Mechanisms of Adhesive and Cohesive Failure in Agglomerate Dispersion
Ica Manas-Zloczower, John Boyle, Donald Feke
Case Western Reserve University
USA

Keywords: agglomerate, dispersion, mixing

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The hydrodynamic dispersion of fine-particle agglomerates into smaller fragments is affected by both the physical and chemical nature of the particle-fluid system as well as processing conditions. Particle characteristics, agglomerate packing structure, and the presence of interstitial liquids primarily affect the cohesive strength of the agglomerate, whereas the flow geometry and intensity determine the hydrodynamic forces that act to disrupt the agglomerate. Fluid infiltration into agglomerates synergistically affects agglomerate cohesivity and distribution of hydrodynamic stress acting on and within the agglomerate. In general, when the applied hydrodynamic force exceeds the cohesivity, dispersion will occur. Dispersion often takes the form of surface erosion of small fragments or bulk rupture into several large fragments. In either case, since the interparticle bonds that are severed are between particles in like states, the process can be classified as a cohesive failure of the agglomerate.

In this paper, we report on a qualitatively different manifestation of dispersion in which the agglomerate breaks by an adhesive failure. Experiments performed with fumed silica agglomerates soaked for different times in poly(dimethyl siloxane) fluids illustrate the different mechanism of dispersion. Fluid infiltration into these agglomerates may cause some structural rearrangement of the agglomerate leading to the formation of a distinct outer shell of highly saturated particles surrounding an essentially dry core. Adhesive failure occurs as the infiltrated shell separates from the dry core. We report on the several factors (such as the agglomerate and fluid characteristics, applied hydrodynamic stress and flow geometry) that trigger dispersion by an adhesive failure