Purbarun Dhar

The role of percolation and sheet dynamics during heat conduction in poly-dispersed graphene nanofluids

A thermal transport mechanism leading to the enhanced thermal conductivity of graphene nanofluids has been proposed. The graphene sheet size is postulated to be the key to the underlying mechanism. Based on a critical sheet size derived from Stokes-Einstein equation for the poly-dispersed nanofluid, sheet percolation and Brownian motion assisted sheet collisions are used to explain the heat conduction. A collision dependant dynamic conductivity considering Debye approximated volumetric specific heat due to phonon transport in graphene has been incorporated. The model has been found to be in good agreement with experimental data.

Effects of interplay of nanoparticles, surfactants and base fluid on the surface tension of nanocolloids

Abstract.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.Graphical abstract

Particle–fluid interactivity reduces buoyancy-driven thermal transport in nanosuspensions: A multi-component Lattice Boltzmann approach

ABSTRACT Severe contradictions exist between experimental observations and computational predictions regarding natural convective thermal transport in nanosuspensions. The approach treating nanosuspensions as homogeneous fluids in computations has been pinpointed as the major contributor to such contradictions. To fill the void, inter-particle and particle–fluid interactivities (slip mechanisms), in addition to effective thermophysical properties, have been incorporated within the present formulation. Through thorough scaling analysis, the dominant slip mechanisms have been identified. A Multi-Component Lattice Boltzmann Model (MCLBM) approach is proposed, wherein the suspension has been treated as a non-homogeneous twin component mixture with the governing slip mechanisms incorporated. The computations based on the mathematical model can accurately predict and quantify natural convection thermal transport in nanosuspensions. The role of slip mechanisms such as Brownian diffusion, thermophoresis, drag, Saffman lift, Magnus effect, particle rotation, and gravitational effects has been accurately described. A comprehensive study on the effects of Rayleigh number, particle size, and concentration revealed that the drag force experienced by the particles is primarily responsible for the reduction of natural convective thermal transport. In essence, the dominance of Stokesian mechanics in such thermofluidic systems is established in the present study. For the first time, as revealed though a thorough survey of the literature, a numerical formulation explains the contradictions observed, rectifies the approach, predicts accurately, and reveals the crucial mechanisms and physics of buoyancy-driven thermal transport in nanosuspensions.

Superior dielectric breakdown strength of graphene and carbon nanotube infused nano-oils

Nano-oils comprising stable and dilute dispersions of synthesized Graphene (Gr) nanoflakes and carbon nanotubes (CNT) have been experimentally observed for the first time to exhibit augmented dielectric breakdown strengths compared to the base transformer oils. Variant nano-oils comprising different Gr and CNT samples suspended in two different grades of transformer oils have yielded consistent and high degrees of enhancement in the breakdown strength. The apparent counter-intuitive phenomenon of enhancing insulating caliber of fluids utilizing nanostructures of high electronic conductance has been shown to be physically consistent thorough theoretical analysis. The crux mechanism has been pin pointed as efficient charge scavenging leading to hampered streamer growth and development, thereby delaying probability of complete ionization. The mathematical analysis presented provides a comprehensive picture of the mechanisms and physics of the electrohydrodynamics involved in the phenomena of enhanced breakdown strengths. Furthermore, the analysis is able to physically explain the various breakdown characteristics observed as functions of system parameters, viz. nanostructure type, size distribution, relative permittivity, base fluid dielectric properties, nanomaterial concentration and nano-oil temperature. The mathematical analyses have been extended to propose a physically and dimensionally consistent analytical model to predict the enhanced breakdown strengths of such nano-oils from involved constituent material properties and characteristics. The model has been observed to accurately predict the augmented insulating property, thereby rendering it as an extremely useful tool for efficient design and prediction of breakdown characteristics of nanostructure infused insulating fluids. The present study, involving experimental investigations backed by theoretical analyses and models for an important dielectric phenomenon such as electrical breakdown can find utility in design of safer and more efficient high operating voltage electrical drives, transformers and machines.

