Characterization of Gels During Drying

Since X-ray tomography keeps the sample intact, it is suitable not only for the characterization of the dried product, but also to follow the evolution of gel characteristics during the drying process. Up to now, this requires an interruption of the process to scan the sample being dried, but, in principle, the tomograph can be built around laboratory drying equipment. For such dynamic measurement, the time for a full scan of the sample is a crucial parameter it depends on the sample size and the desired resolution and may vary from a few minutes, if500 voxels are sufficient, to several hours, for 2000 voxels. 

As most gels shrink during drying, it is essential to determine their surface area evolution to calculate the drying flux from mass loss measurements. With appropriate image analysis, very accurate shrinkage curves (as in Fig. 5.34) can be obtained from X-ray transmission images and reconstructed cross-sections (Fig. 5.12). The 

In a more general context. X-ray microtomography can also help to determine the internal structure of the dried product. Unlike SEM, which requires sample preparation and yields only 2D information for a small part of the surface so that many repetitions are necessary for statistically relevant results, it is a non-destructive technique that gives full 3D information. For these reasons. X-ray microtomography can complement classical measurement techniques, if the dry material exhibits large pores which cannot be characterized by nitrogen adsorption (from 2 to 50 nm) or mercury porosimetry (from 7.5 nm to 150pm). 

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