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.

Presented in an accessible and introductory manner, this is the first book devoted to the comprehensive study of colloidal suspensions.

Essential text on the practical application and theory of colloidal suspension rheology, written by an international coalition of experts.

This thesis is divided into two research aims: The first aim, presented in Chapters 3 and 4, is to test the theoretical framework of friction contact models with rheological and nanotribological measurements on model suspensions with controlled surface properties. In Chapter 3, we first present comprehensive experimental tests of a friction contact model based on correlating simulation results against rheological measurements for both model and industrial colloidal dispersions complemented by independent estimates of the particle-scale friction coefficients from literature surveys. The comparisons emphasize the sensitivity of the first normal stress difference can distinguish between states of shear thickening dominated by hydrodynamic friction or contact friction. Based on the findings in Chapter 3, a systematic exploration of nanotribological measurements using lateral force microscopy (LFM) is presented in Chapter 4. Our systematic studies qualitatively agree with the Stribeck relationship regardless of the solvent environment. It is also confirmed that the friction coefficient obtained from the bulk rheology lies in the high Sommerfeld number regime, suggesting that directly applying the friction coefficient obtained from the nanotribological measurements for predicting the rheology using the friction contact model is not quantitative.

The relationship between colloidal suspension microstructure and rheology is investigated to provide a solid understanding of the nonlinear rheology of concentrated suspensions, with a focus on shear thickening. These suspensions are also studied as a treatment to woven fabric in body-armor applications, where the insight from microstructural measurements is used to attempt to improve the application. The colloid and suspension properties are characterized via SEM, DLS, SANS, USANS, and rheometry. The rheology is mapped onto an effective hard-sphere model with the addition of a yield stress. An additional excluded volume shell, as measured by SANS and USANS structural measurements, accounts for interparticle interactions due to surface forces arising from the stabilizing layer on the particles. The microstructure of these near hard-sphere concentrated, shear-thickening colloidal dispersions are measured via SANS and USANS as a function of volume fraction, shear rate, and particle size. Special Rheo-SANS, flow-SANS, and flow-USANS instruments are developed and validated to measure microstructure in flowing systems. Structures measured via USANS show a cluster peak that arises from hydrocluster formation in concentrated, shear thickening suspensions. Structure measurements in SANS via 1-d averaged analysis and analysis of the 2-d structure corresponds to that expected from Stokesian Dynamics simulations. Micromechanics theory via a Stress-SANS law is used to calculate rheology from the microstructure for comparison. As the rheology calculated from the SANS data qualitatively agrees with that of the suspension and the structures seen correspond to those expected from simulations, strong confirmation of the hydrocluster mechanism for shear thickening is observed. This improved knowledge of the hydrocluster structure is used to develop an elastohydrodynamic theory for the limiting viscosity at high shear stresses due to particle deformations in the hydrocluster. Measurements of particle modulus give consistent application of this elastohydrodynamic model to suspensions studied in this thesis. In addition, the model is applied to widely varying suspensions, including those of hard mineral particles, polymer particles, microgels, and emulsions and consistent results are seen. The results of these fundamental studies of how particle size, concentration, and hardness affect suspension microstructure and rheology are used to engineer shear thickening fluid treated textiles, suitable for various types of protective devices.