Solute strongly impacts freezing under confinement

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 alone on freezing is well known, water is never pure and solutes depressing the freezing point are naturally found. However, the combination of confinement and solute is poorly understood. Here, we study in situ the freezing of water in a model porous medium made of densely packed particles with various salt (KCl) concentrations. We demonstrate a synergistic effect of solute with confinement: the freezing front, initially heterogeneous due to confinement, drives solute enrichment in the remaining liquid, further depressing its freezing point. This increases the local freezing point depression and results in much larger mushy layers where ice and liquid water coexist. We compare our experimental freezing profile with theory and estimate the local solute concentration to increase by more than one order of magnitude through the freezing process. These results imply that even low solute concentrations may have important effects on the distribution of water in frozen porous media and should help explain the variety of freezing patterns observed experimentally. This may be critical for cryo-tolerance of construction materials and organisms and will help understanding solute precipitation and redistribution in soils.