(608b) Aggregate Breakage Under Laminar Conditions

Design and usage of appropriate units for handling of fragile materials requires the knowledge of conditions leading to the breakage of such materials. Unfortunately, this information is commonly obtained using stirring devices with a broad shear rate distribution, and often under conditions where, besides breakage, aggregation is also present. An alternative approach to measure the breakage phenomena is to use aggregates produced under static and dilute conditions and break them in various types of flow fields. In this way, although no aggregation is present, obtained results will be affected by the restructuring of the particles within the aggregates. Therefore, in order to avoid possible artifacts from both aggregation and restructuring, we choose the following approach. In a first step, we prepare compact aggregates using a stirred vessel operated under turbulent conditions. In a second step, such aggregates, diluted to a very low concentration where aggregation is negligible, are broken either in extensional flow using geometry with a sudden contraction or in simple shear flow using a rheometer. The evolution of the aggregates size and structure over the course of the breakage process is measured using a combination of small angle static light scattering and image analysis. The steady state values obtained for the two types of flow are compared to the steady state aggregates sizes collected in various stirring devices operated under turbulent conditions, always using the same size of primary particles.