On the flow curve of colloids presenting shear-induced phase transitions

QUEMADA DANIEL, BERLI CLAUDIO

Pekín, República Popular China.

Congreso; 12th International Conference on Surface and Colloid Science; 2006

International Association of Colloid and Surface Scientists

During the last decade, rheometric measurements in combination with optical techniques allowed to attain detailed information on the flow characteristics of systems that develop fluid phases with different mechanical properties, namely shear bands. In these studies, the shear rate can be regarded as a field variable, since it is used to induce either the formation or disintegration (disruption) of microstructures, thus transforming fluid phases. In situations where two o more phases coexist in the flow domain of the rheometric cell, sharp variations of the shear rate occur, and hence the measured flow curve cannot be directly related to the fluid constitutive model. This fact takes particular relevance when dealing with non-uniform shear rate flows, as the one developed in Couette cells, where the shear rate function is related to measured quantities through an implicit integral equation. The present work discusses some rheological aspects concerning the treatment of experimental data of shear banded flows. Further, a simple shear stress vs. shear rate relation is suggested to interpret steady state flow curves involving shear-induced phase transitions. The relation derives from a phenomenological description of structural changes, and involves the possibility of multivalued shear rates under defined conditions.