This study focuses on the transition from the paste state to the solid state which occurs during the hardening of metakaolinbased Na-geopolymers. The occurrence of primary interactions and the structural properties were investigated using classical oscillatory rheology (OR) methods. A time-frequency-resolved rheology (TF2R) method was used in order to obtain more detailed information about the percolation and aggregation mechanisms. The results obtained show the occurrence of the following process: (i) Elastic behavior predominates initially over the viscous behavior at low frequency. (ii) A percolating process then takes place when the viscoelastic parameters become parallel over more than two frequency decades. At the gel point, a mass-fractal dimension of approximately 2 was determined. (iii) During the formation of the porous network, the viscous behavior predominates over the elastic one due to the occurrence of polycondensation reactions. (iv) Lastly, a solid state is reached, where the elastic modulus shows a plateau and the viscous modulus decreases. These macroscopic mechanical results are compared using the small angle x-ray scattering technique. Scattering experiments were in complete agreement with the rheological measurements and showed the presence of aggregated oligomers and a mass fractal dimension equal to 2.1 at the gel point, increasing up to a surface fractal dimension reflecting the formation of the mesoporous network.
Quantitative ultrasound techniques based on the backscatter coefficient (BSC) have been commonly used to characterize red blood cell (RBC) aggregation. Specifically, a scattering model is fitted to measured BSC and estimated parameters can provide a meaningful description of the RBC aggregates' structure (i.e., aggregate size and compactness). In most cases, scattering models assumed monodisperse RBC aggregates. This study proposes the Effective Medium Theory combined with the polydisperse Structure Factor Model (EMTSFM) to incorporate the polydispersity of aggregate size. From the measured BSC, this model allows estimating three structural parameters: the mean radius of the aggregate size distribution, the width of the distribution, and the compactness of the aggregates. Two successive experiments were conducted: a first experiment on blood sheared in a Couette flow device coupled with an ultrasonic probe, and a second experiment, on the same blood sample, sheared in a plane-plane rheometer coupled to a light microscope. Results demonstrated that the polydisperse EMTSFM provided the best fit to the BSC data when compared to the classical monodisperse models for the higher levels of aggregation at hematocrits between 10% and 40%. Fitting the polydisperse model yielded aggregate size distributions that were consistent with direct light microscope observations at low hematocrits.
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