Random porous media and magnetic separation of magnetic colloids

Abstract

The separation of magnetic nanoparticles from a stable dispersion is a challenging task because of the nanoparticles' thermal motion and relatively small magnetic moments. Strong magnetic gradients are required to capture such particles, which can be achieved in a high-gradient magnetic separator. In this work, several facets of this separation process are studied by simulation and experiment. Our method uses a fibrous porous separation matrix comprising magnetisable metallic fibres acting as a magnetic filter. Once the medium is magnetized externally, a nanoparticle dispersion is passed through. The external field induces strong local magnetic field gradients around the fibres, attracting passing nanoparticles and capturing them. Upon removal of the external field, the matrix demagnetizes and the captured particles are released. The fundamental aspects of the capture process are investigated by means of Brownian dynamics simulations, where we investigate the capture of differently sized nanoparticles by a single cylindrical fibre. To calculate the magnetic forces accurately, an exact mathematical expression for the magnetic field of the cylinder is derived. Particle capture appears most effective for fibres oriented perpendicular to the magnetic field, and for particles with a diameter larger than 50 nm. As expected, smaller particles are more difficult to capture, but at higher concentrations the nanoparticles form chains, enhancing capture. Additional to the simulations, a separation setup is constructed for exploratory experiments. Initial results show that nanoparticles of around 10 nm in size can be captured almost completely, contrary to the simulation results. A fraction of the particles remains irreversibly attached to the fibres, which makes reusing the nanoparticles difficult, for example in a catalytic process. The structure of the fibrous porous medium is investigated by simulation of random dense packings of spherocylinder particles in the confinement of a bounding cylinder. A modified version of the mechanical contraction method is used, and a virtual particle method is developed to account for hard boundaries. A relationship between the aspect ratio of the spherocylinders, the packing density and the diameter of the bounding cylinder is determined. We find that the presence of a cylindrical boundary only causes strong particle alignment for very narrow cylinders. Furthermore we revise and extend the mechanical contraction method addressing some theoretical difficulties with the original method and implementing the simulation of superquadric particles--versatile objects that can take on the shape of spheres, ellipsoids, disks and (rounded) cuboids. This generalization allows for future investigation of yet unexplored particle shapes. Finally, a chemical synthesis route is described to prepare stable aqueous dispersions of sub-micron sized composite colloidal particles based on the protein zein. Via a simple and general heterocoagulation method, spherical zein particles are loaded with different negatively charged nanoparticles. By using superparamagnetic magnetite particles, magnetically susceptible composite particles are formed. We modify these particles by coating them with a thin layer of silica, creating possibilities for further modification to highly specific magnetic carrier particles.