Steady-state dendritic growth ofNH4Br from solution

Abstract

The growth of ${\mathrm{NH}}_4$Br dendrites from supersaturated aqueous solution is investigated to test recent theories of dendritic growth. A key prediction of these theories is that the tip radius ρ and steady-state growth speed v depend strongly on the surface tension anisotropy ɛ. We measure the capillary length d${¯}_0$ and the crystalline anisotropy of ${\mathrm{NH}}_4$Br, along with the radius and speed of steady-state dendritic tips, in order to make a quantitative test of the theory. The stability constant ${\sigma }^{\mathrm{*}}$ (defined to be 2d${¯}_0$D/v${\rho }^2$, where D is the diffusion constant) is found to be 0.081±0.02 for this material, in approximate agreement with the theoretical prediction of 0.065±0.02. The initial side-branch wavelength is found to be slightly larger than predicted. These measurements are consistent with the hypothesis that ‘‘microscopic solvability’’ determines the steady state of a dendrite.