  The presence of liquid water in frozen media impacts the strength of soils,
the growth of frost heave, plant life and microbial activities, or the
durability of infrastructures in cold regions. If the effect of confinement on
freezing is well known, water is never pure and solutes depressing the freezing
point are naturally found. Moreover, the combination of confinement and solute
is poorly understood. We imaged the freezing dynamics of water in a model
porous medium with various salt (KCl) concentrations. We showed that the
freezing front, initially heterogeneous due to confinement, drives salt
enrichment in the remaining liquid, further depressing its freezing point.
Confinement and solute have a synergistic effect that results in much larger
mushy layers and greater freezing point depression. These results should help
understand the distribution of water in frozen porous media, solute
precipitation and redistribution in soils, and cryo-tolerance of construction
materials and organisms.
