Growth kinetics of body centered cubic colloidal crystals

Abstract

A combination of static light scattering and video microscopy is used to perform high precision measurements on the growth velocity of body centered cubic (bcc) crystals in a metastable colloidal melt of monodisperse, highly charged latex spheres. The crystals nucleate heterogeneously at the walls of a flat flow-through shear cell and solidification proceeds without significant disturbance by homogeneous nucleation. The suspension parameters packing fraction Φ of the spheres and the concentration of screening electrolyte c are systematically varied for two kinds of particles with equal diameter but different charge. For all experimental conditions the growth velocities in the 〈110〉 direction collapse on a single curve if plotted against a reduced energy density difference ${\mathrm{\Pi }}^{\mathrm{*}}$ between the melt and the fluid at melting. Close to the phase boundary growth velocities vary linearly with increasing ${\mathrm{\Pi }}^{\mathrm{*}}$, and saturate at large ${\mathrm{\Pi }}^{\mathrm{*}}$ at a value of ${\mathit{v}}_{\mathrm{\infty }}$=9.1 μm ${\mathrm{s}}^{\mathrm{-}1}$. The master curve can be fitted excellently by a Wilson-Frenkel growth law which was suggested to hold for the solidification of highly charged systems. A comparison of coefficients allows for the derivation of a quantitative estimation procedure for the difference in chemical potential Δμ between melt and solid in terms of the thermal energy ${\mathit{k}}_{\mathit{B}}$T: Δμ=${\mathrm{\Pi }}^{\mathrm{*}}$B. The best value for the conversion factor B is found to be B=(6.7±0.1)${\mathit{k}}_{\mathit{B}}$T. In contrast to previous work on homogeneously nucleated crystals the growth velocity of the 〈110〉 face is limited by the reactionlike kinetics of registering preordered layers formed within an interface of finite thickness. We suggest a unified description covering also the growth of the rough interfaces of other crystal faces. (c) 1995 The American Physical Society