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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 (

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.

Experimental investigations were carried out to elucidate the roles of surface wettability and inclination on the post-impact dynamics of droplets. The maximum spreading diameter and spreading time were found to decrease with increasing inclination angle and normal Weber number (We n ) for superhydrophobic (SH) surfaces. The experiments on SH surfaces were found to be in excellent agreement with an existing analytical model, albeit with the incorporation of modifications for the oblique impact conditions. The energy ratios and elongation factors were also determined for different inclination angles. On inclined SH surfaces, different features like arrest of secondary droplet formation, reduced pinch-off at the contact line and inclination dependent elongation behavior were observed. On the contrary, the hydrophilic surfaces show opposite trends of maximum spreading factor and spreading time with inclination angle and We n , respectively. This is caused by the dominance of tangential kinetic energy over adhesion energy and gravitational potential at higher inclination angles. Further, the influence of the surface tension (using surfactant solutions, without significantly changing the viscosity) and viscosity (using colloids, without significantly changing the surface tension) for impact on SH and hydrophilic surfaces are probed. The exercise allows better insight on the exact hydrodynamic mechanisms at play during the impact events. Overall, the article provides a comprehensive picture of post-impact dynamics of droplets on inclined surfaces, encompassing a broad spectrum of governing parameters like Reynolds number (Re), Weber number (We), degree of inclination and surface wettability.

The present article discusses the numerical simulation results of laminar mixed convection flow in vertical tubes. A comparative analysis of the thermal and hydrodynamic features of both buoyancy-assisted and opposed flows was performed for Reynolds number (100 ≤ Re ≤ 2300), Grashof number (103 ≤ Gr ≤ 7.935×106) and Richardson number (0.1 ≤ Ri ≤ 1.5) with uniform heat flux boundary condition. 2-D axisymmetric steady state simulations were carried out for a length-to-diameter (L/D) ratio of ≤ 1000 with water as the working fluid. Numerical simulations were performed by employing SIMPLE scheme for pressure–velocity coupling in momentum equations and second order UPWIND scheme for solving energy equations. In case of assisting flow (Re∼250), at fully developed state the centerline velocity decreases, and velocity is increased near the tube wall due to heat flux induced free convection. With increasing heat flux, the decrement in centerline velocity compared to the no-heat flux condition increases. Further, with increasing heat flux, the increase in friction factor and Nusselt number was observed. Therefore, at the same Re, the variation of heat flux led to unique velocity profiles and temperature gradients. The variation of centerline velocity and temperature in developing region was also studied. While centerline temperature was monotonically increasing with length, the centerline velocity increased up to a maximum in the developing region and then attained the steady state at a lower value in the fully developed state. Subsequently we studied the dependence of Ri on the hydrodynamic and thermal features. For constant Re, friction factor and Nusselt number was observed to increase with increase in Ri from 0.1 to 1.5 range. At fixed Re, variation of heat flux leads to variation of Gr or Ri. Hence, the buoyancy effect has a significant role in the entrance length development. The hydrodynamic entry length after an initial increase, attained an almost constant value with increasing Ri. On the other hand, the thermal entry length exhibited a decreasing trend with increasing Ri. Similarly, at constant Ri, Nusselt number increased with increasing Re for a range of 100–1000. It was evident that for a given Re and Ri, the heat transfer in the developing flow is always higher as that of the fully developed flow. Contrasting observations were observed for buoyancy-opposed flow. Velocity is accelerated at the center as compared to the tube wall in case of opposing flow for same Re and Ri. For a fixed Re, the friction factor and Nusselt number decreased with increasing the Ri. Both the hydrodynamic and thermal entry length increases for opposing flows. Finally, we have developed correlations for fully developed friction factor (f) with Re as well as Ri and Nusselt number (Nu) with Ri for buoyancy-assisting and buoyancy-opposing flows. Two independent correlations are also produced for Nu with Graetz number (Gz) for developing and fully developed regimes in both assisting as well as opposing flows.

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 in 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.

Understanding of impact dynamics of droplets on oblique plane is relevant for applications like pesticides spraying and internal combustion engines. Experiments were performed using three test fluids to investigate the effect of viscosity and surface tension properties on the elongation factor of droplets impacting on oblique planes. The temporal variation of elongation factor primarily depends upon the surface inclination and surface tension. For a given Weber number, emission of secondary droplet takes place at the earliest time for surfactant solution due to reduction of interfacial energy.

