Magnetically-actuated thin films

The application of a magnetic field (MF) to thin films (TFs) containing ferromagnetic nanoparticles (NPs) is explored as a means of acquiring control over the film drainage and stability. Freely-suspended TFs were created on a horizontal glass boundary from mixtures containing water, surfactant, glycerin and magnetite NPs. The film thickness is recorded via interferometry and the motion of aggregates can be traced as they appear white. In the absence of an external MF, drainage of the film is approximately axisymmetric and towards the borders of the film due to capillary suction; however, with an inhomogeneous external MF, there is an increase in the drainage rate towards the region of greater magnetic flux density (by magnitude) due to magnetite-surfactant aggregates moving, and dragging surrounding liquid, towards that region. The average film lifetime was significantly lower in this case. Overall, this may be the starting point for creating externally actuated TFs, which could encapsulate a substance such as a gaseous fuel or drug whose release could be induced through an external field. As a result of these findings, future work will focus on studying whether similar effects can be found with an electric field applied to TFs containing charged or electrically-polarisable NPs.