Quantification of microstructures in stable and gelated suspensions from Cryo-SEM

It is shown that the microstructures of concentrated suspensions can be analyzed in a quantitative way from cryo-SEM images of high-pressure frozen samples, both in the electrostatically stabilized and in the flocculated state. Suspensions of spherical silica particles (40 vol%) in an aqueous solution were used. The average particle radius was 525 nm with a polydispersity of below 7%. The suspensions were high-pressure frozen, which resulted in a quenching of the configuration without apparent change in volume or crack formation. After fracturing the samples at liquid nitrogen temperature, the fracture surface was etched by controlled sublimation of the frozen aqueous phase, coated with 8 nm of platinum, and examined by stereo-cryo-SEM. The 3-dimensional positions of all the visible particles were determined from the SEM images. Assuming an isotropic particle configuration in the sample before cracking, it is possible to extract the 3-dimensional pair correlation function from the particle positions on the fracture surface. A comparison to recent results from Brownian Dynamics simulations shows good agreement between our experiments and the simulations.