Rheological Studies Of Concentrated Model Colloidal Suspensions

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.

Colloids are ubiquitous in the food, medical, cosmetics, polymers, water purification, and pharmaceutical industries. The thermal, mechanical, and storage properties of colloids are highly dependent on their interface morphology and their rheological behavior. Numerical methods provide a convenient and reliable tool for the study of colloids. Accelerated Lattice Boltzmann Model for Colloidal Suspensions introduce the main building-blocks for an improved lattice Boltzmann–based numerical tool designed for the study of colloidal rheology and interface morphology. This book also covers the migrating multi-block used to simulate single component, multi-component, multiphase, and single component multiphase flows and their validation by experimental, numerical, and analytical solutions. Among other topics discussed are the hybrid lattice Boltzmann method (LBM) for surfactant-covered droplets; biological suspensions such as blood; used in conjunction with the suppression of coalescence for investigating the rheology of colloids and microvasculature blood flow. The presented LBM model provides a flexible numerical platform consisting of various modules that could be used separately or in combination for the study of a variety of colloids and biological flow deformation problems.

Astronauts are constantly threatened by impact from micrometeorite and orbital debris (MMOD) when conducting extra-vehicular activities (EVA) in low Earth orbit (LEO). These threats have already become a major challenge to long-term missions and deep space exploration. Shear thickening fluids (STFs) demonstrate an abrupt increase in viscosity with applied high shear stress, improving their ability to dissipate energy and making them good candidates for protective body armor. In my thesis work, a novel STF formulation in ionic liquid has been developed to improve the resistance of EVA suit against threats from ballistic, puncture, and hypervelocity MMOD impacts. Ionic liquids serve as the solvent phase for the STF formulations because of their low volatility and stability over a broad range of temperatures. However, dispersing colloidal particles in ionic liquids can be challenging because the high ionic strength of ionic liquids screens the electrostatic stabilizing forces that are typically important for stabilizing colloidal dispersions in polar solvents. ☐ Stable nanoparticle dispersions in the ionic liquid [C4mim][BF4] are created through surface coatings (e.g., fluorinated alkyl chains, alcohol), which induce solvation layering around the particles. Solvation layers are initiated by hydrogen bonds between the anion groups [BF4]- and the functionalized particle surface, providing a stabilizing steric repulsive inter-particle force. Rheology, electron microscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) are employed to determine the thickness of the solvation layers and the microstructure of dispersions for different coating systems. A quantitative model based on analysis of SANS data is developed to evaluate the inter-particle interactions and the thickness of the solvation layers. Additionally, the rheological behavior of dispersions is controlled by tuning the strength of surface hydrogen bonding with different surface chemistry. The influence of temperature on the thickness of solvation layers and particle interactions is also investigated through rheology, DLS, and SANS studies. Destabilization phenomena (from stable dispersion to unstable gel) are identified due to the change of interfacial structure with increasing temperature. Furthermore, the influence of impurities (i.e., water) on the microstructure and thermodynamic properties of ionic liquid are studies using SANS and small-angle x-ray scattering (SAXS) techniques. A phase diagram for ionic liquid aqueous solutions (microphase separation, phase inversion, and micelle formation) is constructed, revealing similarities to traditional oil-water-surfactant systems. This understanding of ionic liquid phase behavior and formation of solvation layers is critical for the formulation of colloidal dispersions in ionic liquids with a specific rheological profile. Ionic liquids based STF-Kevlar® nanocomposites are shown to provide superior puncture resistance in lab scale quasi-static puncture tests. The fabricated nanocomposites are proven to provide better protection than traditional Kevlar® without compromising the flexibility. The results of the present research demonstrate the feasibility of STF-Kevlar® nanocomposites for astronaut protection and identify technological challenges that still need to be addressed.

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.