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Breakup of aggregated particles in turbulence is an important problem with many technical and environmental applications, i.e. breakup of aggregates plays an important role in drug release systems, homogenization of suspensions, and the dispersion of powders. Also does it affects the transport and evolution of environmental particles such as marine snow. In this talk, I will report our recent experimental results on the breakup of finite size colloidal aggregates in homogeneous isotropic turbulence measured by 3D particle tracking velocimetry (3D-PTV). Aggregates grown in mild shear flow are released, one at a time, into homogeneous isotropic turbulence where their breakup is monitored. The aggregates have an open structure with fractal dimension around 2.2 and their size is 1.4 +/- 0.4 mm which is large compared to the Kolmogorov length scale (eta = 0.15 mm). 3D-PTV allows for the simultaneous measurement of aggregate trajectories and the full velocity gradient tensor along their pathlines. This enables us to access the Lagrangian stress history of individual aggregates up to their breakup. We found that for the large aggregates studied here no consistent pattern that relates breakup to the local flow properties. Also the correlation between the aggregate size and shear stress at the point of breakage is found to be weaker when compared with the correlation between size and drag stress. The analysis suggests that aggregates are mostly broken due to accumulation of drag stress over a time-lag of the order of the Kolmogorov time scale. This finding is explained by the fact that the aggregates are large which gives their motion inertia and which increases the time for stress propagation inside the aggregate. Furthermore, it is found that the scaling of the largest fragment and the accumulated stress at breakup follows an earlier established power law obtained from laminar nozzle experiments. This indicates that despite the large size and the different type of hydrodynamic stress, the microscopic mechanism that cause breakup is consistent over a wide range of aggregate size and stress magnitude.
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