Dendrites Growing in the Presence of an External Flow: The Cases of PVA and NH₄Br

Abstract

The problem of dendritic shape selection in crystal growth processes has been the subject of many theoretical and experimental studies these last years. Theoretical predictions¹ for the values of growth constant, C = 4 ρ²V/Dd₀ are now available (here ρ, V are the tip radius and the growth velocity of the dendrite, and D, d₀ are the diffusion constant and the capillary length). Until now, only a few measurements of the growth constant have been done. Experiments performed with SCN² show good agreement between theory and experiment, whereas those carried out with NH₄Br³ indicate agreement only within ± 25 %. Other measurements of C have also been performed on Pivalic Acid, indicating values much larger than those predicted by the theory; actually they need to be completed by a reliable estimate of the crystal anisotropy⁴. Recently, dendrites of Ammonium Bromide growing in a supersaturated gel have been studied. The values of the growth constant are found significantly below the theoretical prediction. Until now, there is therefore no convincing agreement between theory and experiment concerning the stationary state of dendrites.