Jet or wet? Droplet post-impact regimes on concave contours.

Droplet collision and subsequent spreading or wetting interactions with the solid substrate exhibit rich and interesting physics and are also important for various utilities. The fluid dynamics becomes more interesting and insightful when the wettability and geometry of the surface are tuned and altered. This study investigates the post-impact regimes of droplet impact on hydrophilic and superhydrophobic concave profile grooves (having dimensions comparable to that of the droplet). The post-collision hydrodynamics for such substrate-droplet system is three-dimensional, as in addition to droplet dynamics in the azimuthal direction, liquid jets may also be generated in the axial direction of the groove. Thereby the system may either lead to wetting or jetting, depending on the impact conditions. The effect of the impact Weber number (We) on the jet velocity, non-dimensional spreading width (γ) and non-dimensional south-pole film thickness (h*) has been probed and quantified. The observations reveal that the role of the wettability of the substrate is more profound in the recoiling stage than in the spreading stage, because inertial forces dominate in the latter. It is also noted that the spreading width increases and south-pole height decreases with increasing the impact Weber number. The opposite trend is noted upon increasing the groove concavity by altering just one dimension of the groove. The jet velocity is found to be the highest immediately after the impact and eventually decreases in a nonlinear fashion. Further, it has been found that the jet velocity increases with increasing the impact Weber number and that this effect is more prominent for superhydrophobic surfaces. A semi-analytical framework has been proposed to predict the jet velocity evolution in terms of governing Weber (We) and capillary (Ca) numbers. The predictions of the proposed model are in good agreement with the experimental observations.

Augmenting the Leidenfrost Temperature of Droplets via Nanobubble Dispersion.

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 (TDL). Here we propose and validate a novel paradigm of augmenting the TDL by employing droplets with stable nanobubbles dispersed in the fluid. In this first-of-its-kind report, we show that the TDL 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 TDL. At a fixed impact velocity, the TDL 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.

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 (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.

Symmetric shear banding and swarming vortices in bacterial superfluids.

Bacterial suspensions-a premier example of active fluids-show an unusual response to shear stresses. Instead of increasing the viscosity of the suspending fluid, the emergent collective motions of swimming bacteria can turn a suspension into a superfluid with zero apparent viscosity. Although the existence of active superfluids has been demonstrated in bulk rheological measurements, the microscopic origin and dynamics of such an exotic phase have not been experimentally probed. Here, using high-speed confocal rheometry, we study the dynamics of concentrated bacterial suspensions under simple planar shear. We find that bacterial superfluids under shear exhibit unusual symmetric shear bands, defying the conventional wisdom on shear banding of complex fluids, where the formation of steady shear bands necessarily breaks the symmetry of unsheared samples. We propose a simple hydrodynamic model based on the local stress balance and the ergodic sampling of nonequilibrium shear configurations, which quantitatively describes the observed symmetric shear-banding structure. The model also successfully predicts various interesting features of swarming vortices in stationary bacterial suspensions. Our study provides insights into the physical properties of collective swarming in active fluids and illustrates their profound influences on transport processes.

Elasto-inertial turbulence.

Turbulence is ubiquitous in nature, yet even for the case of ordinary Newtonian fluids like water, our understanding of this phenomenon is limited. Many liquids of practical importance are more complicated (e.g., blood, polymer melts, paints), however; they exhibit elastic as well as viscous characteristics, and the relation between stress and strain is nonlinear. We demonstrate here for a model system of such complex fluids that at high shear rates, turbulence is not simply modified as previously believed but is suppressed and replaced by a different type of disordered motion, elasto-inertial turbulence. Elasto-inertial turbulence is found to occur at much lower Reynolds numbers than Newtonian turbulence, and the dynamical properties differ significantly. The friction scaling observed coincides with the so-called "maximum drag reduction" asymptote, which is exhibited by a wide range of viscoelastic fluids.