Dynamic NMR Relaxometry as a Straightforward Measurement of Concentration Variations in Colloidal Gels

We show that dynamic NMR relaxometry allows one to probe the particle size or the concentration evolution over time in homogeneous colloidal suspensions or the concentration in different regions of heterogeneous suspensions, up to large volume fractions. We first demonstrate that the NMR transverse relaxation time is independent of the gel structure at the particle scale so that it only slightly varies during the gelation of a colloidal suspension. The evolution over time of the NMR transverse relaxation time during gel drying and its analysis with the help of the fast-exchange assumption extended to a partially saturated medium then allowed us to identify three successive regimes: homogeneous shrinkage, desaturation, and molecular film regime. A detailed analysis of the NMR relaxation characteristics provides information on the distribution of the fluid along the solid structure at the particle scale in the two last (partially desaturated) regimes. This in particular shows that, thanks to such analysis of their temporal evolution, such simple, nondestructive, time-resolved, global measurements can be used to follow precisely the solid volume fraction of the system and its state of saturation up to full drying, independently of the exact solid structure.