Simulating the restructuring of colloidal aggregates

Controlling the structural properties of colloidal aggregates is an active research topic for solid-liquid separation in the food, mining, and wastewater industry, is important for colloidal crystal synthesis, and has attracted attention progressively in the pharmaceutical industry for drug delivery vehicles. For such colloidal processing, the investigation of the restructuring behavior of colloidal aggregates by means of numerical methods has been the research subject for many years, utilizing diverse models for interparticle as well as hydrodynamic interactions. First, proper interparticle force models are required to explain the stability in the simulation of restructuring aggregates. External experimental observations of tangential forces between bonded colloidal particles and the capability of these bonds to support bending moments are included in the proposed model. The resulting two particle contact model is set up with resistances against normal, sliding, and torsional displacement, as well as the newly introduced bending resistance. Second, we investigate two methods for the hydrodynamic interaction of a colloidal particle system in the zero-Reynolds-number regime, Stokesian dynamics and the finite element method. Calculating the longranged and many-body nature of hydrodynamic interactions in Stokes flow, Stokesian dynamics is very efficient, while the finite element method provides satisfactory precision of hydrodynamic forces on particles as well as the exact solution of a flow field in and around an aggregate. As the two methods complement each other for a comparison of particle dynamics, their application to restructuring aggregates is described, including numerical setup, origins of calculated particle drag forces, as well as applicability. Through coupling the models for hydrodynamic interactions and the particle contact, the restructuring of colloidal aggregates was finally investigated with the discrete element method. By means of simulating fractal aggregates suspended in shear flows, restructuring rates were studied by tracking mean structural parameters, i.e. the radius of gyration, now with the ability of investigating the full set of contact parameters to restructuring rates. Aggregate restructuring rates from simulations can be transferred to multi-scale formulations, such as involving population balance models, in order to improve the design and processing of colloidal systems.