(92d) Morphology of Nanoparticle Aggregates in Flow through Beds Packed with Spheres: Self- and Flow-Induced Assembly Using Lattice Boltzmann Simulations

The aggregation of nanoparticles as they propagate through porous media is an important process for the environment, since the use of nanotechnology has led to increasing rates of nanoparticle manufacturing that can in turn lead to nanoparticles introduced into the subsurface and possibly into aquifers. In addition, controlled assembly of nanoparticles can lead to the manufacturing of nanoparticles of specific shape and functionality. Understanding the process of nanoparticle aggregation in porous media, and the effects of the flow in the process, can therefore be of practical interest. Electrostatic forces and van der Waals forces drive the nanoparticle aggregation process. The mean size of aggregates as a function of the monomer size and of different operating conditions, and the growth rate of the aggregates have mostly attracted research attention. However, the morphology of the aggregates is also important, as is the final shape of the particles. Aggregate morphology affects the way colloid aggregates pass through the pores, either enhancing or reducing the clogging of pores. In this study, the lattice Boltzmann method (LBM) is used to simulate flow through a porous medium, and a Lagrangian method is used to simulate the transport of individual nanoparticles in the flow field [1,2,3]. The packed sphere beds are represented by packing known-sized spheres in either ideal arrays (i.e., body centered cubic, face centered cubic, and simple cubic) or a random array, using the modified Lubachevsky-Stillinger algorithm [4]. After that, the velocity field of water in these beds is generated, using the LBM simulations. The Lagrangian trajectories of the nanoparticles are then calculated based on convection due to the flow field and due to forces exerted on the nanoparticles. In our simulation, the balance of drag force, random force, buoyancy force, van der Waals force, and electrostatic force is responsible for advancing the nanoparticles. At nano-scale distances (i.e., 0.75 nm), the nanoparticles are pulled strongly together by the van der Waals force, and the aggregates are formed. The addition of each nanoparticle on an aggregate changes the morphology of the nano-aggregate, and we can observe the formation of these aggregates at different hydrodynamic and electrolyte conditions while they propagate in the packed beds. The change in morphology is quantified with calculations of the transient fractal dimensions of the particles, changes in the size of the aggregates with time, and with changes in the asphericity of the particle aggregates. Aggregates that are elongated, like cylinders, or round, like spheres, can be formed under different flow conditions.

ACKNOWLEDGEMENTS

The computational support of XSEDE (CTS090017) is gratefully acknowledged.

REFERENCES

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