This proposal outlines a 3-year plan of integrated research, teaching and outreach centered on complex fluid rheology. Much effort has been devoted to understanding the flow behavior of particulate laden fluids, with the underlying ubiquity of these materials in industry a strong motivator. The recent observation of shear thickening in gel forming colloidal systems presents an opportunity to contribute to the fundamental understanding of the rheology of this important class of materials.
A. Scientific objectives The proposed work is directed towards answering key questions regarding the observed shear thickening, and to exploring several qualitatively new phenomena displayed by shear thickened gels. This proposal describes a detailed plan for (1) the characterization of shear thickening in these materials, (2) the study of internal stress dynamics in stationary states and (3) the study of the evolution of ordered anisotropic microstructures on shear deformation of such colloidal gels.
B. Intellectual merit Significant attention has been given to shear thickening in repulsive and hard sphere systems. By contrast, in flocculating systems, which constitute the majority of industrial applications, shear thickening has neither been predicted nor explored. The proposed study addresses this gap and will elucidate the origin of shear thickening in attractive systems as well as the dependence on interaction strength and volume fraction. It will constitute the first quantitative study of short and long timescale dynamics of stresses that result on system quenches from the liquid to the gel state in soft materials. Finally, it will provide a new look, with unparalleled detail, at the formation of anisotropic vorticity-aligned aggregates in a complex fluid under flow.
C. Broader impact 1. Technical impact Fumed particles are commodity materials with sales measured in millions of tonnes and applications in all manner of formulations and products. The rheology of filled Newtonian and viscoelastic fluids is an active research area for exactly this reason, and scientific exchange between industry and academia is common. The PI maintains a relationship with Cabot Corp., and their personnel often attend meetings of the local complex fluids community, facilitating further interactions. There are also technical exchanges with industry concerning rheometer design and operation. 2. Education and Outreach The PI is a new faculty member with a strong interest in education and outreach. This project will actively seek the participation of underrepresented groups for three undergraduate research projects. Graduate students will gain a broad experimental background in complex fluids and be involved in education through teaching, mentorships of undergraduates and local outreach events. As part of a broader thrust in complex fluids/soft materials research at Yale, the PI will develop coursework incorporating themes from the project in a module on complex fluid rheology. Results will be published on an interactive website and will be regularly presented at the quarterly meetings of the regional New England Complex Fluids Workgroup (NECF) as well as at national meetings.
for CBET-0828905 This project was a three-year investigation of the rheology, or flow behavior of complex fluids. A suspension of clay particles in water, a dollop of shaving foam, or a solution containing dissolved polymers and surfactants, such as dishwashing liquid, are all examples of complex fluids. Such fluids are ubiquitous in the everyday life of individuals as well as various industries and as one might expect, their flow behavior is of great concern in their manufacture. This NSF-sponsored work examined the rheology of complex fluids formed by suspending clay particles (laponite) in water and of fluids formed by suspending carbon black (soot) in mineral oil. These two materials are frequently found in many consumer products. Our work specifically examined two areas. The first was shear thickening and shear induced structure formation in carbon black gels. Here, we found that shear thickening, an increase in the viscosity of the system with increasing flow rate, occurs in this system at a critical stress due to the breakup of clusters of carbon black particles. Normally, shear thickening is not observed in systems where the particles aggregate with one another. However, in this case, shear thickening can be observed, but as we showed, it is expressly due to the very rough nature of the particle surface. Upon subsequent flow, shear thickened suspensions display a very dramatic formation of log-like structures that suddenly appear and then disappear at a characteristic time in the system. This characteristic time is in fact related to the mechanical properties of the log-like structures. The second area concerned the kinetics with which clay suspensions "solidify" or "age" after being fluidized by a high speed flow. We showed conclusively that the application of a shear stress delays the time at which the system solidifies and that the solidification time has a clear mathematical relation to the magnitude of the shear stress. We developed a method whereby we could accurately resolve the evolution of the rheological properties (frequency and strain dependent) during the rapid transition of the system from a fluid-like state to the solid-like regime, in the presence (or absence) of shear stress. The evolution of the properties followed a universal form, which we accounted for in terms of a sigmoidal increase of the characteristic relaxation time of the material. The attached image shows the time evolution of the elastic (solid-like) and viscous (liquid-like) moduli for a clay suspension undergoing liquid-solid transition in the presence of stresses of different magnitudes as indicated in the legend. The arrest of the material occurs when the elastic modulus (filled symbols) becomes larger than the viscous modulus. Notably, the arrest of the system – the transition from a liquid like material to a solid-like material – occurs with a characteristic signature represented by a peak in the viscous modulus of the material. The shape of this peak was shown to be strongly sensitive to the magnitude of any stress applied during the liquid-solid transition. The narrowing of the peak with increasing stress is analogous to the narrowing of relaxation spectra in other disordered materials – polymer and molecular glasses. The results of these experiments are sensitive not only to the manner in which the system is fluidized, but also to the rate at which the system is brought to rest after fluidization. We showed that careful control of this mechanical history of the system results in dramatically different trajectories for the evolution of the rheological properties of the system in the absence of flow. These results have important implications for understanding the way in which consumer products like gels and lotions can recover their mechanical integrity after being perturbed in some manner – for example after being perturbed by vibration during shipping or handling. As part of this project, we developed a project in "Kitchen Rheology" where high school students used a speed-controlled blender as a tool to study the flow behavior of gels. This project was featured in the New Haven Science Fair in which the participating student won the Phillip Orville Memorial Prize for his work. As part of this project, a lab module was developed for a graduate course in Polymer Physics based on the temperature dependent rheology of polymers. This lab was taught twice (in consecutive years) during the duration of the project. The lab experience has become an integral component of the Polymer Physics course at the PI’s institution.
