Report dated January '02 but covers work done in 2001
Introduction
This research focuses on the dispersion of clusters of fine particles through the application of hydrodynamic shear. In many industrial applications, the processing goal is the production of breakdown of the particle agglomerate into small fragments (or possibly complete breakage of the agglomerate into its constituent particles) followed by distribution of the fragments throughout the liquid medium. Our ultimate goal for this work is to obtain a fundamental understanding of the various factors that influence the dispersion behavior of fine- particle agglomerates. Once such an understanding is achieved, chemical or mechanical strategies aimed at improving the outcome of dispersion processes or the design of more efficient dispersion equipment can be attempted.
Our general approach has been to study the dispersion behavior of well-characterized single agglomerates in controlled flow fields. This allows us to establish the links between the fundamental properties of an agglomerate and dispersion characteristics such as critical shear stress for dispersion, mode and kinetics of fragmentation, and the evolution of the fragment size distribution.
A major emphasis of the research supported under this IFPRI grant involves investigation of the role of certain time-dependent (dynamic) phenomena in governing dispersion. Such time- dependent effects are inherent in many aspects of the industrial practice of dispersion. For example, when agglomerates are contacted with processing liquids, wetting and spreading of liquid throughout the agglomerate structure occurs simultaneously with dispersion. Also, particle agglomerates are subject to complex, non-steady shear histories in practical processing equipment. Also, in some cases, dissolution of the particles or binder liquids may occur on a time scale comparable to that for dispersion itself.
In the first year of this IFPRI grant, the bulk of the research effort was devoted to the development of a new experimental approach for the investigation of the influence of dynamic effects on dispersion behavior. This entailed the design (and redesign) of a dynamic dispersion chamber, and construction of it and the ancillary equipment. Preliminary experiments were done to validate the experimental techniques and to refine the analytical procedures. In the second year, we performed additional modifications to the experimental device and refined our experimental procedures. This allowed us to complete additional experimental studies that highlight the dramatic influence of time-dependent effects on the outcome of dispersion. In addition, we performed a thorough modeling study of the flow and shear fields present within our experimental device. This modeling enables us to analyze the results of our experiments, and to understand the limitations of our dispersion experiments with oscillatory flow. In the third year, we continued to perform experimental studies that shed light on the fundamentals of dispersion processes. These studies are leading us toward a mapping of dispersion regime on which different types of dispersion behavior can be related to agglomerate and flow characteristics. In addition, we attempted to study the dispersion behavior of agglomerate samples that were obtained from IFPRI member companies, and found that our experimental methods divulged a significant non-uniformity in the agglomerate structure. Such non-uniformity makes difficult the interpretation of the outcome of dispersion experiments with these materials.