Description
The agglomeration of nanoparticles is of growing importance in industrial production processes of nanomaterials, in which the growth of the agglomerates defines essential product characteristics. Therefore monitoring and control of agglomerates and their structure is of major importance and subject of many research studies. Simulation and modelling of agglomerate formation in real flows is a typical multi-scale problem with respect to the relative size of nanoparticles and the larger scales of the flow, and most studies focus either on individual collisions of particles neglecting the larger scales or resolve the large scale flow motion and model agglomeration by semi-empirical closures. For the latter, assumptions need to be made on particle collision coefficients that strongly depend on size and shape of the agglomerates. The motion of very small particles will be subject to Brownian motion while larger particles and their collisions are rather determined by turbulent convective transport and the agglomerates' relative inertia. A priori assumptions are usually made for the size of the particles and their collision kernels, and the effect of particle and agglomerate growth is usually neglected. It is well known, however, that particles -such as black carbon particles- with primary particle sizes of a few nanometers can form agglomerates that are substantially larger and collision efficiencies will change substantially during the course of a simulation. How the collision efficiencies changes and how this can be modeled is largely unknown and will be the subject of this project.
This subproject will model agglomeration of nanoparticles over a wide range of scales to investigate the transition of particle agglomeration due to Brownian motion or turbulent transport. The particle transport is simulated by solving the Langevin equations using the software ESPRESSO for molecular dynamics simulations. The flow field is obtained from the Navier-Stokes equations using pseudo-spectral methods and both codes need to be coupled to account for interactions between particles and turbulence. The collision dynamics and formation of agglomerates are studied by calculating forces between the particles, particles and agglomerate and multiple agglomerates in three dimensions with not only considering atomistic scales but also mesoscales coupled with micro-turbulent scales. For many industrially relevant production processes of nanoparticles this allows for the first time a consistent analysis of the growth of the agglomerates from particle nucleation at nanometer scale to complex structures in the micrometer range.