Browsing by Author "Harikrishnan, A.R."

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Acknowledgment to the Reviewers of Fluids in 2022
(MDPI, 2023) Harikrishnan, A.R.
High-quality academic publishing is built on rigorous peer review. Fluids was able to uphold its high standards for published papers due to the outstanding efforts of our reviewers. Thanks to the efforts of our reviewers in 2022, the median time to first decision was 42 days and the median time to publication was 17 days. Regardless of whether the articles they examined were ultimately published, the editors would like to express their appreciation and thank the following reviewers for the time and dedication that they have shown Fluids:
Adverse impact of macro-textured superhydrophobicity on contact time reduction at high Weber numbers
(Elsevier, 2022-11) Harikrishnan, A.R.
Macro textured superhydrophobic surfaces are reported to be highly effective in contact time reduction and thus a potential candidate for anti-icing applications. However, we find that the macro textured superhydrophobic cylindrical surfaces have higher contact time compared to its counterpart without any macro texturing at high impact Weber numbers (We)for the various range of curvature ratios of the cylinder and aspect ratios of the macro texturing. The asymmetric spreading is aided by the preferential jetting and flow redirection results in a stable lamella in the azimuthal direction resulting in the contact time enhancement for intermediate We number regimes. The retraction velocity is also adversely influenced by the presence of ribs in these We number regimes thus resulting in enhanced residence time. However, the dynamics at the high We regimes are governed by the hole nucleation and film rupturing. The presence of ribs is found to reduce the nucleation inception and nucleation density resulting in higher contact time compared to the non-ribbed superhydrophobic surfaces. An outcome based map was developed based on the systematic experimental observations over a wide spectrum of parametric variation. Detailed analysis is presented to explain the counter-intuitive observations.
Amplifying thermal conduction calibre of dielectric nanocolloids employing induced electrophoresis
(Elsevier, 2019-09) Harikrishnan, A.R.
Electrophoresis has been shown as a novel methodology to enhance heat conduction capabilities of nanocolloidal dispersions. A thoroughly designed experimental system has been envisaged to solely probe heat conduction across nanofluids by specifically eliminating the buoyancy driven convective component. Electric field is applied across the test specimen in order to induce electrophoresis in conjunction with the existing thermal gradient. It is observed that the electrophoretic drift of the nanoparticles acts as an additional thermal transport drift mechanism over and above the already existent Brownian diffusion and thermophoresis dominated thermal conduction. A scaling analysis based on the thermophoretic and electrophoretic velocities from classical Huckel-Smoluchowski formalism is able to mathematically predict the thermal performance enhancement due to electrophoresis. It is also inferred that the dielectric characteristics of the particle material is the major determining component of the electrophoretic amplification of heat transfer. Influence of surfactants has also been probed into and it is observed that enhancing the stability via interfacial charge modulation can in fact enhance the electrophoretic drift, thereby enhancing heat transfer calibre. Also, surfactants ensure colloidal stability as well as chemical gradient induced recirculation, thus ensuring colloidal phase equilibrium and low hysteresis in spite of the directional drift in presence of electric field forcing. The findings may have potential implications in enhanced and tunable thermal management of micro-nanoscale devices and in thermo-bioanalysis within lab-on-a-chip devices.
Bouncing–pinning criterion for a drop impacting on a superhydrophobic surface
(AIP, 2025-03) Harikrishnan, A.R.
Drop impact on non-wetting surfaces has garnered significant interest due to its potential applications in water repellency, drag reduction, self-cleaning, and anti-icing. However, there are instances where a droplet fails to rebound from a superhydrophobic surface. It has been reported that the combined effect of gravito-capillary length and visco-capillary length determines the pinning–bouncing criteria. While the fluid properties, such as viscosity and weight, are often considered primary factors influencing droplet rebound, this study highlights the crucial role of surface characteristics, particularly the contact angle hysteresis, in determining post-impact behavior. We propose a modified criterion that predicts droplet bouncing and pinning on superhydrophobic surfaces by integrating both fluid properties and the contact angle hysteresis of the surface. The findings emphasize the importance of surface morphology in droplet dynamics, providing a more comprehensive understanding of droplet behavior on non-wetting surfaces.
