Bulk and micro-scale rheology of an aging, yield stress fluid, with application to magneto-responsive systems

Abstract

Understanding the ways that matter deforms and flows, which is the focus of the branch of science known as rheology, is essential for the efficient processing and proper function of such practically and technologically important materials as plastics, paints, oil-drilling fluids, and consumer products. Rheology is also powerful from a scientific perspective because of the correlation between rheological properties and the structure and behavior of matter on microscopic and molecular scales. The developing sub-field of microrheology, which explicitly examines flow and deformation behavior on microscopic length scales, provides additional clarity to this connection between rheology and microstructure. Aging materials, whose rheological properties evolve over time, are one class of materials that are of significant scientific and practical interest for their rheological behavior. Also, the unique field-responsive rheological properties of magnetorheological (MR) suspensions, which can be tuned with an applied magnetic field, have been used to create active vibration damping systems in such diverse applications as seismic vibration control and prosthetics. A material that undergoes rheological aging and that has received much attention from soft matter researchers is the synthetic clay Laponite® . This material is attractive as a rheological modifier in industrial applications and consumer products because a rich array of rheological properties, including a yield stress, viscoelasticity, and a shear-thinning viscosity, can be achieved at very low concentrations in aqueous dispersions (~ 1 w%). Though this behavior has been investigated extensively using traditional 'bulk' rheology, a number of important questions remain regarding the nature of the dispersion microstructure. The techniques of microrheology, in which rheological properties are extracted from the motion of embedded microscopic probe particles, could help to elucidate the connection between microstructure and rheology in this material. Microrheological studies can be performed using passive techniques, in which probes are subject only to thermal motion, and active techniques, in which external forces are applied to probes. Because aqueous Laponite® dispersions exhibit a significant yield stress, they could be beneficial as novel matrix fluids for magnetorheological suspensions. MR fluids consist of a suspension of microscopic magnetizable particles in a non-magnetic matrix fluid. When an external magnetic field is applied, the particles attract each other and align in domain-spanning chains of particles, resulting in significant and reversible changes in rheological properties. Because of the typically large density difference between the matrix fluid and the suspended magnetic particles, however, sedimentation is often problematic in MR fluids. A yield stress matrix fluid such as an aqueous Laponite® dispersion could help address this issue. In this thesis, bulk rheology and microrheology experiments are combined in order to provide a thorough characterization of the rheological properties of aqueous Laponite® dispersions. Multiple Particle Tracking (MPT), a passive microrheology technique, is used to explore the gelation properties of dilute dispersions, while an active magnetic tweezer microrheology technique is used to examine the yield stress and shear-thinning behavior in more concentrated dispersions. MPT results show strong probe-size dependence of the gelation time and the viscoelastic moduli, implying that the microstructure is heterogeneous across different length scales. We also demonstrate the first use of magnetic tweezers to measure yield stresses at the microscopic scale, and show that yield stress values determined from bulk and micro-scale measurements are in quantitative agreement in more concentrated Laponite® dispersions. With a thorough understanding of the clay rheology, we study the magnetorheology of MR suspensions in a yield stress matrix fluid composed of an aqueous Laponite® dispersion. Sedimentation of magnetic particles is prevented essentially indefinitely, and for sufficient magnetic field strengths and particle concentrations the matrix fluid yield stress has negligible effect on the magnetorheology. Using particle-level simulations, we characterize the ability of the matrix fluid yield stress to arrest the growth of magnetized particle chains. The methods and results presented in this thesis will contribute to the fundamental understanding of the rheology and microstructure of aqueous Laponite® dispersions and provide researchers with new techniques for investigating complex fluids on microscopic length scales. Additionally, our characterization of the effects of a matrix fluid yield stress on magnetorheological properties will aid formulators of MR fluids in achieving gravitationally stable field-responsive suspensions, and provide a new method for manipulating the assembly of particle building blocks into functional structures.