Caustics formed from iron nanoparticles coalescing into chains aligned with the lines of the magnetic field from an adjacent magnet

Many people will be familiar with an iron filings and magnet experiment often performed in high school physics classes.  Iron filings on a sheet of paper with a magnet underneath, when the paper is gently tapped, align with the lines of the magnetic field sometimes forming ‘chains’.  In our research group, we have performed a version of this experiment at the nanoscale using iron nanoparticles smaller than 200nm in diameter floating in deionised water on a microscope slide.  We brought a magnet into close proximity with the slide and the nanoparticles formed chains aligned along the lines of magnetic field which we visualised using caustics [see ‘Holes in fluids’ on October 22, 2014].  When we removed the magnet, the nanoparticles resumed their Brownian motion.  Not a ground-breaking experiment but it’s exciting to be able to demonstrate that the phenomenon occurs at the nanoscale and that we can observe it.