Imaging magnetic flux lines with iron oxide nanoparticles using a “fossilized liquid assembly”

Abstract

Directed self-assembly of nanomaterials via external fields is an attractive processing tool for industry as it is inherently inexpensive and flexible. Direct observations of this process are however challenging due to the nano and meso length scales involved. The self-assembly of magnetic nanoparticles in particular has gained much recent interest for applications ranging from biomedical imaging and targeted cancer therapy to ferrofluid mechanical damping devices, that rely on the state of aggregation and alignment of the nanoparticles. We utilize an oil–water platform to directly observe directed self-assembly of magnetic nanoparticles that are field ordered into two-dimensional mesostructures through the fossilized liquid assembly (FLA) method. Our system consisted of polymer-coated iron-oxide nanoparticles (25 nm) which were assembled in the vicinity of the interface between a crosslinkable hydrophobic monomer (UV-polymerizable) oil, and water through the use of external magnetic fields, and then cured with UV light. This flash curing system effectively provides a snapshot of the assembly process and allows for direct visualization of assemblies through the use of both atomic force and optical microscopy. In this study, entire magnetic flux field lines in various geometrical configurations were successfully modelled and mapped out by the magnetic nanoparticles, both in-plane and in perpendicular orientations utilizing FLA. The assemblies showed strong directional selectivity and alignment with the flux field lines and provided evidence of strong dipole interactions which partially caused aggregate sedimentation.