arXiv subjects

Purbarun Dhar

Publications and source records attributed to Purbarun Dhar.

At least 19 recordsLinked to original sources

arXiv2026

Elasto-hydrodynamics of droplet-pool-interactions

In Newtonian fluids, impact of a droplet on a liquid pool births a cavity, crown, capillary waves, and Worthington jet. The corresponding hydrodynamic events for elastic or Boger fluids, however, remain an uncharted domain of comprehension and exploration. We thoroughly investigate, via experiments, theory, and simulations, how elastic energy storage, fluid relaxation, and competitive inertio elasto capillarity govern the spatio temporal evolution of the cavity, the crown, and the ensuing Worthington jet in polymeric elastic fluids. The events are systematically explored over a wide range of impact Weber and Deborah numbers, considering varied Newtonian and elastic fluid droplet pool combinations, and revealing new, and distinct morphological regimes compared to Newtonian counterparts. We illustrate that these new findings are purely driven by fluid elasticity, and not by viscosity or interfacial tension. We derive a theory for cavity radius evolution, using energy conservation within potential-flow framework. We show that 30-40 % of the droplets kinetic impact energy may be stored as elastic energy by the stretching polymer chains during cavity expansion. Appealing to the FENE P model, we derive a theory for the temporal evolution of the radius of the elongated Worthington jet. We show that in elasto capillary regime, competitive elastic and capillary stresses lead to exponential decay of the jet radius. The role of elastic stresses and the local velocity field in governing cavity evolution, morphology, and jet formation are further elucidated through computer simulations. Our findings significantly advance the uncharted paradigm of interplay between inertia, capillarity, and elasticity in droplet-pool interaction elastohydrodynamics.

physics.flu-dyn↗

arXiv2026

Interfacial-Thermo-Fluid-Adhesion Dynamics of Evaporating Capillary Bridges between Curved Surfaces

We probe the evaporation mechanism, and the associated adhesion dynamics of liquid capillary bridges connecting two curved, solid substrates. The coupled thermo fluid species transport and the transient evolution of capillary adhesion during evaporation are systematically examined. An accurate, fully coupled transient numerical framework is developed, wherein the equilibrium capillary profiles are first determined from level set method. Next, the evaporation is simulated via Arbitrary Lagrangian Eulerian ALE framework to accurately track the moving liquid vapor interface. The combined influence of substrate curvature, surface wettability, and solid thermal conductivity on evaporation and capillary adhesion character is comprehensively analysed. The simulation methodology is robustly validated against published literature for capillary profiles, evaporation rates, and capillary forces, demonstrating good agreement. Our results reveal that the evaporation characteristics of both hydrophilic and superhydrophobic SH liquid bridges are strongly governed by substrate curvature and thermal conductivity, and increasing values pose favourable condition for augmented interfacial mass transfer rate. The innately non uniform vapour flux generates spatially varying evaporative cooling, producing surface tension gradients that drive internal thermo capillary circulation. A non dimensional scaling analysis shows that Marangoni flow dominates buoyancy induced flow throughout. Also, increasing substrate curvature decreases the overall capillary force, owing to the reduced curvatures of the liquid bridge, while the temporal evolution of the adhesion force is strongly influenced by both substrate curvature and wettability.

physics.flu-dyn↗

arXiv2026

The magneto-Leidenfrost effect in ferrofluid droplets

The dynamic Leidenfrost effect LFE and behaviour of impinging colloidal droplets is strongly influenced by the impact and spreading paradigms. LFE actuated rebound and levitation occurs due to enhanced spreading and near-frictionless recoil over the intervening vapour layer, providing opportunities for external field stimulus aided modulation and control of impact outcomes, and the resulting boiling-LFE behaviour. Magnetic field modulated LFE onset, dynamics and boiling transport of stable aqueous nano Fe2O3 based ferrofluid droplets was studied using high speed imaging. The interplay between magnetic, inertia, and viscocapillary forces on droplet spreading, magneto LFE-driven rebound conditions, residence time, and post-impact regimes was analysed using dimensionless parameters maximum spread factor, Weber number, and magnetic Bond number. We report a purely new phenomenon, namely magneto Leidenfrost effect MLFE, wherein magnetic field induces LFE aided onset of droplet rebound at substrate temperatures Ts below the zero-field dynamic Leidenfrost temperature LFT. The critical for the onset of MLFE decreases with increasing . Increasing the nanoparticle concentration permits the onset even at considerably lower . At elevated Ts , the residence time is noted as dependent. At much higher Ts, increasing promotes formation of radial filamentous structures, leading to complete droplet fragmentation. We also propose a theoretical framework that explains magnetic field driven spreading enhancement and rebound, and predicts of MLFE droplets in agreement with experiments. Our findings provide valuable insights into the novel realm of field dictated LFE, and hold significant implications towards the design of frictionless, rapid colloid droplet transport systems, and targeted droplet manipulation or activation for advanced thermal management.