Using the discrete vortex method we perform numerical simulations of the Anguilliform mode of swimming dynamics of a one-dimensional flexible filament. Various parameters are varied to quantify the coefficient of thrust and swimming efficiency. We show that wake vortices are in a Benard--von Karman (BvK) configuration in the drag producing regime and rearrange to reverse-BVK (rBvK) when thrust producing. The resultant wake vortex distribution contour map and associated velocity field clarify differences between BvK in drag regime, the transition region, and rBvK in thrust regime. Optimal parameters for high thrust and swimming efficiency of two-dimensional flexible filaments are identified.

This article highlights the role of non-Newtonian (elastic) effects on the droplet impact phenomenon at temperatures considerably higher than the boiling point, especially at or above the Leidenfrost regime. The Leidenfrost point (LFP) was found to decrease with an increase in the impact Weber number (based on the velocity just before the impact) for fixed polymer (polyacrylamide) concentrations. Water droplets fragmented at very low Weber numbers (approx. 22), whereas the polymer droplets resisted fragmentation at much higher Weber numbers (approx. 155). We also varied the polymer concentration and observed that, up to 1000 ppm, the LFP was higher than that for water. This signifies that the effect can be delayed by the use of elastic fluids. We have shown the possible role of elastic effects (manifested by the formation of long lasting filaments) during retraction in the increase of the LFP. However, for 1500 ppm, the LFP was lower than that for water, but had a similar residence time during the initial impact. In addition, we studied the role of the Weber number and viscoelastic effects on the rebound behaviour at 405°C. We observed that the critical Weber number up to the point at which the droplet resisted fragmentation at 405°C increased with the polymer concentration. In addition, for a fixed Weber number, the droplet rebound height and the hovering time period increased up to 500 ppm, and then decreased. Similarly, for fixed polymer concentrations like 1000 and 1500 ppm, the rebound height showed an increasing trend up to certain a certain Weber number and then decreased. This non-monotonic behaviour of rebound heights was attributed to the observed diversion of the rebound kinetic energy to rotational energy during the hovering phase. Finally, a relationship between the non-dimensional Leidenfrost temperature and the associated Weber and Weissenberg numbers is developed, and a scaling relation is proposed.

In this present article, we have investigated the magnetohydrodynamics of ferrofluid droplets impacting on different wettability surfaces, in the presence of a vertical magnetic field. The spreading dynamics was studied for a wide spectrum of magnetic Bond number (Bom), Hartmann number (Ha) and Weber number (We). In absence of any magnetic field, the droplets exhibited secondary droplet pinch-off during rebound. In the presence of magnetic field, the field modulated Rayleigh-Plateau instability delays the droplet pinch off as Bom increases. For a fixed We, the rebound of the droplet was suppressed on a superhydrophobic (SH) surface for varying the magnetic field strength (manifested through Bom). An analytical model based on the principle of conservation of energy was formulated to explain the magnetic field modulated droplet pinch-off. We have also investigated the influence of Bom on the temporal spreading dynamics of different Ha ferrofluid droplets impacting on hydrophilic and SH surfaces. With an increase in Bom, the magneto-visco-capillarity of high Ha droplets inhibits the capillary waves and motion of the contact line after impingement onto hydrophilic surface, compared to low Ha ferrofluid drops. Instead, the droplet rim fragments into secondary droplets during retraction event, leading to the onset of rim instability for low Ha ferrofluid drops with an increase in the magnetic field strength. We have also proposed a theoretical formulation based on energy conservation principle to predict the experimentally measured maximum spreading diameters under influence of magnetic field. The findings may hold significance in ferrofluid based droplet microfluidics systems with magnetic control or actuation.

