The coupling of wall slip and thixotropy in colloidal gel rheology

Attractive colloidal gels appear across numerous industries, including food, pharmaceuticals, cosmetics, mining, and oil and gas. In such systems, full rheological characterisation is essential to the optimisation of both product properties and processing. It has previously been shown that colloidal gels with a yield stress tend to exhibit flow instabilities at low shear rates, notably shear banding and apparent wall slip. We employ rheometry to investigate these flow instabilities in two attractive gels: kaolinite suspensions and cellulose nanocrystal suspensions. Both systems display the hallmark features of attractive gels: shear thinning, viscoelasticity, yield stress, and thixotropy. This thesis investigates the relationship between apparent/true wall slip and thixotropy through rheometry. A range of surface roughnesses and rheometer geometries is used to delineate the effects of surface-dependent apparent wall slip from those of bulk-dependent thixotropy. For the kaolinite suspension, a clear relationship is established between the onset of wall slip and thixotropic recovery: the more the structure recovers, the more likely the system is to show flow instability at low shear rates. We attribute slip onset to the growth of aggregates beyond the scale of the wall asperities and validate this picture by coupling a simple slip law to a structure-parameter model. For the cellulose nanocrystal suspension, the interplay between thixotropy, yielding, shear banding, and wall slip is examined. Creep experiments distinguish the true yield stress from the lower maintenance stress required to sustain flow, and a physical explanation for the appearance of discrete-regime shear banding is developed. These studies inform the final chapter of this thesis, in which a modified thixotropic elasto- viscoplastic (TEVP) model captures both the viscoelastic and plastic response of the cellulose nanocrystal gel while depending on the degree of structural recovery and apparent wall slip. The results show that a measured yield stress is not an intrinsic property but a geometry and interface-dependent quantity, and that adding a simple slip term captures much of the physical behaviour.