The competing effects of high zeta potential and finite ionic size on the thermal behaviour of pseudoplastic flows through confined spaces with hydrophobic surfaces
(Begell House, 2021-12) Harikrishnan, A.R.
We present Galerkin Finite Element computations of the temperature profile and Nusselt Number associated with the mixed electroosmotic and pressure driven flow of a power law non - Newtonian fluid through a micro/nano channel with velocity slip at the wall. The geometry considered is a parallel plate microchannel and the mathematical model used incorporates the effects of high zeta potential, steric effect, viscous dissipation and Joule heating. Based on the temperature profile and Nusselt Number, we determine the qualitative effect of various hydrodynamic and thermal parameters on the heat transfer performance of the flow and in particular, the influence of the zeta potential and the steric factor. We observe that the zeta potential and steric factor have competing effects that are most prominent near the wall of the microchannel. It is also observed that velocity slip at the wall can enhance the overall convective heat transfer in the fluid. The results of this study offer insight into the thermal design of micro/nano-fluidic systems.
Competitive Electrohydrodynamic and Electrosolutal Advection Arrests Evaporation Kinetics of Droplets
(ACS, 2020-07) Harikrishnan, A.R.
This article reports the hitherto unreported phenomenon of arrested evaporation dynamics in pendant droplets because of electric field stimulus. The evaporation kinetics of pendant droplets of electrically conducting saline solutions in the presence of a transverse, alternating electric field is investigated experimentally. While the increase of field strength reduces the evaporation rate, increment in field frequency has the opposite effect. The same has been explained on the solvation kinetics of ions in polar water. Theoretical analysis reveals that change in surface tension and the diffusion-driven evaporation model cannot predict the decelerated evaporation. With the aid of particle image velocimetry, suppression of internal circulation velocity within the droplet is observed under electric field stimulus, which directly affects the evaporation rate. A mathematical scaling model is proposed to quantify the effects of electrohydrodynamic circulation and electrothermal and electrosolutal advection on the evaporation kinetics. The analysis encompasses major governing parameters, namely, the thermal and solutal Marangoni numbers, the electrohydrodynamic number, the electro-Prandtl and electro-Schmidt numbers, and their respective contributions. It has been shown that the electrothermal Marangoni effect is suppressed by the electric field, leading to deteriorated evaporation rates. Additionally, the electrosolutal Marangoni effect further suppresses the internal advection, further reducing the evaporation rate by a larger proportion. Stability analysis reveals that the electric body force retards the stable internal advection. The stability mapping also illustrates that if the field strength is high enough for the electrosolutal advection to overshadow the solutal Marangoni effect completely, it can lead to improvement in evaporation rates.
Comprehensive analysis of factors leveraging bio-inspired conical designs for efficient fog harvesting
(ACS, 2025-03) Harikrishnan, A.R.
Environmental fog accumulation is a sustainable source of clean water, particularly in humid and arid regions. Many organisms have evolved passive microstructures to aid in fog droplet nucleation, accumulation, and transport. Researchers have developed various fog collectors, utilizing strategies like wire mesh, conical geometries, micronano texturing, and wettability modifications to enhance water collection. Despite conical geometry being the desirable configuration for accumulating water from atmospheric fog and condensation, a uniform framework must be established to estimate, quantify, and evaluate the water collection efficiency (WCE) of a conical geometry with distinct wettabilities at varying flow rates. In the present study, we determine the WCE of multiple cones ranging from 5° to 45° with four distinct wettabilities, namely, hydrophilic (HPH), mild hydrophobic (HPB), highly hydrophobic (HHPB), and superhydrophobic (SHPB) at three different fog flow velocities. The mechanism of fog deposition and water collection for different cases is thoroughly investigated with the help of onset time. Theoretical analysis of the aerodynamic and deposition efficiencies is conducted for the conical geometry pertaining to the actual fog conditions and shows a similar variation to that observed under experimental test conditions. The flow patterns over the conical substrate are visualized using high speed imaging. The WCE of smaller cone angles (5° and 10°) is observed to be higher than the larger cone angles for all the ranges of wettabilities. SHPB wettable cones have a shorter onset time due to minimal contact angle hysteresis and, hence, have the highest water collection rate among all wettabilities. The onset time of fog collection is largely influenced by the fog velocity and the wettability of the surface material. The current study presents the basis for developing an efficient fog collector employing conical arrays.
