Dendritic Crystal Growth From Aqueous Solutions of Ammonium Chloride and Ammonium Nitrate

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. Specifically, we present measurements of the tip radius ρ and growth speed v, along with initial estimates of the stability constant σ*=2d₀D/vρ², where D is the chemical diffusion constant and d₀ is the capillary length. These results use a new model for the dendritic tip shape that attempts to capture the slight deviations from a pure parabolic shape. Finally, preliminary results for ammonium nitrate at higher temperatures show non-parabolic dendrites, where the tip is approximately a hemispherical cap followed almost immediately by a large set of sidebranches.