Ferrofluid droplet impingement cooling of modified surfaces under the influence of a magnetic field

This paper documents the spreading characteristics and heat transfer of a single ferrofluid droplet impinging onto dry solid surfaces with and without a magnetic field. Three surfaces were investigated: a pure silicon substrate, a silicon substrate with a thin titanium film, and a superhydrophobic coating. Fluid dynamics and cooling results of a ferrofluid droplet were compared to that of pure water and water-surfactant droplets for the same Weber number (We = 61) and initial surface temperature (T0 = 95∘C). Our results show that the magnetic field (magnetic Bond number Bom = 2186) increased the droplet spreading diameter up to 12% and suppressed droplet rebound, thus improving the cooling performance of a ferrofluid droplet. The highest heat transfer rates were measured for the pure silicon substrate under a magnetic field with an average value of 14 W. The lowest cooling performance was observed for the superhydrophobic surface without a magnetic field with a peak value of 0.82 W. However, using the magnetic field, the superhydrophobic surface achieved 263% higher heat transfer rate than the non-magnetic case because of the suppression of droplet rebound. Moreover, the surface did not foul from ferrofluid deposition, an advantage for spray cooling applications when using any nanofluid.