Dendritic Growth of Ammonium Nitrate Crystals

Dendritic crystal growth is an important example of nonequilibrium pattern formation that involves both nonlinear dynamics and noise-driven effects. It is commonly observed in the growth of metal alloys, but can also be observed in the solidification of some transparent organic and inorganic compounds. In particular, the growth of ammonium nitrate dendrites are of interest because ammonium nitrate is essential for creating fertilizers. The resulting large-scale structures exhibit growth facilitated by chemical or thermal diffusion and are sensitively dependent on relatively small effects, such as surface tension, and also on small anisotropies in those quantities. In this work, we present results for phase II ammonium nitrate dendrites grown from supersaturated aqueous solution. This new system has been studied previously by van Driel et al.[1]. Specifically, we present new measurements of the tip radius ρ, growth speed v, and sidebranch spacing λ, along with initial estimates of the stability constant σ*=2d₀D/vρ², where D is the chemical diffusion constant and d₀ is the capillary length. We also discuss the large-scale structure of the dendritic patterns, and make qualitative comparisons between ammonium nitrate dendrites and crystals of other chemistries that demonstrate dendritic growth.

[1] Van Driel, C.A., Van der Heijden, A.E.D.M, Van Rosmalen, G.M., "Growth of Ammonium-Nitrate Phase-I and Phase-II Dendrites." J. Cryst. Growth 128 (1993) 229–233. https://doi.org/10.1016/0022-0248(93)90324-P