Towards a mesoscale rheology model for aqueous particulate suspensions

Particulate suspensions are ubiquitous and diverse; pharmaceutical formulations, biological fluids, magma and foodstuffs are just few of numerous examples. In many cases, the flow behaviour (rheology) of the suspension is critical to its function. A key rheological property is viscosity; a measure of a substance’s resistance to flow. This work aims to understand molecular-level mechanisms responsible for determining flow behaviour in moderately dense suspensions; 35% particles by volume (i.e., volume fraction 0.35). The industrial application of interest to this thesis is catalysis; namely, the ‘washcoat’, a key component in the performance of catalytic converters. A typical washcoat formulation is an aqueous suspension, comprising a high surface-area support powder, an active catalyst material, together with organic additives and certain salts used to optimise properties of the washcoat; including its flow behaviour. Of these components, this work investigates ‘salt-specific effects’; i.e. the influence of differing salt-types. Investigation is conducted at molecular and macroscopic resolution via simulations and experiments, respectively. The research approach probes the constituents of a suspension: the aqueous phase, the particle-aqueous phase interface, and particle interactions. Molecular dynamics simulations are employed as the foundation of this analysis, with experiments - rheology, nuclear magnetic resonance and dynamic light scattering - utilised alongside. A final set of rheology experiments is conducted on particulate suspensions of 35% volume fraction, in pure water and the aqueous salt solutions of interest. At all stages of analysis, results suggest that macroscopic behaviours are a cumulative manifestation of phenomena at molecular resolution. However, such phenomena are varied; the challenge lies in identifying which mechanisms are relevant to the behaviour of interest, how they work together, and how they manifest cumulatively. Towards a mesoscale rheology model for aqueous particulate suspensions, results are discussed in terms of input for such a model, which would predict rheology as a function of particle loading, ionic strength and possibly other factors, in future work

Synthesis and Functionalization of Silver Nanoparticles for the Preparation of High Permittivity Nanocomposites

Over the last decade, a growing interest in nanochemistry has emerged due to the interesting features of nanomaterials that vary with size, shape and surface structure. In particular, metal nanoparticles have received much attention due to their properties that enable their use in various scientific disciplines. Although metal nanoparticles exhibit a number of properties that differ from bulk, some properties, such as their infinite permittivity, remain unchanged. As a result, metal nanoparticles have also been used to prepare nanocomposites with polymers in order to provide dielectric materials featuring high permittivities which can be used for applications such as energy storage (capacitors) or as materials for the conversion of electrical energy into mechanical motion (actuators). Despite the large number of publications on the preparation of nanocomposites exhibiting high permittivities which have emerged over the years, there is still room for further improvement in the current materials properties. For instance, dielectric losses are still quite high in some materials, and the use of certain types of filler lead to a large deterioration in the mechanical properties of the nanocomposites, especially with increasing filler content. In addition, a large number of the fillers used for the preparation of the nanocomposites feature poor size and shape control as well as poorly defined surface properties thus adding to the complexity of understanding the resulting material properties. This work tries to address some of the current issues concerning the preparation of dielectric materials. Therefore, silver nanoparticles (AgNPs) were used as filler, while polydimethylsiloxane (PDMS) was employed as the polymeric matrix. The advantages of using AgNPs as filler consist of their relative facile preparation, as well as the possibility of controlling their surface properties due to their resistance towards oxidation and corrosion. The possibility of preparing AgNPs in large amounts with control over the average size of the particles was realized by conducting the polyol synthesis of AgNPs in a Segmented Flow Tubular Reactor (SFTR). A SiO2 layer was grown around the AgNPs to prevent the loss of the insulating nature of the composite due to the formation of conductive pathways, and the thickness dependency of the dielectric properties of the core-shell particles was also investigated in this work. Furthermore, the SiO2 shell also provided the possibility of further surface functionalization, which was conducted in order to compatibilize the core-shell particles with the PDMS matrix. PDMS was chosen as the polymeric matrix due to its good electromechanical properties, which include high elasticity, low viscosity as well as low conductivity and low tangent losses (tan ÎŽ). The resulting nanocomposites featured enhanced permittivities compared to PDMS, while further optimization in the reaction conditions as well as in the processing procedure yielded nanocomposites with high flexibility that can undergo strains as high as 800 % at a silver content of 20 vol%. Other properties such as electric conductivity and the tan ÎŽ were kept low which emphasizes the potential of the nanocomposites to being used as flexible dielectric materials