The effects of initial seed size and transients on dendritic crystal growth

The transient behavior of growing dendritic crystals can be quite complex, as a growing tip interacts with a sidebranch structure set up under an earlier set of conditions. In this work, we report on two observations of transient growth of NH$_4$Cl dendrites in aqueous solution. First, we study growth from initial nearly-spherical seeds. We have developed a technique to initiate growth from a well-characterized initial seed. We find that the approach to steady state is similar for both large and small seeds, in contrast to the simulation findings of Steinbach, Diepers, and Beckermann[1]. Second, we study the growth of a dendrite subject to rapid changes in temperature. We vary the dimensionless supersaturation $\Delta$ and monitor the tip speed $v$ and curvature $\rho$. During the transient, the tip shape is noticeably distorted from the steady-state shape, and there is considerable uncertainty in the determination of the curvature of that distorted shape. Nevertheless, it appears that the ``selection parameter'' $\sigma^* = 2 d_0 D / v \rho^2$ remains approximately constant throughout the transient. [1] I. Steinbach, H.-J. Diepers, and C. Beckermann, \textit{J. Cryst. Growth}, \textbf{275}, 624-638 (2005).