2025 AIChE Annual Meeting

(600a) Microstructure and Rheology of Colloidal Gels with Bidisperse Particles

Authors

Lilian Hsiao - Presenter, North Carolina State University
We report the ensemble-averaged and pairing-specific microstructure formed by short-range depletion attractions in hard sphere-like colloidal systems, along with rheological characterization performed using oscillatory amplitude sweeps. Gelation is induced by adding polystyrene molecules at a fixed concentration to colloids with different bidispersity ratios (α = 1, 0.70, and 0.62) at a range of volume fractions that span the gelation threshold (0.10 ≤ φ ≤ 0.40). 3D confocal microscopy imaging combined with a scale-invariant feature transform algorithm show that monodisperse colloids pack more efficiently, whereas increasing the size disparity leads to looser, more disordered, and sub-isostatic packings. Categorizing the structures formed by small and large particles reveal that certain cluster configurations may be favored due to a lower degree of symmetry with greater rotational entropy. These triplet bonds between small and large particles affect the density of tetrahedral and poly-tetrahedral clusters in bidisperse systems. Above the gelation threshold, increasing the gel volume fraction leads to an increase in the number of nearest neighbors. However, the internal density within each cluster decreases, possibly due to kinetic arrest from the deeper potential wells of tetrahedral clusters at low volume fractions in which vertices are primarily made out of larger particles. Confocal rheometry performed on these systems suggest that the shear-induced cluster structures are strain- and bidispersity-dependent.