The study reports the aspects of post-impact hydrodynamics of ferrofluid droplets on superhydrophobic (SH) surfaces in the presence of a horizontal magnetic field. A wide gamut of dynamics was observed by varying the impact Weber number (We), the Hartmann number (Ha) and the magnetic field strength (manifested through the magnetic Bond number (Bo m )). For a fixed We~60, we observed that at moderately low Bo m ~300, droplet rebound off the SH surface is suppressed. The noted We is chosen to observe various impact outcomes and to reveal the consequent ferrohydrodynamic mechanisms. We also show that ferrohydrodynamic interactions leads to asymmetric spreading; and the droplet spreads preferentially in a direction orthogonal to the magnetic field lines. We show analytically that during the retraction regime, the kinetic energy of the droplet is distributed unequally in the transverse and longitudinal directions due to the Lorentz force. This ultimately leads to suppression of droplet rebound. We study the role of Bo m at fixed We~60, and observed that the liquid lamella becomes unstable at the onset of retraction phase, through nucleation of holes, their proliferation and rupture after reaching a critical thickness only on SH surfaces, but is absent on hydrophilic surfaces. We propose an analytical model to predict the onset of instability at a critical Bo m . The analytical model shows that the critical Bo m is a function of the impact We, and the critical Bo m decreases with increasing We. We illustrate a phase map encompassing all the post-impact ferrohydrodynamic phenomena on SH surfaces for a wide range of We and Bo m .

In this study, the spreading characteristics of water droplets impacted on a solid spherical target have been investigated experimentally and theoretically. Droplet impact and postimpact feature studies have been conducted on hydrophilic and superhydrophobic spherical surfaces. Effects of the impact Weber number and target-to-drop diameter ratio on the spreading hydrodynamics have been discussed. Postcollision dynamics are explored with side and top views of impaction phenomenon using a high speed imaging technique. The morphological outcome of this impingement process has been quantitatively discussed with three geometric parameters, namely, liquid film thickness at the north-pole of the target surface, spread factor, and the maximum spread angle. Observations revel that spread factor and the maximum spread angle increases with the decrease in the size of the spherical target, whereas opposite of this is true for liquid film thickness at the north-pole of the target surface. Temporal variations of liquid film thickness at the north pole of the target have been plotted and found in agreement with the theoretical predictions made in the earlier studies. Finally, a mathematical model based on the energy balance principle has been proposed to predict the maximum spread angle on spherical targets. The theoretical values are found in good agreement with the experimental results for a wide range of spherical diameters studied. The findings may have implications toward a better understanding of fluid wetting, spraying, and coating behavior of complex shapes and geometries.

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Droplet impact, dynamics, wetting and spreading behavior on surfaces imposes rich and interesting physics, in addition to extensive understanding of processed employing droplets and sprays. The physics and mechnisms become richer and more interesting in the event the geometry, morphology and wettability of the surface provides additional constraints to the fluid dynamics. Post-impingement morphology and dynamics of water droplets on convex cylindrical surfaces of variant radii has been explored experimentally. Droplet impact and post-impact feature studies have been conducted on hydrophillic and superhydrophobic (SH) cylindrical surfaces. Effects of the impact Weber number (We) and target-to-drop diameter ratio on the wetting and spreading hydrodynamics has been studied and discussed. The postimpact hydrodynamics have been quantified employing dedicated non-dimensional variables, such as the wetting fraction, the spreading factor and non-dimensional film thickness at the north pole of the target. The observations reveal that the wetting fraction and spread factor increases with an increase in the impact We and decrease in the target-to-drop diameter ratio. An opposite trend is noted for the non-dimensional film thickness at the 2 target's north pole. It is also deduced that the spread factor is independent of the target wettability whereas the wetting fraction is remarkably low for SH targets. The lamella dynamics post spreading has also been observed to be a strong function of the wettability, impact We and the diameter ratio and the same has been explained based on wetting and inertial principles. An analytical expression for temporal evolution of film thickness at north pole of the cylinderical target is derieved from first priciples. The article also proposes a theoretical model based on energy conservation for predicting the maximum wetting fraction for variant cylindrical targets in terms of the governing Weber number (We) and Capillary number (Ca). It is observed that the experimental measurements are in good agreement with the theoretical predictions.

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