Disorder-to-Order Transition in Settling Suspensions of Colloidal Silica: X-ray Measurements

Dispersions of colloidal particles exhibit thermodynamic properties similar to those of molecular systems, including a hard sphere disorder-to-order transition. In experiments with organophilic silica in cyclohexane, gravity settling was used to concentrate the particles. With small particles the slow sedimentation permits rearrangement into the iridescent ordered phase, but larger particles form amorphous sediments instead. Scanning electron microscopy of the crystalline sediment indicates hexagonally closepacked layers. X-ray attenuation measurements reveal a discontinuity coincident with the observed boundary between iridescent and opaque regions. Sediments accumulating faster than the maximum rate of crystallization produce a glass, in accord with the classical theory for crystal growth.