In this article, we report the experimental and semi-analytical findings to elucidate the electrohydrodynamics (EHD) of a dielectric liquid droplet impact on superhydrophobic (SH) and hydrophilic surfaces. A wide range of Weber numbers (We) and electro-capillary numbers (Cae) are covered to explore the various regimes of droplet impact EHD. We show that for a fixed We ∼ 60, droplet rebound on a SH surface is suppressed with increase in electric field intensity (increase in Cae). At high Cae, instead of the usual uniform radial contraction, the droplets retract faster in an orthogonal direction to the electric field and spread along the direction of the electric field, inducing large electrical stresses at the liquid rim facing the electrodes. This prevents the accumulation of sufficient kinetic energy to achieve the droplet rebound phenomena. For certain values of We and Ohnesorge number (Oh), droplets exhibit somersault-like motion during rebound. Subsequently, we propose a semi-analytical model to explain the field induced rebound phenomenon on SH surfaces. Above a critical Cae ∼ 4.5, EHD instability causes a fingering pattern via evolution of a spire at the rim. Further, the spreading EHD on both hydrophilic and SH surfaces is discussed. On both wettability surfaces and for a fixed We, the spreading factor shows an increasing trend with increase in Cae. We have formulated an analytical model based on energy conservation to predict the maximum spreading diameter. The model predictions hold reasonably good agreement with the experimental observations. Finally, a phase map was developed to explain the post impact droplet dynamics on SH surfaces for a wide range of We and Cae.

In this article, we report the post-collision elasto-hydrodynamics of non-Newtonian elastic or Boger fluid droplets [polyacrylamide (PAAM) solution in water] on convex or cylindrical targets of various diameters. Both hydrophilic and superhydrophobic (SH) surfaces were studied to deduce the role of wettability. Different governing parameters, such as cylinder diameter, Weber number, and fluid elasticity (different polymer concentrations), were systematically varied to understand various hydrodynamic outcomes. In contrast to the Newtonian water droplets on hydrophilic surfaces, PAAM droplets resisted capillary breakup and exhibited formation of long lasting, slender, fluid filaments. In certain cases, these filaments showed the existence of satellite beads during stretching, which are generated through blistering or pearling instability (known as beads-on-a string). In the case of SH surfaces, PAAM droplets rebound at larger cylindrical diameters and higher Weber number compared to water. Thin transient filaments attached to the cylinder surface eventually suppress droplet rebound. Such rebound suppression is essentially a non-Newtonian feature, as water droplets on a cylindrical SH surface always exhibited rebound and fragmentation. Finally, we illustrate phase maps where the different regimes of post-impact elasto-hydrodynamics are correlated as functions of a proposed elastic Weber number (which incorporates the effects of both the Weber and the Weissenberg numbers) and the non-dimensional diameter D*. We show that distinct scaling regimes appear in the elasto-hydrodynamic behavior of the post-impact droplets of elastic fluids.

Experiments find that millimetric drops of a viscoelastic fluid impacting on a superhydrophobic surface at several meters per second do not rebound above a critical Weissenberg number of 1.

In this article, we report a new class of colloidal, micrometer-scale agarose powder based organic electrorheological (ER) fluids and its ER and viscoelastic characteristics. The steady shear ER characteristic of the colloids shows enhancements in the yield stress of the fluid, and yield stress values approaching ∼1 kPa have been noted. The ER hysteresis and electro-thixotropy illustrate that the microstructure of the colloids under field effects is able to withstand dynamic and impact stresses with good repeatability. The electro-creep strain and stress relaxation characteristics of the colloids show transition to the elastoviscous state with an increase in electric field strength. The oscillatory shear ER characteristic of the colloids shows field induced transition from a fluid-like nature to solid-like nature. Atypical regimes of loss and regain in viscoelastic nature are noted for the colloids under different field constraints. The viscoelastic dissipation and complex viscosity characteristics are also discussed for utilitarian aspects. Mathematical analysis reveals that the electric field induced viscoelastic, creep strain, and stress relaxation signatures of the colloids conform to fractional derivative elastoviscous models. The present findings may find significant implications toward the design and development of organic particle based ER fluids.

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 magnetic field strength (manifested through the magnetic Bond number (Bom), which is defined as the ratio of magnetic force to surface tension force), and the Hartmann number (Ha), defined as the ratio of magnetic force to the viscous force. For a fixed We ∼ 60, we observed that at moderately low Bom ∼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 lead to asymmetric spreading due to variation in magnitude of the Lorentz force, 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 (orthogonal to the external horizontal magnetic field) and longitudinal (along the direction of the magnetic field) directions. This ultimately leads to the suppression of droplet rebound. We studied the role of Bom 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 Bom. The model shows that the critical Bom is a function of the impact We, and the critical Bom 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 Bom.

