Abstract
The impact of magnetic field ramp rate and the resulting aggregation kinetics on thermal transport in three different oil based ferrofluids of similar properties but with different particle size distributions is reported here. The volume fraction of the surfactant capped magnetite nanoparticles, in all three cases were kept constant at 0.037(20 wt%). For 17 G/s ramp thermal conductivity enhancement of 180% at 200 G improved to 230% with 33 G/s ramp in the ferrofluid system with the least fraction of larger particles. Our results show that faster magnetic field ramps have a profound influence on thermal conductivity enhancement. The phase contrast optical microscopy images show that the slower ramps resulted in thicker aggregates leading to a substantial decrease in their number density. On the other hand, faster ramps resulted in simultaneous formation of large number density of finer aggregates which proved to be extremely beneficial for thermal transport. In a ferrofluid with moderately high number of larger particles, faster ramps were found to be very effective in improving thermal transport. On sudden exposure to magnetic field, particularly for higher strengths, substantial improvement in thermal conductivity was observed initially, which decreased with time due to coarsening of aggregates and reduced number density of chains, but leveled off to a lower time independent value. Our findings provide a way to improve field induced thermal transport of a ferrofluid without altering their physical properties. Alternate magnetic field on-off cycles and faster field ramps were found to be particularly helpful in improving thermal transport of a ferrofluid possessing modest amount of larger particles, which will enable development of superior smart cooling devices.
Published Version
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