Though suspensions comprised of anisotropic particles are ubiquitous in industry, little has been done to elucidate the effects of particle anisotropy on concentrated suspension rheology, or the mechanism responsible for the reversible shear thickening observed in these systems. This dissertation explores the rheology and shear-induced microstructure of anisotropic particle suspensions through the shear thickening transition, and provides the first account of anisotropic particle alignment during shear thickening. For this investigation, Poly(ethylene glycol) (PEG)-based suspensions of acicular precipitated calcium carbonate (PCC) particles of varying particle aspect ratio (nominal L/D & sim; 2, 4, 7) are generated that demonstrate both continuous and discontinuous reversible shear thickening with increasing applied shear rate or stress. The critical volume fraction for the onset of discontinuous shear thickening decreases as the average particle aspect ratio is increased. However, the critical stress for shear thickening is found to be independent of particle anisotropy and volume fraction, and can be predicted based on the minor axis dimension of the particles in agreement with the critical stress scaling for hard-sphere suspensions. Small angle neutron scattering during shear flow (Rheo-SANS) demonstrates that long-axis particle alignment with the flow direction is maintained throughout the range of shear stresses investigated, including the shear thickening regimes for both continuous and discontinuous shear thickening PCC/PEG suspensions. Investigations of particle flow alignment following flow cessation provide evidence that the critical volume fraction for shear thickening may be associated with an isotropic-nematic transition within the anisotropic particle suspensions. Rheo-SANS investigations of concentrated kaolin clay suspensions demonstrate that disk-shaped particles exhibit particle alignment with the face surfaces orthogonal to the gradient direction during both continuous and discontinuous shear thickening. The critical stress at the onset of shear thickening for discontinuous shear thickening clay suspensions is observed to scale with the particle thickness dimension. The rheology and Rheo-SANS observations for both the acicular PCC and disk-like kaolin clay suspensions invalidate earlier hypothesis suggesting that shear thickening behavior in anisotropic particle dispersions results from increased particle rotations out of flow alignment potentially leading to particle jamming. Rather, the observations suggest that shear thickening in anisotropic particle suspensions is a consequence of short range hydrodynamic lubrication forces resulting in the formation of hydroclusters at higher shear rates, analogous to the behavior established for spherical particle suspensions. Lastly, anisotropic particle suspensions are used to successfully develop of shear thickening fluid (STF)/ballistic fabric composites. Shape anisotropy imparts the advantage of lower solids loading required to achieve energy dissipative improvements compared to spherical particle STFs. The observed improvements in ballistic and stab resistance response of these composites over that of ballistic fabrics alone suggests that they could potentially be used in the development of personal body armors with improved, multi-threat protective capabilities.

Many fundamental issues in classical condensed matter physics can be addressed experimentally using systems of individually visible mesoscopic particles playing the role of “proxy atoms”. The interaction between such “atoms” is determined by the properties of the surrounding medium and/or by external tuning. The best-known examples of such experimental model systems are two different domains of soft matter — complex plasmas and colloidal dispersions.The major goal of this book — written by scientists representing both complex plasmas and colloidal dispersions — is to bring the two fields together. In the first part of the book the basic properties of the two systems are summarized, demonstrating huge conceptual and methodological overlap of the fields and emphasizing numerous cross-connections between them and their essential complementarity. This “introductory part” should serve to help each community in understanding the other field better. Simultaneously, this provides the necessary basis for the second part focused on particle-resolved studies of diverse generic phenomena in liquids and solids — all performed with complex plasmas and/or colloidal dispersions. The book is concluded with the discussion of critical open issues and fascinating perspectives of such interdisciplinary research.

This thesis explores the dispersion stability, microstructure and phase transitions involved in the nanoclay system. It describes the recently discovered formation of colloidal gels via two routes: the first is through phase separation and second is by equilibrium gelation and includes the first reported experimental observation of a system with high aspect ratio nanodiscs. The phase behavior of anisotropic nanodiscs of different aspect ratio in their individual and mixed states in aqueous and hydrophobic media is investigated. Distinct phase separation, equilibrium fluid and equilibrium gel phases are observed in nanoclay dispersions with extensive aging. The work then explores solution behavior, gelation kinetics, aging dynamics and temperature-induced ordering in the individual and mixed states of these discotic colloids. Anisotropic ordering dynamics induced by a water-air interface, waiting time and temperature in these dispersions were studied in great detail along with aggregation behavior of nanoplatelets in hydrophobic environment of alcohol solutions.

* Physical chemists will find this book comprehensive. Topical reviews on all aspects of colloidal ordering and related phase transitions will be covered. It provides a good blend of experimental and theoretical investigations. * Useful to materials scientists and chemical engineers, the book includes a discussion of stability, important from the point of view of applications of colloidal crystals. * Physicists will be interested in the book, because it highlights the controversy over effective interparticle interaction in charged colloids.