physics.flu-dyn↗

arXiv2026

Evaporative thermo-fluidics and deposition patterns in surface-active droplets

We investigate the thermo solutal transport phenomena and deposition patterns during the evaporation of surfactant laden droplets experimentally and through theoretical scaling based analysis. Experiments were conducted using the sessile droplet configuration in the acrylic chamber for both hydrophilic and hydrophobic substrates. Infrared thermography and particle image velocimetry measurements were conducted during evaporation to illustrate the temperature and velocity distributions, respectively. Sodium dodecyl sulphate SDS surfactant molecules enhanced the evaporation rate with an increase in concentration for the hydrophobic surface. In contrast, the evaporation rate increased up to 0.5 CMC and then decreased for droplets on a hydrophilic substrate. The evaporation rates computed from the shadowgraphy imaging were explained using the average velocities obtained from the PIV analysis. It was found that advection within the droplet is strongly dependent on surfactant concentration and wettability. Further, the theoretically obtained Marangoni velocities were in close agreement with the experimental values. It was found that Marangoni solutal advection dominates other advection mechanisms, such as Marangoni thermal advection and buoyancy driven flow. However, surfactant crowding and viscous resistance with increasing surfactant concentration can dampen the increase in solutal advection. The surface tension and viscosity measurements were also conducted with variation in surfactant concentration to understand the suppression of advection by viscous forces. The computation of contact line velocities showed sudden fluctuations, illustrating stick slip behaviour during droplet drying, complementing microscopic visual observations.

physics.flu-dyn↗

arXiv2025

Ferro-hydrodynamics of droplet necking filaments

We explore the necking, filament thinning, and pinchoff dynamics of ferrofluid droplets within a magnetic field, via a simple and low-cost experimental method. In our studies, both the Ohnesorge number Oh and the Deborah number De are O1, a typically inaccessible regime with conventional extensional rheometers. Under magnetic forcing, the nanoparticles assemble into field aligned, chainlike structures, that generate a tunable magnetoelastic response, and markedly alter the extensional flow. Although behaving as Newtonian liquids in the absence of a magnetic field, the field induces extensional thickening, and the emergence of beads on a string BOAS structures in the ferrofluid filaments, a non-Newtonian signature. By combining controlled elongation with high speed imaging, we directly quantify the magnetic field-dependent extensional viscosity and relaxation time. Our findings underscore how magnetically induced microstructures govern filament stability and extensional dynamics in ferrofluids.

physics.flu-dyn↗

arXiv2025

The fluid dynamics of liquid mushrooms

Droplets that impact the surface of a deep liquid pool may form a vertical jet after the cavity formation event, provided they have sufficient impact energy. Depending on the associated time scales and the effect of the Rayleigh Plateau instability, this jet may either continue to rise, or may form satellite droplets via necking. Collision of these structures with a second incoming droplet, ejected from the same dispensing tip as the first droplet, may result in the formation of various lamellar patterns, depending on the impact conditions, giving rise to liquid mushroom and or umbrella structures. In this research, we experiment for the first time with hydrodynamics of such liquid mushrooms, and study the effect of droplet impact height, surface tension, and viscosity on the dynamics of such lamellar formations. We further explore the role of the orientation of incoming droplet impact, ie whether head on or offset collision with the rising jet or satellite droplet. We discuss the spatiotemporal evolution of the lamella diameters, and its susceptibility to surface tension, viscosity, and droplet impact height. We put forward a theoretical model based on energetics, to predict the maximum spread diameter of the lamellae, which yields accurate predictions with respect to our experiments. Our findings may help to provide important insights towards a fluid dynamic phenomenon observed often in nature and may be important in niche utilities as well.

