Settling suspensions of colloidal silica: observations and X-ray measurements

Suspensions of colloidal particles execute Brownian motion and thus exhibit thermodynamic properties analogous to those of molecular systems. Hard-sphere colloidal silica suspensions undergo a disorder–order transition, i.e. freezing or crystallization, at high volume fractions. In suspensions of small particles the slow sedimentation permits the transition to occur at the bottom where the bulk of the crystalline sediment is formed by one-dimensional crystallization, as illustrated with photographs and scanning electron microscopy. X-Ray tomography measurements reveal a volume fraction discontinuity coincident with the observed crystal boundary occurring between the sediment and hindered setting region of the Kynch theory. Larger particles, however, form amorphous sediments because their rate of accumulation at the bottom exceeds the maximum crystal growth rate, in accord with classical kinetic theory. This theory is paired with Kynch's kinematic theory using an experimental sedimentation coefficient to interpret observations of settling suspensions.