Study of shear-induced deformation and fragmentation of laboratory-cultured marine aggregates*

Mass transport of marine aggregates is one of the remaining uncertainties for understanding the oceanic carbon cycle. Marine hydrodynamics alter the size of these aggregates, regulating the total mass flux. Different from droplets and colloidal aggregates, marine aggregates have unique bonding forces among biological primary particles, which causes distinct mechanical properties relevant to deformation and fragmentation. To understand that, we conducted breakup experiments with lab-made aggregates from cultured diatoms. In this study, a novel rotating/oscillating tank provided calibrated laminar shear, in which the shear rate was comparable to that in the surface ocean. We also developed a particle matching and breakup detection algorithm that allowed for individual aggregate tracking. With high-speed imaging techniques, we observed aggregates deforming and disrupting under varying shear. The aggregate morphological evolutions can be coupled to their local shear exposure. With our database, we summarize the necessary hydrodynamic conditions for breakup and discuss the unique morphological changes that precede breakup in terms of aggregate shape, orientation, and Taylor deformation parameter.