Fractal Aggregations at Low Driving Force with Strong Anisotropy

In this Letter we report a unique fractal aggregation behavior of ${\mathrm{NH}}_4\mathrm{Cl}$ crystallites grown in agarose gel. The fractal branches consist of faceted crystallites with remarkable correlations in their crystallographic orientations, which are formed by successive heterogeneous nucleations. Unlike the conventional fractal growth, this aggregation process occurs at a low growth driving force with strong crystallographic anisotropy, which leads to the ordering in the fractal branches.