(75a) Experimental Determination of Breakage Rate of Colloidal Aggregates in Axisymmetric Extensional Flow

Aggregates prepared under fully destabilized conditions by the action of Brownian motion were exposed to an extensional flow generated at the entrance of the sudden contraction. Two non-invasive techniques were used to monitor their breakup process, i.e. light scattering and 3D particle tracking velocimetry (3D-PTV). While the first one can be used to measure the size and the morphology of formed fragments after the breakage event, the latter is capable to resolve trajectories of individual aggregates up to the breakage point as well as the trajectories of formed fragments. Furthermore, measured velocity gradients were used to determine the local hydrodynamic conditions at the breakage point. All these information was combined to experimentally determine for the first time the breakage rate (K_B) of individual aggregates as a function of the applied strain rate as well as the properties of the formed fragments, i.e. the number of formed fragments and the size ratio between the largest fragment and the original aggregate. It was found that K_B scales with the applied strain rate according to a power law with the slope being dependent on the initial fractal dimension only, while the obtained data indicates a linear dependency of K_B with the initial aggregate size. Furthermore, the probability distribution function (PDF) of the number of formed fragments and the PDF of the size ratio between the largest fragment and original aggregate indicate that a breakage will with high probability (75%) result in the formation of two to three fragments with a rather asymmetric ratio of sizes of about 0.8. The obtained results are well in line with those published in the literature evaluated from numerical simulations.