Ferrofluid droplets in uniform magnetic fields: evidence for field-dependent interfacial tension

Ferrofluids are fluids which become strongly magnetised in magnetic fields and thus can be manipulated by moderate magnetic fields. They consist of magnetic particles of the order of 10nm in diameter, which are coated with a surfactant and suspended in a carrier liquid. Applications of ferrofluids are diverse, including their use as coolants in loud-speakers, seals for rotating shafts, and components of micro-pumps for lab-on-a-chip technologies. For some applications, it is important to understand the response of a ferrofluid droplet to an applied magnetic field or field gradient. An instrument capable of applying a combination of uniform magnetic fields and uniform magnetic field gradients to a ferrofluid droplet suspended in an immiscible, viscous medium has been designed and built. When a static uniform field is applied to a ferrofluid droplet, the droplet elongates in the direction of the applied field. This elongation is a result of a change in the balance between the surface energy and the self-demagnetizing energy of the droplet. By comparing experimentally measured aspect ratios with those predicted theoretically from independently measured magnetic properties, it was found that contrary to the assumptions of standard theoretical models, the interfacial tension of a droplet depends on the applied magnetic field. The interfacial tension was found to increase linearly with applied magnetic field and saturate at fields well below the fields at which the magnetisation of the fluids saturates. Increases in interfacial tension were found to range from 14 to 70% on application of saturating magnetic fields, depending on the composition of the ferrofluid.