Confined Evaporation-Mediated Enhanced Residence Time of Levitated Water Drops over Deep Oil Pools
(ACS, 2021-11) Harikrishnan, A.R.
We observe the impact of bouncing and floating of water drops on a pool of immiscible volatile oil pools at low Weber numbers. The residence time of the impacting drop ranges from a few milliseconds to a few seconds before it sinks into the lighter oil phase. It is hypothesized that the confined evaporation from the volatile oil pool replenishes the thin film draining and results in prolonged floating and delayed sinking of drops into the oil pool. Water drops are released from a low height to impact on volatile hydrocarbon oil deep pools of various volatilities. The floating dynamics and residence times are captured using high-speed imaging. A theoretical model for the residence time has been developed to evaluate the hypothesis. The drop residence time is found to be directly proportional to the volatility of the oil pool in accordance with the hypothesis. The mathematical model incorporating the coupled confined evaporation and film draining dynamics is found to be in well agreement with the experimentally observed residence time. The bouncing–sinking regime map has been developed based on the experimental data. Supporting Information
Correlating contact line capillarity and dynamic contact angle hysteresis in surfactant-nanoparticle based complex fluids
(AIP, 2018-04) Harikrishnan, A.R.
Dynamic wettability and contact angle hysteresis can be correlated to shed insight onto any solid-liquid interaction. Complex fluids are capable of altering the expected hysteresis and dynamic wetting behavior due to interfacial interactions. We report the effect of capillary number on the dynamic advancing and receding contact angles of surfactant-based nanocolloidal solutions on hydrophilic, near hydrophobic, and superhydrophobic surfaces by performing forced wetting and de-wetting experiments by employing the embedded needle method. A segregated study is performed to infer the contributing effects of the constituents and effects of particle morphology. The static contact angle hysteresis is found to be a function of particle and surfactant concentrations and greatly depends on the nature of the morphology of the particles. An order of estimate of line energy and a dynamic flow parameter called spreading factor and the transient variations of these parameters are explored which sheds light on the dynamics of contact line movement and response to perturbation of three-phase contact. The Cox-Voinov-Tanner law was found to hold for hydrophilic and a weak dependency on superhydrophobic surfaces with capillary number, and even for the complex fluids, with a varying degree of dependency for different fluids.
Droplet Collision and Nucleation Hydrodynamics on Superhydrophobic Cylindrical Surfaces
(Springer, 2023-04) Harikrishnan, A.R.
Water drop impact onto hydrophobic cylindrical surfaces with four different curvature ratio were experimentally investigated. At lower Weber number impact droplet asymmetrically bounces from all curvature cases with increase in Weber number droplet starts splitting/splashing. On higher striking velocity, the stretched lamella shatters into several small droplets. The high velocity impact droplets ruptures rapidly by formation of nucleation holes on the film as a result of small scale roughness on contact surface. The small scale roughness on test surface causes hole nucleation/film rupturing and reduces the contact time. As the impinging velocity reaches the maximum of our experimental study, the contact time was observed to be even less that the capillary time (tc < τ0). Due to complete shattering of water drop, the retraction time is absent in these cases and results in reduced contact time. It was found that the number of nucleations is in proportion with velocity of impact and contact area on striking.
Effect of Interaction of Nanoparticles and Surfactants on the Spreading Dynamics of Sessile Droplets
(ACS, 2017) Harikrishnan, A.R.
