(95n) Normal Field Instability in Mr Fluids

A ferrofluid is a liquid which contains magnetic nanoparticles such as magnetite and iron and which responds to externally applied magnetic fields. Ferrofluids remain in a liquid state when exposed to a magnet because of the characteristic small size of the particles (~10nm). A magnetorheological fluid (MR fluid) is similar to a ferrofluid in that it responds to applied magnetic fields but has larger particles (~1-10µm). This increase in size and the higher particle volume fractions used causes the fluid to solidify when exposed to a magnetic field. It is commonly known that a ferrofluid forms spikes when a magnetic field is applied perpendicular to a free surface when the magnetic surface force exceeds the stabilizing effects of fluid weight and surface tension. This phenomenon is termed the normal field instability. Our research seeks to investigate a similar but more abrupt effect observed in the surface of a magnetorheological fluid exposed to a normal magnetic field. In order to identify the critical field for peak formation, and patterns in magnetorheological fluid, we carried out a series of experiments with various dilutions of a water-based MR Fluid (Lord Corporation, MRF-241ES) in a container with as flat a surface as possible. Images of the instability patterns will be presented, as well as comparison with the classical theory applicable to ferrofluids.