Behavior of Fluid Systems Under Shear: Characterization and Metrology

C. D. Muzny and H.J.M. Hanley

Objective: To understand the relationship between fluid properties and shear and subsequently to control nanoscale structural properties through the application of shear.

Problem: The flow properties of a complex system can be predicted or controlled if the relationship between its structure and rheological characteristics is understood. We argue that the structure can be determined by radiation scattering, but that such data must be correlated with rheological data obtained simultaneously from a rheometer or viscometer.

Approach: We recently modified a constant stress rheometer adapted so that a Couette cell can be placed in the neutron beam. The apparatus is capable of high accuracy measurements with viscosities that can range over about ten orders of magnitude. This apparatus is a powerful, generic contribution to the metrology required to investigate systems out of equilibrium. We have also adapted our light scattering equipment to extract dynamic information from a particular shear-modified gelling sample. The results from all of these experiments are co-ordinated with computer simulation data of model systems under shear.

Results and Future Plans: Studies with our neutron scattering adapted rheometer include correlating the alignment of macromolecules with their viscosity in solution and extensive investigations relating structure changes with viscometry of gelling colloidal silica. Our light scattering system has recently be used in studies of gelling systems under oscillatory shear, the results of which can lead to a better understanding of the chemistry of the process. Computer simulation results on the morphology and structure factor of a two-dimensional system of particles interacting through a Lennard-Jones potential, modified to include a long-range repulsive component, have also been reported. It was shown that gel formation can be regarded as the competition between the short-range attractive forces (cause aggregation) and the long-range repulsion forces (keep particles separate) that encourage the formation of space-filling networks.

Future plans include extending our investigations to x-ray scattering, and to extend the current small angle neutron and light facilities. If successful, we will then have the potential not only to investigate materials over the wide range of length scales, which is our objective, but also will have the flexibility to investigate systems with the most appropriate scattering tool.

Publications:

Straty, G.C., Muzny, C.D., Butler, B.D., Lin, M.Y., Slawecki, T.M., Glinka, C.J., and Hanley, H.J.M., "An in situ Rheometric Shearing Apparatus for SANS," Physica B: Condens. Matter 241-243, 74 (1998).

Butler, B.D., Muzny, C.D., and Hanley, H.J.M., "Scaling of Small-Angle Neutron Scattering Intensities from Gelling Colloidal Silica," Int. J. Thermophys. 20, 35 (1999).

Hanley, H.J.M., Muzny, C.D., Butler, B.D., Straty, G.C., Bartlett, J., and Drabarek, E., "Shear-Induced Restructuring of Concentrated Colloidal Silica Gels," J. Phys: Condens. Matter 11, 1369 (1999).

Butler, B.D. and Hanley, H.J.M., "Aggregation in Quenched Systems Interacting via a Short-Range Attractive, Long-Range Repulsive Potential," J. Sol. Gel Sci. and Tech. 15, 161 (1999).