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. Two distinct steady-state modes of periodic ice-banding were observed in the range of freezing rates examined. At slow freezing rates, the ice segregates from the suspension into crack-like ice lenses and there are dark- and light-colored layers visible in the boundary-layer of rejected particles directly ahead of the ice lenses. At fast freezing rates, ice bands appear qualitatively different and there is no visible structure in the suspension ahead of the segregated ice. There is a transition between these two modes of ice banding at intermediate freezing rates, marked by disordered of ice segregation and coinciding with the disappearance of the dark boundary-layer. For each mode of banding, the bandwidth between successive layers of ice decreases with increasing freezing rate. The boundary-layer structure formed by the rejected particles is described in detail, given the apparent importance of this structure on the resultant mode of ice segregation. 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 2C 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 an established theoretical framework for periodic ice-lensing. I’ll describe a model for ice lensing in qualitative agreement with our experimental observations.