Rheology of granular-colloidal gel composite suspensions
Granular-colloidal gel composite suspensions composed of a mixture of large repulsive particles (LRPs) and small attractive particles (SAPs) are encountered in a variety of industrial applications such as the manufacturing of Lithium (Li)- ion battery cathodes. While granular suspensions and colloidal gels have been well-studied individually, there has been relatively little work on mixtures of these two systems, “granular-colloidal gel composite” suspensions. Although the previous studies reported that the internal structure of a composite system in a quiescent state changes by the interplay between granular and gel phases, the rheological behaviour under flow and the link between the rheology and internal structure remain open challenges. In this thesis, we investigate the rheology and internal structure of model granular-colloidal gel composite systems which are composed of larger non-functionalised silica and much smaller hydrophobic fumed silica in low molecular-weight polyethylene glycol (PEG). In the first part of the thesis, we find a unique transition between solid and liquid states driven by applied pre-shear σpre, the shear-driven state transition. We map out the different rheological states in a (ϕₗ, σₚᵣₑ) diagram, showing that the shear-driven state transition occurs in 0.1 ≲ ϕₗ ≲ 0.5, and also that this range can be divided into three different regimes: the bistable, the sol, and the gel regimes, with two volume-fraction dependent boundaries: σᵧ(ϕₗ), σb(ϕₗ). The first boundary σᵧ, which divides the bistable state and the sol state, corresponds to the yield stress of the gel state. The second boundary σb, divides the sol state and the gel state. We connect the unique rheological state transition to changes in the suspension microstructure observed by cryogenic-Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (EDS) mappings. Shearing the sample under σₚᵣₑ ≥ σb cause the SAPs to be well-dispersed among the LRPs, which themselves can form a gel network upon shear cessation. By contrast, the gel network melts into disjoint “blobs” of the SAPs by shearing the sample under σy ≤ σₚᵣₑ < σb, leading to a liquid state at rest. The dense blobs are abruptly broken up at σₚᵣₑ ≈ σb, the stress at the second boundary. In addition, we find that the blobs show a core-shell structure, where the higher-volume-fraction shell implies that the blobs are compressed from the outside due to the motion of the LPRs. In the second part of the thesis, we explore how the boundaries on the state diagram can be tuned by changing experimental parameters such as the concentration of the gel phase, the particle size of the LRPs, the surface coatings on the SAPs, and the choice of solvent. We show that the higher ϕₗ or σᵧ was, more likely the shear-driven state transition occurs. We also show that both σy and σb can be tuned by changing the surface coatings and the choice of solvent, suggesting that σy of the background gels might be predictive of the shear-driven state transition behaviour. On the other hand, the results with lower attraction SAPs show that the relationship between σy and σb is not so simple and might depend on a number of other details, such as gel network/micro-structure, tribology of SAP-SAP contacts, or the interaction between SAP surfaces and solvent molecules. In addition, we examine how the LRPs influence the blob strength and structure by measuring the volume fraction of the SAPs inside blobs, ϕᶦⁿₛ, using EDS. We show that ϕᶦⁿₛ increases with ϕₗ, suggesting that the blobs are compressed by the LRPs and thus they are reinforced. We suggest that our LRPs generated particle pressure and this local pressure could reinforce the mechanical strength of the blobs. In the final part of the thesis, we develop a more industrially-relevant composite system composed of the same large non-functionalised silica particles and carbon black particles in PEG. We show that the shear-driven state transition occurs in different systems: trimethyl coated fumed silica composite, octyl coated fumed silica composite, and carbon black composite, demonstrating that the state transition may be generic in such granular-colloidal gel composite systems. Therefore, we believe that the results of this study can provide important strategies to control the rheology and internal structure in industrial applications such as the processing of Li-ion battery cathodes.