Colloidal suspension rheology under shear and compression
Creators: name: ISLAM, Mohammad Manjiul
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)

description: Suspensions of colloidal particles are central to a wide range of industries and materials such as minerals processing, pharmaceuticals, coatings and paints, and knowledge of their rheology is critical to both suspension processing and material performance. Whilst much is known regarding the rheology of strongly flocculated colloidal gels under shear or compression alone, almost all applications involve an amount of combined shear and compression. Although the combination of these deformation modes is known to vastly alter material behaviour, the rheology of strong colloidal gels under such arbitrary tensorial tress states, termed the tensorial rheology, is largely unknown. This project aims to investigate the tensorial rheology of strong colloidal gels at the microscopic (particle) level and develop quantitative macroscopic descriptions of material behaviour. We use strain-controlled particle scale Discrete Element Method (DEM) simulations to probe the behaviour of strong colloidal gels under combined shear and compression. These simulations uncover the fundamental mechanisms of failure and consolidation of these disordered materials under a range of tensorial strain states.

We first consider the evolution of normal stress differences (NSDs) during uniaxial consolidation, which is capable of quantifying the tensorial state of the colloidal network. The consideration of NSDs during uniaxial consolidation is one step towards the development of tensorial rheology. We investigate the evolution of the transverse stress that then complete the evolution NSDs during uniaxial consolidation. We demonstrate that during consolidation, the Poisson ratio rises from zero close to the gel point to nearly unity at close-packing and reveals the particle-scale mechanisms that support these observations. This observation of NSDs establishes that the rheology of strong colloidal gels is inherently tensorial under compressive loadings which demands a tensorial modelling framework to resolve the complex rheology of these gels.

We then extend the consolidation of strong colloidal gels under arbitrary compressive stresses. This study provides an understanding of how strong colloidal gels deform and consolidate over the range from isotropic to uniaxial consolidation. Consolidation appears to differ fundamentally between uniaxial and biaxial systems in both micro-scale and macro-scale. Macroscopic analysis of strong colloidal gel demonstrates that the maximum normal stress is invariant between uniaxial and isotropic consolidation. These insights are used to develop a macroscopic constitutive model under arbitrary compressive loadings, which is consistent with the conventional constitutive model for uniaxial consolidation. So, these observations endorse a simplified constitutive model that capable of predicting multidimensional consolidation under arbitrary compressive loadings without requiring additional characterization beyond uniaxial consolidation.

Finally, we investigate the fundamental mechanism of colloidal suspensions under combined shear and compression. The deformation and failure of the strong colloidal gels are investigated under strain-controlled conditions ranging from pure compression to shear dominated consolidation. This investigation provides the knowledge of how shear acts to destabilize the colloidal network and how does the applied shear and compression stress act to promote failure of colloidal network under superposed shear and compression. In the macroscopic analysis, the energy required to consolidate the colloidal gels is found to pass through a minimum that corresponds to the weak shear strain. These findings suggest that a small amount of shear can play a key role in weakening the colloidal particle network, which provides important insights into the tensorial rheology of colloidal gels.

These results generated in this study provide deep insights into the rheology of strong colloidal gels under combined shear and compression and so represents a critical step towards the development of a tensorial rheology of these complex materials. This is the problem of significance for industrial and natural applications, and it is anticipated that future research shall work toward a more comprehensive understanding of this tensorial rheology, including the development of robust characterization techniques and constitutive models.