Microscopic investigations of homogeneous nucleation in charged sphere suspensions

We studied the homogeneous nucleation kinetics of an aqueous suspension of charged colloidal spheres under de-ionized conditions. Samples of equilibrium crystalline structure were shear molten and the metastable melt left to solidify after cessation of shear. At low particle number densities $n$⁠, corresponding to low metastability of the melt, nucleation was monitored directly via video microscopy. We determined the nucleation rates $Γ(t)$ by counting the number of newly appearing crystals in the observation volume per unit time. Using a suitable discrete adaptation of Avrami's [J. Chem. Phys. 7, 1003 (1939); J. Chem. Phys.8, 212 (1940); J. Chem. Phys.9, 177 (1941)] model for solidification via homogeneous nucleation and subsequent growth, we calculate the remaining free volume $VF(t)$ to obtain the rate densities $J(t)=Γ(t)∕VF(t)$⁠. We observe $J(t)$ to rise steeply, display a plateau at a maximum rate density $Jmax$⁠, and to decrease again. With increased $n$ the plateau duration shrinks while $Jmax$ increases. At low to moderate number densities fully solidified samples were analyzed by microscopy to obtain the grain-size distribution and the average crystallite size $⟨L⟩$⁠. Under the assumption of stationarity, we obtained the nucleation rate density $JAvr$⁠, which increased strongly with increasing $n$⁠. Interestingly, $JAvr$ agrees quantitatively to $Jmax$ and to $JAvr$ as obtained previously from scattering data taken on the same sample at large $n$⁠. Thus, by combination of different methods, reliable nucleation rate densities are now available over roughly one order of magnitude in $n$ and eight orders of magnitude in $J$⁠.