While a body of literature on the spreading dynamics of surfactants and a few studies on the spreading dynamics of nanocolloids exist, to the best of the authors’ knowledge, there are no reports on the effect of presence of surfactants on the spreading dynamics of nanocolloidal suspensions. For the first time the present study reports an extensive experimental and theoretical study on the effect of surfactant impregnated nanocolloidal complex fluids in modulating the spreading dynamics. A segregation analysis of the effect of surfactants alone, nanoparticle alone, and the combined effect of nanoparticle and surfactants in altering the spreading dynamics have been studied in detail. The spreading dynamics of nanocolloidal solutions alone and of the surfactant impregnated nanocolloidal solutions are found to be grossly different, and particle morphology is found to play a predominant role. For the first time the present study experimentally proves that the classical Tanner’s law is disobeyed by the complex fluids in the case of particle alone and combined particle and surfactant case. We also discuss the role of imbibitions across the particle wedge in the precursor film in tuning spreading dynamics. We propose an analytical model to predict the nature of dependency of contact radius on time for the complex colloids. A detailed theoretical examination of the governing factors, the interacting forces at the three phase contact line, and the effects of interplay of surfactants and the nanoparticles at the precursor film in modulating the spreading dynamics has been presented for such complex colloids.
Effects of interplay of nanoparticles, surfactants and base fluid on the surface tension of nanocolloids
(Springer, 2017-05) Harikrishnan, A.R.
A systematically designed study has been conducted to understand and demarcate the degree of contribution by the constituting elements to the surface tension of nanocolloids. The effects of elements such as surfactants, particles and the combined effects of these on the surface tension of these complex fluids are studied employing the pendant drop shape analysis method by fitting the Young-Laplace equation. Only the particle has shown an increase in the surface tension with particle concentration in a polar medium like DI water, whereas only a marginal effect of particles on surface tension in weakly polar mediums like glycerol and ethylene glycol has been demonstrated. Such behaviour has been attributed to the enhanced desorption of particles to the interface and a theory has been presented to quantify this. The combined particle and surfactant effect on the surface tension of a complex nanofluid system showed a decreasing behaviour with respect to the particle and surfactant concentration with a considerably feeble effect of particle concentration. This combined colloidal system recorded a surface tension value below the surface tension of an aqueous surfactant system at the same concentration, which is a counterintuitive observation as only the particle results in an increase in the surface tension and only the surfactant results in a decrease in the surface tension. The possible physical mechanism behind such an anomaly happening at the complex fluid air interface has been explained. Detailed analyses based on thermodynamic, mechanical and chemical equilibrium of the constituents and their adsorption-desorption characteristics as extracted from the Gibbs adsorption analysis have been provided. The present paper conclusively explains several physical phenomena observed, yet hitherto unexplained, in the case of the surface tension of such complex fluids by segregating the individual contributions of each component of the colloidal system.
Electrohydrodynamic fibrillation governed enhanced thermal transport in dielectric colloids under a field stimulus
(RSC, 2018-05) Harikrishnan, A.R.
Electrorheological (ER) fluids are known to exhibit enhanced viscous effects under an electric field stimulus. The present article reports the hitherto unreported phenomenon of greatly enhanced thermal conductivity in such electro-active colloidal dispersions in the presence of an externally applied electric field. Typical ER fluids are synthesized employing dielectric fluids and nanoparticles and experiments are performed employing an in-house designed setup. Greatly augmented thermal conductivity under a field's influence was observed. Enhanced thermal conduction along the fibril structures under the field effect is theorized as the crux of the mechanism. The formation of fibril structures has also been experimentally verified employing microscopy. Based on classical models for ER fluids, a mathematical formalism has been developed to predict the propensity of chain formation and statistically feasible chain dynamics at given Mason numbers. Further, a thermal resistance network model is employed to computationally predict the enhanced thermal conduction across the fibrillary colloid microstructure. Good agreement between the mathematical model and the experimental observations is achieved. The domineering role of thermal conductivity over relative permittivity has been shown by proposing a modified Hashin–Shtrikman (HS) formalism. The findings have implications towards better physical understanding and design of ER fluids from both ‘smart’ viscoelastic as well as thermally active materials points of view.
Electromagnetic field orientation and characteristics governed hydrodynamics within pendent droplets
(APS, 2018-12) Harikrishnan, A.R.
