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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.

Droplet impact on a heated substrate is an important area of study in spray cooling applications. On substrates significantly hotter than the saturation temperature, droplets immediately hover on its vapor cushion, exhibiting the Leidenfrost phenomenon. Here, we report the phenomena wherein addition of Al2O3 nanoparticles to water significantly increases the onset of dynamic Leidenfrost temperature ( TDL) and suppresses the overall Leidenfrost regime. We experimentally revealed that the onset of TDL delays with increasing the nanoparticle concentration of the colloidal dispersions at a particular Weber number ( We). But, for a constant concentration, the onset of TDL decreases with an increase of impact We. In contrast to water droplets, the colloid droplets exhibit vigorous spraying behavior due to the nanoparticulate residue deposition during the spreading and retraction stages. Further, the residue on the heated substrate changes the departure diameter of the vapor bubbles during boiling, prevents bubble coalescence and vapor layer formation, and reduces the propensity to attain dynamic Leidenfrost regime. With the aid of scaling analysis of TDL with impact We, we have explored the thermo-hydrodynamic behavior of impacting colloid droplets on a superheated substrate. Finally, we have also segregated the different boiling regimes of colloid droplets over various impact We.

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Droplets may rebound/levitate when deposited over a hot substrate (beyond a critical temperature) due to the formation of a stable vapor microcushion between the droplet and the substrate. This is known as the Leidenfrost phenomenon. In this article, we experimentally allow droplets to impact the hot surface with a certain velocity, and the temperature at which droplets show the onset of rebound with minimal spraying is known as the dynamic Leidenfrost temperature (T DL ). Here we propose and validate a novel paradigm of augmenting the T DL by employing droplets with stable nanobubbles dispersed in the fluid. In this first-of-itskind report, we show that the T DL can be delayed significantly by the aid of nanobubble-dispersed droplets. We explore the influence of the impact Weber number (We), the Ohnesorge number (Oh), and the role of nanobubble concentration on the T DL . At a fixed impact velocity, the T DL was noted to increase with the increase in nanobubble concentration and decrease with an increase in impact velocity for a particular nanobubble concentration. Finally, we elucidated the overall boiling behaviors of nanobubble-dispersed fluid droplets with the substrate temperature in the range of 150− 400 °C against varied impact We through a detailed phase map. These findings may be useful for further exploration of the use of nanobubble-dispersed fluids in high heat flux and high-temperature-related problems and devices.