Abstract:
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 (Bom),
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 Bom 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 Bom. In the absence of the magnetic nanoparticles, the non-Newtonian droplets do not show rebound suppression for
the range of Bom investigated. Likewise, the Newtonian ferrofluids show rebound suppression at large Bom. 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 (Wim) 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.
