anisotropy for the fractal aggregation to be very low.