(474h) Rheology and Flow-Induced Structure of a Model Nanoparticle System with Adhesive Hard Sphere Interactions
Nanoparticle and colloidal gels, flocculated suspensions, and attractive driven glasses are of fundamental scientific interest and pose challenges to industrial formulation and application because of the hierarchy of structures that connect particle properties to bulk material properties. In many cases the processing of these materials relies on the fact that the underlying structure may be reversibly broken down by flow. As a result their rheological behavior is of technological relevance to process design optimization, and can exhibit complex behaviors such as solid-like linear viscoelasticity, thixotropy, aging, yielding, hysteresis, and shear localization. Much is known about the fractal and fractal-like microstructure of low density gels under static conditions and the dissolution of the network as a result of flow, but less is known about concentrated systems. In this work, we explore the shear induced structural reordering of a nanoparticle dispersions with tunable attractions using small-angle neutron scattering (SANS) in combination with a rheometer (rheo-SANS) and a novel 1-2 plane shear cell (flow-SANS). The model system is composed of silica spheres which have a relatively thin grafted oligomeric surface layer that provides steric stability in a good solvent, but contributes to a reversible, short range attraction in poor solvents (Eberle et al., Langmuir, 2010). When suspended in tetradecane (C14) the solvent quality can be “tuned” with temperature such that, upon quenching, the system transitions from a fluid to a stable physical gel that occurs without competition for macroscopic phase separation for concentrations, Φ ≥ 0.05 (Eberle et al., PRL, 2011). Rheo-SANS and flow-SANS scattering profiles are analyzed in combination with the steady shear rheology. The strength of attraction is found to have a profound impact on both the rheology and local nanostructure. The goals of this study are to identify the hierarchical structures that quantitatively relate the state or phase of the system, interparticle potential, and particle properties to the bulk rheology.