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Non-Proliferative Microbes for Water Processing Dr EWC Lim (Department of Chemical and Biomolecular Engineering)
A modified Discrete Element Method (DEM) has been successfully developed and applied towards simulating the agglomeration behaviours of magnetic nanoparticles during gelation processes in this project. Gels were formed by salt-induced double layer compression of magnetic nanoparticles in the absence or presence of an external magnetic field. The internal pore structures of gels obtained were characterized using cryosectioning and Field Emission Scanning Electron Microscopy techniques. Although gels formed in this way may appear to exhibit low porosities based on visual inspection of their external morphologies, the interior structures were observed to be highly porous and composed of large spaces among the branches of a convoluted network. When the gelation process was allowed to occur in the absence of an external magnetic field, the branches of such a network were observed to be oriented in no particular direction. On the other hand, these branches appeared to be oriented predominantly in one direction when gelation occurred in the presence of an external magnetic field, indicating that network growth has occurred along the direction of the magnetic field. This opens the possibility of manipulating the internal pore structures of gels formed from magnetic nanoparticles non-invasively via an external magnetic field which may find novel applications in such areas as controlled drug delivery and tissue engineering. The process of gelation with and without the application of an external magnetic field giving rise to the different internal pore structures could be understood mechanistically by results of the simulations performed using the modified DEM developed in this project. Gelation occurred by the formation of random aggregates of nanoparticles within the domain which then joined with one another to form a network. However, in the presence of anisotropic magnetic forces, these aggregates were rotated to align along the direction of the magnetic field. Elongation of aggregates occurred and the final network formed consisted largely of such elongated branches of magnetic nanoparticles arranged more or less parallel to one another.
Figure 1a shows the aggregation patterns of magnetic nanoparticles formed in the absence of an external magnetic field obtained from computer simulations with the modified DEM methodology. Small isolated aggregates of nanoparticles were observed at low solid volume fractions while at high solid volume fractions, an extended network of nanoparticles usually referred to as a percolated network was observed to form spontaneously. In contrast, Figure 1b shows that in the presence of an external magnetic field, the aggregates of nanoparticles were aligned along the direction of the magnetic field due to the anisotropic nature of the magnetic forces exerted on each nanoparticle and aggregate. At low solid volume fractions, individual elongated strands of aggregates were formed while at high solid volume fractions, such aggregates were capable of joining together due to smaller distances between aggregates. In comparison with the previous case where an external magnetic field was absent, the branches of the network formed here were composed of more particles and were thus longer.
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