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 however are more complicated (e.g. blood, polymer melts or paints), they exhibit elastic as well as viscous characteristics and the relation between stress and strain is nonlinear. We here demonstrate for a model system of such complex fluids that at high shear rates turbulence is not simply modified as previously believed but it is suppressed and replaced by a new type of disordered motion, elasto-inertial turbulence (EIT). EIT is found to occur at much lower Reynolds numbers than Newtonian turbulence and the dynamical properties differ significantly. In particular the drag is strongly reduced and the observed friction scaling resolves a longstanding puzzle in non-Newtonian fluid mechanics regarding the nature of the so-called maximum drag reduction asymptote. Theoretical considerations imply that EIT will arise in complex fluids if the extensional viscosity is sufficiently large.

International audience; The law of the wall and the log law rule the near-wall mean velocity profile of three-dimensional turbulent flows. These well-known laws, which are validated by legions of experiments and simulations, may be universal. Here, using a soap-film channel, we report the first experimental test of these laws in quasi-two-dimensional turbulent channel flows under two disparate turbulent spectra. We find that despite the differences with three-dimensional flows, the laws prevail, albeit with notable distinctions: the two parameters of the log law are markedly distinct from their three-dimensional counterpart; further, one parameter (the von Kármán constant) is independent of the spectrum whereas the other (the offset of the log law) depends on the spectrum. Our results suggest that the classical theory of scaling in wall-bounded turbulence is incomplete wherein a key missing element is the link with the turbulent spectrum.

In shear flows, turbulence first occurs in the form of localized structures (puffs/spots) surrounded by laminar fluid. We here investigate such spatially intermittent flows in a pipe experiment showing that turbulent puffs have a well-defined interaction distance, which sets their minimum spacing as well as the maximum observable turbulent fraction. Two methodologies are employed. Starting from a laminar flow, puffs are first created by locally injecting a jet of fluid through the pipe wall. When the perturbation is applied periodically at low frequencies, as expected, a regular sequence of puffs is observed where the puff spacing is given by the ratio of the mean flow speed to the perturbation frequency. At large frequencies however puffs are found to interact and annihilate each other. Varying the perturbation frequency, an interaction distance is determined which sets the highest possible turbulence fraction. This enables us to establish an upper bound for the friction factor in the transitional regime, which provides a well-defined link between the Blasius and the Hagen-Poiseuille friction laws. In the second set of experiments, the Reynolds number is reduced suddenly from fully turbulent to the intermittent regime. The resulting flow reorganizes itself to a sequence of constant size puffs which, unlike in Couette and Taylor–Couette flow are randomly spaced. The minimum distance between the turbulent patches is identical to the puff interaction length. The puff interaction length is found to be in agreement with the wavelength of regular stripe and spiral patterns in plane Couette and Taylor–Couette flow.

The process of transition from laminar to turbulent regime in shear driven flows is still an unresolved issue. Localized turbulent regions or spots occur in pipe flow for Reynolds numbers around 2000. Typically in this regime an intermittent change between laminar and turbulent flow is observed (Wygnanski). Indeed, even if a large section of the laminar flow is uniformly perturbed localized turbulent spots emerge rather than an extended region of turbulence. A good understanding of this localization process is crucial for the comprehension of the transition to turbulence. We investigate the interaction of such turbulent spots in pipe flow for Reynolds numbers from 1900 to 2500. Turbulence is created locally by injecting a jet of water through a small hole in the pipe wall. For small perturbation frequencies the spacing of the turbulent spots downstream is inversely proportional to the frequency. It is observed that for distances less than approximately 20 pipe diameters turbulent spots start to interact and annihilate each other. The interaction distance is measured as a function of Reynolds number. We are also studying the effect of amplitude of the perturbations on the mutual interaction of the puffs. This investigation is closely related to spatially turbulent laminar periodic patterns which were earlier observed in other shear driven flows like Taylor-Couette or plane Couette (Prigent et al), (D. Barkley and L. Tuckerman).