Dynamic micromechanical behavior of agglomerates under compression and dispersion conditions

The relationships between the mechanical response of particle agglomerates to compressive forces, and the hydrodynamic dispersion behavior of such agglomerates have been studied. Structural changes within the agglomerate, rates of stress relaxation following loading of the agglomerate, and the mechanical hardening of particle agglomerates under repeated loading and unloading cycles of uniaxial compression provide information useful to the interpretation of hydrodynamic dispersion phenomena.; Dispersion experiments were performed under controlled flow conditions in a rotating cone-and-plate device which produces a steady simple-shear flow. Specifically, dispersion tests were performed on individual calcium carbonate agglomerates suspended and sheared in polydimethylsiloxane (PDMS). Several binder liquids (water, glycerol, ethanolamine, and PDMS) were used in varying concentrations to impart different interfacial properties (and hence different mechanical behaviors) to the starting agglomerates.; Compression experiments were performed in a controlled motor-micrometer system which allowed for uniaxial compressive forces that could be applied to the agglomerates at controlled loading rates. Stress-strain data were measured. Experiments involving compressing the agglomerate to fracture, loading to a certain stress and monitoring stress relaxation, and application of multiple cycles of loading and unloading have been performed.; Generally, the agglomerates exhibit elastoplastic (or viscoelastic) behavior, with the stress-strain curve for the initial loading showing a sequence of elastic and plastic yield responses. A two-parameter non-linear Maxwell model has been proposed and validated to represent the overall mechanical response of the agglomerate. Under conditions of low strain rate (0.0001 ∼ 0.0125 s-1), the structural relaxation of an agglomerate (of a given packing density and binder content) can be described by a single relaxation time. Relaxation time scales associated with the loading portion of the compression experiment are different than those for the unloading segment of the compression cycle. In addition, shifts in the relaxation time scale have been observed in experiments in which multiple loading-unloading cycles are applied to the agglomerate. These results shed light on the mechanical origins of the dispersion behavior.; The fracture energy associated with the cohesive strength of the agglomerate is employed as an index of the expected dispersion behavior. The cohesive strength of the agglomerate is determined by the van der Waals and inelastic forces that act between constituent particles. Thus the agglomerate packing density and nature and composition of the binder used (if any) are important parameters in this analysis.; The tendency of an agglomerate to disperse can further be analyzed in terms of two parameters: the structural relaxation time and the period of the hydrodynamic stress cycle. The ratio of these two time scales (the Deborah number) can be used to predict dispersion tendencies. Direct measurement of the structural relaxation time scale, under stress loading conditions similar to that encountered in hydrodynamic dispersion, were performed in stress-relaxation experiments. The experimental results are consistent with the theoretical notion that if the structural relaxation time of the agglomerate under compression is long compared to the one-quarter period of the hydrodynamic stress cycle, dispersion will take place.