Abstract

Colloidal suspensions do not freeze uniformly—- rather, the frozen phase (ice) becomes segregated, trapping bulk regions of colloid within. Ice segregation in freezing colloidal suspensions is important in a number of applications and is a fascinating example of pattern formation. However, the physical processes behind ice segregation in concentrated suspensions are still poorly understood. To better understand these physics, controlled freezing experiments were performed with concentrated colloidal alumina dispersions using a directional solidification apparatus that provides independent control of the freezing rate and temperature gradient. Distinct modes of periodic ice-banding were observed depending on the freezing rate. For each mode of banding, the distance between successive layers of ice decreases with increasing freezing rate. At slow freezing rates, the rejected particles are found to be irreversibly aggregated by cryosuction forces to form a close-packed cohesive layer which is visibly darker than the surrounding suspension. The temperature in this aggregated layer is depressed below the bulk freezing point by more than 2 degrees Celsius before the ice lenses are encountered; moreover, this undercooled region appears as a light-colored layer. The magnitude of the undercooling and the color change in this region both suggest the presence of pore ice and the formation of a frozen fringe. As well as resolving discrepancies in our experimental observations, the frozen-fringe hypothesis also leads to a theoretical framework for periodic ice-lensing. I shall describe a model for ice lensing in qualitative agreement with our experimental observations.