physics.flu-dyn↗

arXiv2024

Droplet impact and splitting behaviour on superhydrophobic wedges

We report an extensive computational and experimental investigation of droplet impact and subsequent splitting hydrodynamics on superhydrophobic wedges. 2D and necessary 3D simulations using the volume of fluid method, backed with experimentations, have been performed to predict the droplet impact, spreading, split up, retraction against sliding, and daughter droplet lift off events from the SH wedge. In particular, we examine how the wedge angle , wedge asymmetry , Weber number and normalized Bond number influence the post-impact dynamics. We observe that for symmetric wedges, the maximum spread factor of the droplet decreases with an increase in wedge angle at a fixed We. At high wedge angles, the sharp steepness of the wedge causes less contact area for the droplet to spread. For the asymmetric wedges, it has been noted that beta max increases with an increase in the We due to the higher inertial forces of the droplet against sliding. Furthermore, the increases with an increase in Bo at a fixed We due to the dominance of the gravitational force over the capillary force of the droplet. It has been also found that at the same Bo, the increases with an increase in We due to the dominance of inertial forces over the capillary forces. The split volume of daughter droplets during the split up stage for different symmetric and asymmetric wedge angles has been discussed. In general, our 2D simulations agree well with the experiments for a major part of the droplet lifetime. Further, we have conducted a detailed 3D simulation based energy budget analysis to estimate the temporal evolution of the various energy components at different post impact hydrodynamic regimes.

physics.flu-dyn↗

arXiv2023

Offset coalescence behaviour of impacting low-surface tension droplet on high-surface-tension droplet

Impact of droplets of varying surface tension and subsequent spreading over a solid surface are inherent features in printing applications. In this regard, an experimental study of impact of two drops of varied surface tension is carried out where the sessile water droplet on a hydrophilic substrate is impacted upon by another droplet of sequentially lowered surface tension. The impacts are studied for different impact velocities and offsets with respect to the mid-plane of the two colliding droplets. Sodium Dodecyl Sulfate (SDS) is used to alter the surface tension without altering the viscosity, to study the various parameters affecting the spreading length viz. the surface tension, offset between the drops, and impact velocity. The spreading lengths are obtained through image processing of the captured footage of the impact dynamics by a high-speed camera. It is found out that upon lowering the surface tension, the maximum and equilibrium spreading length varies to a significant extent also the nature of the spreading dynamics changes. Both side and top-view imaging are performed to understand the overall hydrodynamics. There is also a substantial change in drawback when dissimilarity is surface tension between the impacting droplets exist. Finally, a fit model is obtained to predict the maximum spread length of the various cases.

physics.flu-dyn↗

arXiv2023

Deformation transients of confined droplets within interacting electric and magnetic field environment

A theoretical exploration and an analytical model for the electro-magneto-hydrodynamics (EMHD) of leaky dielectric liquid droplets, suspended in an immiscible confined fluid domain has been presented. The analytical solution for the system, under small deformation approximation, in creeping flow regime, has been put forward. Study of the droplet deformation suggests that its temporal evolution is exponential, and dependents on the electric and magnetic field interaction. Further, the direction of the applied magnetic field with respect to the electric field decides whether the contribution of magnetic forces opposes or aids the interfacial net electrical force due to the electric field. Validation of the proposed model at the asymptotic limits of vanishing magnetic field show that the model accurately reduces to the case of transient electrohydrodynamic model. We also propose a magnetic discriminating function to quantify the steady-state droplet deformation in the presence of interacting electric and magnetic fields. The change of droplets from spherical shape to prolate, and oblate spheroids, correspond to magnetic discriminating function >0 and <0 regimes, respectively. It is shown that with the aid of low magnitude magnetic field, a substantial augmentation in the deformation parameter, and the associated EMHD circulation within and around the droplet is achieved. The analysis also reveals the deformation lag and specific critical parameters that aid or suppressed this lag behaviour; discussed in terms of relevant non-dimensional parameters.

physics.flu-dyn↗

arXiv2021

Delay of Leidenfrost point during drop impact of surfactant solutions

In this article, a novel method of increasing the dynamic Leidenfrost temperature is proposed through the addition of both anionic (SDS) and cationic (CTAB) surfactants to water droplets. We focus on understanding the hydrodynamics and thermal aspects of droplet impact Leidenfrost behaviour of surfactant solutions, and aim to delay the onset of the Leidenfrost regime. The effects of Weber number (We), Ohnesorge number (Oh) and surfactant concentration on dynamic Leidenfrost temperature were experimentally studied in details, covering a wide gamut of governing parameters. At a fixed impact velocity, increased with the increase of surfactant concentration. decreased with increase of impact velocity for all solutions of surfactant droplets at a fixed surfactant concentration. We proposed a scaling relationship for in terms of We and Oh. At temperatures (~ 400oC) considerably higher than , droplets exhibit trampoline like dynamics or central jet formation, associated with fragmentation, depending upon the impact velocity. Finally, a regime map of the different boiling regimes such as transition boiling, Leidenfrost effect, trampolining and explosive behaviour is presented as function of impact We and substrate temperature (Ts). The findings may hold strong implications in thermal management systems operating at high temperatures

physics.flu-dyn↗

arXiv2020

Magnetoelastic effect in impact dynamics of nonNewtonian ferrofluid droplets

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 (SH) surfaces in 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 manifestation of 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.

