  The freezing of colloidal suspensions is encountered in many natural and
engineering processes such as the freezing of soils, food engineering, and
cryobiology. It can also be used as a bio-inspired, versatile, and
environmentally-friendly processing route for porous materials and composites.
Yet, it is still a puzzling phenomenon with many unexplained features, due to
the complexity of the system and the space and time scales at which the process
should be investigated. We demonstrate here the interest of fast X-ray computed
tomography to provide time-lapse, three-dimensional, in situ imaging of ice
crystal growth in a colloidal silica suspension. The experimental measurements
show that the local increase of colloid concentration does not affect the
growth kinetics of the crystals, until the colloidal particles become closely
packed. For particles much smaller than ice crystals, the concentrated
colloidal suspension is equivalent to a simple liquid phase with higher
viscosity and a freezing point determined by the concentration of colloidal
particles.
