Research in the laboratory of Professor Fuller has centered on applica- tions of techniques in optical rheometry to understand the structure and dynamics of flowing, dense suspensions. This project has involved the design, construction, and implementation of new methods that are thought to be particularly suited to highly turbid systems that are normally difficult to handle using optical methods. The solution to this difficulty has been to develop techniques that can accommodate very thin specimens and still acquire sufficient information to properly access flow-induced anisotropy of a suspension’s structure.
The scattering of light is the dominant optical interaction with most sus- pensions and this phenomena has been used in the present work. For this rea- son, scattering dichroism and small angle light scattering have been applied to several systems of concentrated materials through collaborative arrangements. Furthermore, the experiments have been designed so that these optical measurements can be accomplished in combination with more standard, mechanical rheometry measurements. In this way, the coupling of rheological responses to flow-induced structure can be more easily appreciated.
These techniques have been applied to two types of dense suspensions during the past three year funding period. In a collaboration with Professor Jan Mewis of Leuven, Belgium, a complete study of flow-induced structure in dense suspensions of soft spheres has been made. This work cxamincd the consequences of systematic changes in the stabilizing surfactants that were used to control aggregation. These surfactants were demonstrated to also affect the “softness” of the particles, Using scattering dichroism and small angle light scattering, it was found that hydrodynamics forces were able to induce the formation of “stringlike” structures when the shear stress surpassed a critical value. By simultaneously measuring the mechanical prop- erties of the suspensions and their optical properties, it was determined that this structural transition occurred at a stress where the viscosity undergoes a strong shift in its shear thinning behavior.
The second project involved a joint study with the research group of Prof. Piau of Grenoble, Prance. This collaboration considered dense suspensions of rodlike, sepiolite clay particles. The principal question pursued in these experiments was whether this colloidal system undergoes a isotropic to nematic phase transition as its concentration is incrcascd. Mcasurcmcuts using scattering dichroism and small angle light scattering were demonstrated to produce determinations of the order parameter in flowing suspensions. By subjecting suspensions of varying concentrations to extensional flows, direct evidence of liquid crystalline behavior was obtained.