Dynamic NMR Relaxometry Highlights Drying-Induced Heterogeneity and Structural Reconfiguration in Colloidal Gels

Abstract

In this work, we introduce a dynamic NMR relaxometry approach to monitor gel drying not as a function of time, but through the evolution of the transverse relaxation time (T2) as a function of gel saturation. This formulation enables us to track, with high temporal resolution and in a noninvasive manner, how homogeneity evolves during the drying of deformable colloidal systems. The originality of the approach resides in the coupling between three complementary elements: experimentally measured hydric profiles, numerically simulated hydric profiles obtained from a multiparameter model, and the corresponding distributions of T2. This combined framework reveals distinct drying regimes and physical instabilities, each characterized by robust power-law relationships between transverse relaxation time (T2) and gel water content. A key outcome of the study is the ability to quantify the gel reconfiguration capacity, that is, its ability to rearrange its internal network structure during drying, and to define simplified, equivalent hydric profiles that offer clear insight into drying-induced heterogeneities. Notably, we show that gel rigidity is highly sensitive to sodium hydroxide concentration, with optimal particle reconfiguration and homogeneity preservation observed near a standard concentration. This integrated approach offers a powerful new tool for characterizing the global behavior of gels during drying, without the need to consider in detail the local water content, and provides a versatile framework for optimizing formulation, processing, and performance in a wide range of practical and industrial applications.