RESUMEN
Magnetic density separation is an emerging recycling technology by which several different waste materials-from plastic products, electronics, or other-can be sorted in a single continuous processing step. Larger-scale installations will require ferrofluids that remain stable at several teslas, high magnetic fields at which colloidal stability was not investigated before. Here we optically monitor the concentration profile of iron oxide nanoparticles in aqueous ferrofluids at a field of 10 T and a gradient of 100 T/m. The sedimentation velocities and equilibrium concentration profiles inform on maintenance or breakdown of colloidal stability, which depends on the concentration and magnetic coupling energy of the nanoparticles. Comparison with results obtained with a small neodymium magnet indicate that stability at moderate fields is predictive of stability at much higher fields, which facilitates the development of new ferrofluids dedicated to magnetic density separation.
RESUMEN
Dilute ferrofluids have important applications in the separation of materials via magnetic levitation. However, dilute ferrofluids pose an additional challenge compared to concentrated ones. Migration of the magnetic nanoparticles toward a magnet is not well counteracted by a buildup of an osmotic pressure gradient, and consequently, homogeneity of the fluid is gradually lost. Here, we investigate this phenomenon by measuring and numerically modeling time-dependent concentration profiles in aqueous ferrofluids in the field of a neodymium magnet and at 10 T in a Bitter magnet. The numerical model incorporates magnetic, frictional, and osmotic forces on the particles and takes into account the polydispersity of the particles and the spatial dependence of the magnetic field. The magnetic sedimentation rate in our most stable ferrofluids can be understood in terms of the magnetophoresis of separate nanoparticles, a best-case scenario when it comes to applications.