Quantification of porous microstructures in partially frozen drops using magnetic resonance techniques

Nuclear magnetic resonance (NMR) measurements were shown to be able to non-invasively quantify the microstructure formed following freeze-concentration of (2 mm diameter) drops composed of a range of aqueous solutions (sucrose, fructose, coffee solution and NaCl). Specifically the surface-to-volume ratio and tortuosity of the resultant porous ice structures were uniquely quantified using pulsed field gradient (PFG) NMR measurements of restricted self-diffusion of the remaining unfrozen solution. These measurements were performed as a function of both solute concentration and freezing temperature. Validation was achieved via random walk self-diffusion simulations on SEM micrograph images of fracture planes through selected drops. Reasonably good agreement was produced between the NMR measurements and these simulations. A rapid version of the PFG measurement method was subsequently used to enable quantitative microstructure evolution measurements during the drop freezing process, as well as during subsequent ripening of the porous ice structure.