physics.flu-dyn↗

arXiv2020

Thermal Response of Dielectric Nanoparticle Infused Tissue Phantoms during Microwave Assisted Hyperthermia

Hyperthermia has been in use for many years; as a potential alternative modality for cancer treatment. In this paper, an experimental investigation of microwave assisted thermal heating (MWATH) of tissue phantom using a domestic microwave oven has been reported. Computer simulations using finite element method based tools was also carried out to support the experimental observations and probe insight on the thermal transport aspects deep within the tissue phantom. A good agreement between predicted and measured temperature were achieved. Furthermore, experiments were conducted to investigate the efficacy of dielectric nanoparticles viz. alumina (Al2O3) and titanium oxide (TiO2) during the MWATH of nanoparticle infused tumor phantoms. A deep seated tumor injected with nanoparticle solution was specifically mimicked in the experiments. Interesting results were obtained in terms of spatiotemporal thermal history of the nanoparticle infused tissue phantoms. An elevation in the temperature distribution was achieved in the vicinity of the targeted zone due to the presence of nanoparticles, and the spatial distribution of temperature was grossly morphed. We conclusively show, using experiments and simulations that unlike other nanoparticle mediated hyperthermia techniques, direct injection of the nanoparticles within the tumor leads to enhanced heat generation in the neighb oring healthy tissues. The inhomogeneity of the hyperthermia event is evident from the lo cal occurrence of hot spots and cold spots respectively. The present findings may have far reaching implications as a framework in predicting temperature distributions during MWA.

physics.med-ph↗

arXiv2020

Electrohydrodynamics of dielectric droplet collision with variant wettability surfaces

In this article, we report 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 is covered to explore the various regimes of droplet impact EHD. We show that for a fixed We 60, droplet rebound on SH surface is suppressed with increase of electric field intensity. At high Cae, instead of the usual uniform radial contraction, the droplets retract faster in orthogonal direction to the electric field and spread along the direction of the electric field. 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 4.0, EHD instability causes fingering pattern via evolution of spire at the rim. Further, the spreading EHD on both hydrophilic and SH surfaces are 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.

physics.flu-dyn↗

arXiv2020

Control and modulation of droplet vaporization rates via competing ferro- and electro-hydrodynamics

Modification and control over the vaporization kinetics of microfluidic droplets may have strong utilitarian implications in several scientific and technological applications. The article reports the control over the vaporization kinetics of pendent droplets under the influence of competing internal electrohydrodynamic and ferrohydrodynamic advection. Experimental and theoretical studies are performed and the morphing of vaporization kinetics of electrically conducting and paramagnetic fluid droplets using orthogonal electric and magnetic stimuli is established. Analysis of the observations reveals that the electric field has a domineering influence compared to the magnetic field. While the magnetic field is noted to aid the vaporization rates, the electric field is observed to decelerate the same. Neither the vapour diffusion dominated kinetics nor the field induced modified surface tension can explain the observed vaporization behaviours. Velocimetry within the droplet shows largely modified internal ferro and electrohydrodynamic advection, which is noted to be the crux of the mechanism towards modified vaporization rates. A mathematical treatment is proposed and takes into account the roles played by the governing Hartmann, electrohydrodynamic, interaction, the thermal and solutal Marangoni, and the electro and magneto Prandtl and Schmidt numbers. It is observed that the morphing of the thermal and solutal Marangoni numbers by the electromagnetic interaction number plays the dominant role towards morphing the advection dynamics. The model is able to predict the internal advection velocities accurately. The findings may hold significant promise towards smart control and tuning of vaporization kinetics in microhydrodynamics transport paradigms.

