Capillary length measurements and noise-driven sidebranches in the dendritic crystal growth of ammonium chloride.

Dendritic crystal growth is an important example of nonequilibrium pattern formation that involves both nonlinear and noise-driven effects. The large-scale structures are sensitively dependent on relatively small effects, such as surface tension, and on small anisotropies in those quantities. Testing theoretical models requires careful measurements of the relevant materials parameters. For the growth of ammonium chloride crystals from aqueous solution, previous published estimates of the capillary length have varied by over a factor of 20. We report the results of a new technique for non-faceted materials. This method uses a nearly spherical crystal held near unstable equilibrium in an oscillating temperature field. We find that the product of the chemical diffusion constant D and the capillary length d₀ is approximately 0.5 μm³/s. We also consider noise-driven models of sidebranch growth. No simple power law describes either the growth of the average sidebranch amplitude or the average sidebranch envelope. Instead, the effective power law exponent appears to increase as a function of distance from the dendritic tip. Based on our new capillary length measurement, the sidebranch amplitude is larger than predicted by models of noise-driven sidebranching.