MEAN SHAPE OF 3-DIMENSIONAL DENDRITES - A COMPARISON OF PIVALIC ACID AND AMMONIUM-CHLORIDE

The mean shape of free dendrites of pivalic acid growing from solution is determined. For a three-dimensional axisymmetric needle crystal, the mean width wBAR of the dendrites would scale with distance z from the tip according to w0 = square-root 2rhoz, where rho is the tip radius. For pivalic acid growing from a solution of pivalic acid and ethanol, we find that wBAR does not grow as a simple power law in z, and is always larger than w0. This result reflects the concentration of material into the sidebranching planes due to crystalline anisotropy. A comparison with NH4Cl in aqueous solution, which has a larger surface tension anisotropy, shows that the magnitude of the crystalline anisotropy does not affect the mean shape. The connections with current three-dimensional theories of dendritic growth are also discussed.