Irregular Fractal-Like Crystal Growth of Ammonium Chloride

Abstract

Irregular dendritic crystals of ammonium chloride (NH₄Cl) are grown two-dimensionally between two parallel glasses, one of which has rough surface. The pattern clearly reminds us of those of Witten-Sander diffusion-limited aggregation (DLA) model. The radius of gyration is measured to give the fractal dimension d_f=1.67±0.002, which is in good agreement with two-dimensional DLA. Our result implies that any random perturbations strong enough to overcome crystalline anisotropy always give rise to random DLA-like crystals, instead of otherwise regular dendritic crystals.