NMR Characterization of unfrozen brine vein distribution and structure in model packed beds

When a brine mixed with particles is frozen, some liquid water persists due to the freezing point depression caused by the solute impurity, surface energy, and disjoining pressure (wetting forces). This unfrozen water forms a complex "liquid vein network" (LVN). However, details of the freezing process are still not fully understood, including the permeability/tortuosity of the LVN, and the unfrozen water content at a given temperature. Here, we have applied nuclear magnetic resonance (NMR) relaxation, self-diffusion measurements and magnetic resonance imaging (MRI) to investigate the distribution and structure of LVNs. Magnesium chloride (MgCl2) salt concentrations of 15, 30, and 60 mM were investigated with and without poly-methyl methacrylate (PMMA) particles of diameter 0.4, 9.9, and 102.2 mu m, allowing us to quantify unfrozen water content and the structure of the LVN as a function of temperature, MgCl2 concentration, and PMMA particle size. The results of magnetic resonance imaging (MRI) and self-diffusion confirm that the inhibition of ice recrystallization is a function of particle size. To gain information on LVN structure, we compared NMR results to Monte Carlo simulations of freezing in brine-particle systems. Comparisons between laboratory and simulation data suggest that, for our experimental range of temperature (-17.4 degrees C to-0.9 degrees C +/- 0.5 degrees C), solutes make the dominant contribution to the unfrozen liquid fraction for particle sizes larger than a few microns, whereas in the finest grained porous media we tested, the unfrozen liquid fraction is controlled primarily by the films that wet particle-ice interfaces.