This article reports the dominant governing role played by the direction of electric and magnetic fields on the internal advection pattern and strength within salt solution pendent droplets. The literature shows that solutal advection drives circulation cells within salt based droplets, even in the absence of any external field. An experimental study is performed, where electric and magnetic fields are applied across pendent droplets of salt solutions and their internal flow dynamics is observed. Flow visualization and velocimetry (two-dimensional) reveals that the direction of the applied field governs the enhancement or reduction in circulation velocity and the directionality of circulation inside the droplet. Further, it is noted that while magnetic fields augment the circulation velocity (with respect to the solutal advection already present in salt solution droplets at zero field) the electric field leads to deterioration of the same. The concepts of electro- and magnetohydrodynamics of droplets are appealed to and a Stokesian stream function based mathematical model to deduce the field mediated velocities has been proposed. The roles of the governing Hartmann, Stuart, Reynolds, and Masuda numbers are revealed by the proposed model. The theoretical predictions are observed to be in agreement with the experimentally determined averaged spatiotemporal circulation velocities. The present findings may have strong implications in microscale and interfacial electro- and/or magnetohydrodynamics.
Electromagnetic field orientation and dynamics governs advection characteristics within pendent droplets
(ARXIV, 2018-07) Harikrishnan, A.R.
The article reports the domineering governing role played by the direction of electric and magnetic fields on the internal advection pattern and strength within salt solution pendant droplets. Literature shows that solutal advection drives circulation cells within salt based droplets. Flow visualization and velocimetry reveals that the direction of the applied field governs the enhancement/reduction in circulation velocity and the directionality of circulation inside the droplet. Further, it is noted that while magnetic fields augment the circulation velocity, the electric field leads to deterioration of the same. The concepts of electro andmagnetohydrodynamics are appealed to and a Stokesian stream function based mathematical model to deduce the field mediated velocities has been proposed. The model is found to reveal the roles of and degree of dependence on the governing Hartmann, Stuart, Reynolds and Masuda numbers. The theoretical predictions are observed to be in good agreement with experimental average spatio-temporal velocities. The present findings may have strong implications in microscale electro and/or magnetohydrodynamics.
Evaporation kinetics of laser modulated pendant nanocolloidal droplet
(Begell House, 2023) Harikrishnan, A.R.
While a body of literature is there on the sessile evaporation of droplets, literatures dealing with the evaporation characteristics of the complex nanocolloidal systems are scarce. While a few literatures deals with the evaporation kinetics of such colloids the effect of the external optical irradiation in modulating the evaporation kinetics are not talked in literature. The present study analyses the effect of laser as an external optical source in modulating the evaporation characteristics of the hanging nanocolloidal droplets which are free from surface effects so as to capture the physics behind the interfacial mass transport. The current study analyses the effect of the power of laser, nature and concentration of the particle on evaporation rate of such complex colloidal systems. Evidence of internal circulation was observed with PIV technique in colloidal systems together with volumetric heat generation which can be attributed to be the causes behind the enhanced evaporation rate. Theoretical analysis of the evaporation rate with the classical mass transfer model for droplets falls short in predicting the evaporation rate in colloidal systems. Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found to induce the circulation in such systems.
Ferro-advection aided evaporation kinetics of ferrofluid droplets in magnetic field ambience
(AIP, 2020-08) Harikrishnan, A.R.
