Nanoparticle induced self-assembly

Self-assembly has for the large part focused on the assembly of molecules without guidance or management from an outside source. However, self-assembly is in principle by no means limited to molecules or the nanoscale. A particularly interesting method to the self-assembly of micro- to millimetre sized components is the use of the 'magnetic hole' effect. In this method, nonmagnetic particles can be manipulated by external magnetic fields by immersing them in a dispersion of colloidal, magnetic nanoparticles, denoted ferrofluids. Nonmagnetic particles in magnetized ferrofluids are in many ways ideal model systems to test various forms of particle self-assembly and dynamics. When microspheres are confined to a monolayer between two parallel plates and subjected to static or oscillating magnetic fields they show a variety of dynamical behaviours and assemblages, depending on the frequency and direction of the external fields. A single pair of magnetic holes oscillating in a ferrofluid layer may be used to measure the viscosity of tiny volumes of the fluid. We have also observed ordering of dilute dispersions of macromolecules and nanoparticles in magnetized ferrofluids. The self-assembly at this length scale results from structural correlations between these nanostructures and ferrofluid particles rather than from the macroscopic magnetostatic effect for the magnetic holes.