Engineering applications of ferrofluids: A review

Ferrofluids are stable colloidal suspensions of nanometric sized ferromagnetic, or their oxide particles, in conventional basefluids like water, oil, etc. The property of ferromagnetism is changed to superparamagnetism by size reduction of the particles to nanometer level. It opens the possibility to tune the thermophysical properties of ferrofluids by the application of an external magnetic field. Ferrofluids were invented by Steve Papell of NASA in 1963 with the intention to make a suitable liquid rocket fuel which could be attracted by an external magnetic field towards the inlet of a pump in a zero-gravity environment. Subsequently, intense research has led to diverse applications of these fluids, from computer hard discs, clean room robots to dynamic loudspeakers, exploiting their unique operational behavior. Ferrofluid lubrication and film bearing have emerged as better alternatives to conventional systems. Contemporary times have also witnessed a plethora of bio-medical applications of ferrofluids, ranging from location-specific drug delivery, treatment of tumor cells, cell separation, tagging, and in diagnostic systems like Magnetic Resonance and Particle Imaging, to name a few. New vistas have opened in the context of thermal management of miniaturized systems. Ferrofluids, with enhanced thermal conductivity, and possibility of external control and manipulation of heat transfer coefficient, can potentially compete with other standard heat transfer solutions. In this background, this article attempts a comprehensive review of the gamut of existing and emerging applications of ferrofluids. Relevant insights of the underlying physics and transport mechanisms, which are harnessed in these applications, is also delineated.