The deformation of particle aggregates due to hydrodynamic shear and its role in disaggregation

Much of the particulate matter in natural aquatic environments exists in a state of aggregated clumps. This clumping influences the suspended particle buoyancy and drag, which in turn greatly alters its transport characteristics. Disaggregation, where particle aggregates break apart into smaller sub-aggregates, is a complex process that depends heavily on the aggregate morphology, interparticle bonds, and microscale hydrodynamic forces. This process is further complicated by the fact that much of the particulate matter in natural aquatic environments is biological (e.g. phytoplankton). To clarify the disaggregation process of marine phytoplankton aggregates, we conducted disaggregation experiments with two types of phytoplankton species; Odontella aurita and Skeletonema grethae. We compare their disaggregation characteristics to those of abiotic particles (sulfate polystyrene and polyethylene microspheres) with distinct interparticle bond forces, which we quantified using the extended DLVO theory. Experiments were conducted by using Lagrangian particle tracking in a custom-built, laminar-flow disaggregation tank. Here, we present a force balance analysis using the Maxey-Riley equation to determine the coupling force relevant to aggregate fragmentation. We found that the breakup strength of an aggregate can be derived from the inter-particle bonding force and its porous structure. Moreover, this study also revealed that individual aggregates display a shear-thickening behavior when subjected to tensile stress.