Dendritic crystal growth: A comparison of ammonium nitrate and ammonium chloride

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. The resulting large-scale structures are sensitively dependent on relatively small effects, such as surface tension, and also on small anisotropies in those quantities. In this work, we present new results for ammonium nitrate dendrites grown from supersaturated aqueous solution, and compare them with previous results for the well-studied ammonium chloride system. 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. Preliminary results at higher temperatures show non-parabolic dendrites, where the tip is approximately a hemispherical cap followed almost immediately be a large set of sidebranches. We will discuss similarities and differences between the two materials.

[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