Formation of ironsand stalactites in a magnetic field

Granular flows can exhibit many surprising features, with one of the most familiar examples being the flow of sand in an hourglass. Titanomagnetite sand (known more popularly as ironsand) is a type of granular material which contains high amounts of iron in the form of magnetite, as well as other metal impurities such as calcium and titanium. In this article, we consider a version of the hourglas where ironsand is used for the granular material, finding that the behavior of the granular mound in the bottom chamber is strikingly different for low and high field strengths. We argue that this effect can be seen as a granular analogue of the Rosensweig instability for ferrofluids: the mound separates into chains once the field strength reaches a critical value such that gravity and friction can be overcome. This interpretation is supported with particle-based simulations which show that magnetizable grains cluster more loosely for weak fields, but start to form chains for strong fields. The experiment can be introduced as part of an undergraduate electromagnetism course and used to elucidate various concepts related to magnetic fields and magnetization. As a pedagogical demonstration, the experiment builds on the classic Faraday visualization using iron filings and a horseshoe magnet, but involves more advanced concepts which are suitable for undergraduate level. The demonstration can be reproduced with simple equipment.