The present article discusses the physics and mechanics of evaporation of pendant, aqueous ferrofluid droplets, and modulation of the same by an external magnetic field. We show experimentally and by mathematical analysis that the presence of a horizontal magnetic field augments the evaporation rates of pendant ferrofluid droplets. First, we tackle the question of improved evaporation of the colloidal droplets compared to water and propose physical mechanisms to explain the same. Experiments show that the changes in evaporation rates aided by the magnetic field cannot be explained on the basis of changes in surface tension or based on classical diffusion driven evaporation models. Probing via particle image velocimetry shows that the internal advection kinetics of such droplets plays a direct role toward the augmented evaporation rates by modulating the associated Stefan flow. Infrared thermography reveals changes in thermal gradients within the droplet and evaluating the dynamic surface tension reveals the presence of solutal gradients within the droplet, both brought about by the external field. Based on the premise, a scaling analysis of the internal magneto-thermal and magneto-solutal ferroadvection behavior is presented. The model incorporates the role of the governing Hartmann number, the magneto-thermal Prandtl number, and the magneto-solutal Schmidt number. The analysis and stability maps reveal that the magneto-solutal ferroadvection is the more dominant mechanism, and the model is able to predict the internal advection velocities with accuracy. Furthermore, another scaling model to predict the modified Stefan flow is proposed and is found to accurately predict the improved evaporation rates.
Governing Influence of Thermodynamic and Chemical Equilibria on the Interfacial Properties in Complex Fluids
(ACS, 2018-03) Harikrishnan, A.R.
We propose a comprehensive analysis and a quasi-analytical mathematical formalism to predict the surface tension and contact angles of complex surfactant-infused nanocolloids. The model rests on the foundations of the interaction potentials for the interfacial adsorption–desorption dynamics in complex multicomponent colloids. Surfactant-infused nanoparticle-laden interface problems are difficult to deal with because of the many-body interactions and interfaces involved at the meso-nanoscales. The model is based on the governing role of thermodynamic and chemical equilibrium parameters in modulating the interfacial energies. The influence of parameters such as the presence of surfactants, nanoparticles, and surfactant-capped nanoparticles on interfacial dynamics is revealed by the analysis. Solely based on the knowledge of interfacial properties of independent surfactant solutions and nanocolloids, the same can be deduced for complex surfactant-based nanocolloids through the proposed approach. The model accurately predicts the equilibrium surface tension and contact angle of complex nanocolloids available in the existing literature and present experimental findings.
Impact dynamics of droplets on inclined superhydrophobic cylindrical surfaces: Maximum spreading in axial and azimuthal directions
(AIP, 2025-07) Harikrishnan, A.R.
Droplet impact on surfaces is a fundamental phenomenon in many engineering applications. The asymmetry induced by surface curvature during impact has garnered significant attention due to its relevance in anti-icing strategies for cables and other curved interfaces. While previous studies have extensively examined droplet dynamics on superhydrophobic cylinders oriented horizontally under low Weber number (We) impacts, real-world scenarios often involve high Weber number impacts (⁠ ⁠) and varying obliqueness, leading to complex post-impact behavior. This study systematically investigates the effect of inclination on both axial and azimuthal orientations of the asymmetric post-impact lamella. It is observed that the typical elliptical lamella formed on horizontal cylinders becomes increasingly distorted as the inclination angle, ⁠, increases. Both axial and azimuthal spreading lengths exhibit a decreasing trend with an increase in ⁠. Furthermore, the low hysteresis characteristic of the surface results in reduced adhesion forces, promoting a sliding motion of the lamella along the cylinder's axis. Various post-impact phenomena, including asymmetric bouncing, receding breakup, nucleation-induced film rupture, and fluid lamella splashing, were documented. A modified scaling relation incorporating the inclination angle is proposed to predict the azimuthal spreading length at maximum extension, while axial elongation is modeled using mass and energy balance considerations. The predictive models exhibit strong agreement with experimental results, offering valuable insight into the complex droplet impact dynamics on inclined superhydrophobic cylindrical surfaces.
Influence of temperature and particle concentration on the pH of complex nanocolloids
(Springer, 2017-06) Harikrishnan, A.R.
The pH of colloids is an important electrokinetic property which determines phase stability. We report the effect of temperature and nanoparticle concentration on pH of different nanocolloids of nanomaterials of varied morphologies and sizes. Measurements over a temperature range show that the pH of nanocolloids is a strong function of temperature and the concentration of the dispersed phase. Charge transport mechanisms leading to changes in the effective proton population are discussed. The mannerism in which the electric double layer (EDL) at the particle-fluid interface affects the pH of nanocolloids is presented by appealing to the DLVO theory of electrokinetics dispersion.