physics.flu-dyn↗

arXiv2020

Surface declination governed asymmetric sessile droplet evaporation

The article reports droplet evaporation kinetics on inclined substrates. Comprehensive experimental and theoretical analyses of the droplet evaporation behaviour for different substrate declination, wettability and temperatures have been presented. Sessile droplets with substrate declination exhibit distorted shape and evaporate at different rates compared to droplets on the same horizontal substrate and is characterized by more often changes in regimes of evaporation. The slip stick and jump stick modes are prominent during evaporation. For droplets on inclined substrates, the evaporative flux is also asymmetric and governed by the initial contact angle dissimilarity. Due to smaller contact angle at the rear contact line, it is the zone of a higher evaporative flux. Particle image velocimetry shows the increased internal circulation velocity within the inclined droplets. Asymmetry in the evaporative flux leads to higher temperature gradients, which ultimately enhances the thermal Marangoni circulation near the rear of the droplet where the evaporative flux is highest. A model is adopted to predict the thermal Marangoni advection velocity, and good match is obtained. The declination angle and imposed thermal conditions interplay and lead to morphed evaporation kinetics than droplets on horizontal heated surfaces. Even weak movements of the TL alter the evaporation dynamics significantly, by changing the shape of the droplet from ideally elliptical to almost spherical cap, which ultimately reduces the evaporative flux. The life time of the droplet is modelled by modifying available models for non-heated substrate, to account for the shape asymmetry. The present findings may find strong implications towards microscale thermo-hydrodynamics.

physics.flu-dyn↗

arXiv2020

Advection kinetics induced self assembly of colloidal nanoflakes into microscale floral structures

This article explores the governing role of the internal hydrodynamics and advective transport within sessile colloidal droplets on the self assembly of nanostructures to form floral patterns. Water acetone binary fluid and Bi2O3 nanoflakes based complex fluids are experimented with. Microliter sessile droplets are allowed to vaporize and the dry out patterns are examined using scanning electron microscopy. The presence of distributed self assembled rose like structures is observed. The population density, structure and shape of the floral structures are noted to be dependent on the binary fluid composition and nanomaterial concentration. Detailed microscopic particle image velocimetry analysis is undertaken to qualitatively and quantitatively describe the solutal Marangoni advection within the evaporating droplets. It has been shown that the kinetics, regime and location of the internal advection are responsible factors towards the hydrodynamics influenced clustering, aggregation and self-assembly of the nanoflakes. In addition, the size of the nanostructures and the complex fluids.

cond-mat.soft↗

arXiv2020

Suppressed Leidenfrost phenomenon during impact of elastic fluid droplets

The present article highlights the role of non-Newtonian (elastic) effects on the droplet impact phenomenology at temperatures considerably higher than the boiling point, especially at or above the Leidenfrost regime. The Leidenfrost point (LFP) was found to decrease with increase in the impact Weber number (based on velocity just before the impact) for fixed polymer (Polyacrylamide, PAAM) concentrations. Water droplets fragmented at very low Weber numbers (~22), whereas the polymer droplets resisted fragmentation at much higher Weber numbers (~155). We also varied the polymer concentration and observed that till 1000 ppm, the LFP was higher compared to water. This signifies that the effect can be delayed by the use of elastic fluids. We have showed the possible role of elastic effects (manifested by the formation of long lasting filaments) during retraction in the improvement of the LFP. However for 1500 ppm, LFP was lower than water, but with similar residence time during initial impact. In addition, we studied the role of Weber number and viscoelastic effects on the rebound behaviour at 405o C. We observed that the critical Weber number till which the droplet resisted fragmentation at 405o 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 rebound kinetic energy to rotational energy during the hovering phase.

physics.flu-dyn↗

arXiv2020

Competing advection decelerates droplet evaporation on heated surfaces

In this article we report the atypical and anomalous evaporation kinetics of saline sessile droplets on surfaces with elevated temperatures. In a previous we showed that saline sessile droplets evaporate faster compared to water droplets when the substrates are not heated. In the present study we discover that in the case of heated surfaces, the saline droplets evaporate slower than the water counterpart, thereby posing a counter-intuitive phenomenon. The reduction in the evaporation rates is directly dependent on the salt concentration and the surface wettability. Natural convection around the droplet and thermal modulation of surface tension is found to be inadequate to explain the mechanisms. Flow visualisations using particle image velocimetry PIV reveals that the morphed advection within the saline droplets is a probable reason behind the arrested evaporation. Infrared thermography is employed to map the thermal state of the droplets. A thermosolutal Marangoni based scaling analysis is put forward. It is observed that the Marangoni and internal advection borne of thermal and solutal gradients are competitive, thereby leading to the overall decay of internal circulation velocity, which reduces the evaporation rates. The theoretically obtained advection velocities conform to the experimental results. This study sheds rich insight on a novel yet anomalous species transport behaviour in saline droplets.

physics.flu-dyn↗