Breakup of dense colloidal aggregates under hydrodynamic stresses

TL;DR

This paper develops a fracture-mechanics model and conducts experiments to understand how hydrodynamic stresses cause breakup of fractal colloidal aggregates, revealing the influence of fractal structure on aggregate stability.

Contribution

It introduces a model linking aggregate fractal properties to breakup behavior under hydrodynamic stresses, supported by experimental validation.

Findings

Fractal aggregates resist breakup more as they become denser upon fragmentation.

Hydrodynamic stress effects are size-dependent for fractal aggregates, unlike non-fractal ones.

The model accurately predicts experimental results across different flow regimes.

Abstract

Flow-induced aggregation of colloidal particles leads to aggregates with fairly high fractal dimension () which are directly responsible for the observed rheological properties of sheared dispersions. We address the problem of the decrease of aggregate size with increasing hydrodynamic stress, as a consequence of breakup, by means of a fracture-mechanics model complemented by experiments in a multi-pass extensional (laminar) flow device. Evidence is shown that as long as the inner density decay with linear size within the aggregate (due to fractality) is not negligible (as for), this imposes a substantial limitation to the hydrodynamic fragmentation process as compared with non-fractal aggregates (where the critical stress is practically size-independent). This is due to the fact that breaking up a fractal object leads to denser fractals which better withstand stress. In turbulent flows, accounting for intermittency introduces just a small deviation with respect to the laminar case, while the model predictions are equally in good agreement with experiments from the literature. Our findings are summarized in a diagram for the breakup exponent (governing the size versus stress scaling) as a function of fractal dimension.