Magneto-Elastic Effect in Non-Newtonian Ferrofluid Droplets Impacting Superhydrophobic Surfaces

In this article, we propose, with the aid of detailed experiments and scaling analysis, the existence of magneto-elastic effects in the impact hydrodynamics of non-Newtonian ferrofluid droplets on superhydrophobic surfaces in the presence of a magnetic field. The effects of magnetic Bond number (Bo_m), Weber number (We), polymer concentration, and magnetic nanoparticle (Fe₃O₄) concentration in the ferrofluids were investigated. In comparison to Newtonian ferrofluid droplets, addition of polymers caused rebound suppression of the droplets relatively at lower Bo_m for a fixed magnetic nanoparticle concentration and We. We further observed that for a fixed polymer concentration and We, increasing magnetic nanoparticle concentration also triggers earlier rebound suppression with increasing Bo_m. In the absence of the magnetic nanoparticles, the non-Newtonian droplets do not show rebound suppression for the range of Bo_m investigated. Likewise, the Newtonian ferrofluids show rebound suppression at large Bo_m. This intriguing interplay of elastic effects of polymer chains and the magnetic nanoparticles, dubbed as the magneto-elastic effect, is noted to lead to the rebound suppression. We establish a scaling relationship to show that the rebound suppression is observed as a manifestation of the onset of magneto-elastic instability only when the proposed magnetic Weissenberg number (Wi_m) exceeds unity. We also put forward a phase map to identify the various regimes of impact ferrohydrodynamics of such droplets and the occurrence of the magneto-elastic effect.