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Abstract:
Aerogels are solid materials with a porous structure filled with air and are among the best thermal insulation materials. During ambient pressure drying (APD), silica gels endure significant drying shrinkage due to the capillary pressure generated by pore liquid evaporation. Silylated gels can recover the drying shrinkage through a phenomenon called the spring-back effect (SBE). However, the underlying structural mechanisms and the evolution of the amount of liquid and gaseous phases in the pores of a gel undergoing APD remain unexplored. Here, we use in operando X-ray micro-computed tomography (μCT) with quantitative imaging to monitor the progression of the liquid, gaseous, and solid phases of silica gels during APD, i.e., during the drying shrinkage and the subsequent SBE. Silica gels modified by trimethylchlorosilane shrank to 17% of their original volume and sprung back to 29% of their original volume. We found that a mixture of gaseous and liquid phases is already present in gels before the maximum shrinkage, which challenges the common assumption that gas penetrates the pores in parallel to the SBE. The emergence of the SBE was correlated to an equal volume fraction of solid, liquid, and gas in the gels. The evaporation rate decreased near and after the maximum shrinkage, suggesting a shift from convection-limited to diffusion-limited transport of hexane vapor. Our results show that fluid movements and volume change can be monitored in operando by μCT imaging during ambient pressure drying and spring-back of an aerogel, showing the combined effects of evaporation of fluid and diffusion of gas within and out of the aerogel.