Colloidal graphite/graphene nanostructures using collagen showing enhanced thermal conductivity

In the present study, the exfoliation of natural graphite (GR) directly to colloidal GR/graphene (G) nanostructures using collagen (CL) was studied as a safe and scalable process, akin to numerous natural processes and hence can be termed “biomimetic”. Although the exfoliation and functionalization takes place in just 1 day, it takes about 7 days for the nano GR/G flakes to stabilize. The predominantly aromatic residues of the triple helical CL forms its own special micro and nanoarchitecture in acetic acid dispersions. This, with the help of hydrophobic and electrostatic forces, interacts with GR and breaks it down to nanostructures, forming a stable colloidal dispersion. Surface enhanced Raman spectroscopy, X-ray diffraction, photoluminescence, fluorescence, and X-ray photoelectron spectroscopy of the colloid show the interaction between GR and CL on day 1 and 7. Differential interference contrast images in the liquid state clearly reveal how the GR flakes are entrapped in the CL fibrils, with a corresponding fluorescence image showing the intercalation of CL within GR. Atomic force microscopy of graphene-collagen coated on glass substrates shows an average flake size of 350 nm, and the hexagonal diffraction pattern and thickness contours of the G flakes from transmission electron microscopy confirm ≤ five layers of G. Thermal conductivity of the colloid shows an approximate 17% enhancement for a volume fraction of less than approximately 0.00005 of G. Thus, through the use of CL, this new material and process may improve the use of G in terms of biocompatibility for numerous medical applications that currently employ G, such as internally controlled drug-delivery assisted thermal ablation of carcinoma cells.

Large electrorheological phenomena in graphene nano-gels

Large-scale electrorheology (ER) response has been reported for dilute graphene nanoflake-based ER fluids that have been engineered as novel, readily synthesizable polymeric gels. Polyethylene glycol (PEG 400) based graphene gels have been synthesized and a very high ER response (∼125 000% enhancement in viscosity under influence of an electric field) has been observed for low concentration systems (∼2 wt.%). The gels overcome several drawbacks innate to ER fluids. The gels exhibit long term stability, a high graphene packing ratio which ensures very high ER response, and the microstructure of the gels ensures that fibrillation of the graphene nanoflakes under an electric field is undisturbed by thermal fluctuations, further leading to mega ER. The gels exhibit a large yield stress handling caliber with a yield stress observed as high as ∼13 kPa at 2 wt.% for graphene. Detailed investigations on the effects of graphene concentration, electric field strength, imposed shear resistance, transients of electric field actuation on the ER response and ER hysteresis of the gels have been performed. In-depth analyses with explanations have been provided for the observations and effects, such as inter flake lubrication/slip induced augmented ER response. The present gels show great promise as potential ER gels for various smart applications.

Anomalously Augmented Charge Transport Capabilities of Biomimetically Transformed Collagen Intercalated Nanographene-Based Biocolloids

Collagen microfibrils biomimetically intercalate graphitic structures in aqueous media to form graphene nanoplatelet-collagen complexes (G-Cl). Synthesized G-Cl-based stable, aqueous bionanocolloids exhibit anomalously augmented charge transportation capabilities oversimple collagen or graphene based colloids. The concentration tunable electrical transport properties of synthesized aqueous G-Cl bionanocolloids has been experimentally observed, theoretically analyzed, and mathematically modeled. A comprehensive approach to mathematically predict the electrical transport properties of simple graphene and collagen based colloids has been presented. A theoretical formulation to explain the augmented transport characteristics of the G-Cl bionanocolloids based on the physicochemical interactions among the two entities, as revealed from extensive characterizations of the G-Cl biocomplex, has also been proposed. Physical interactions between the zwitterionic amino acid molecules within the collagen triple helix with the polar water molecules and the delocalized π electrons of graphene and subsequent formation of partially charged entities has been found to be the crux mechanism behind the augmented transport phenomena. The analysis has been observed to accurately predict the degree of enhancement in transport of the concentration tunable composite colloids over the base colloids. The electrically active G-Cl bionanocolloids with concentration tunability promises find dual utility in novel gel bioelectrophoresis-based protein separation techniques and advanced surface charge modulated drug delivery using biocolloids.

Cascaded collision lattice Boltzmann model (CLBM) for simulating fluid and heat transport in porous media

ABSTRACT The present paper reports a cascaded collision lattice Boltzmann model for the simulation of an incompressible two-dimensional fluid flow in a porous media regime. The cascaded model is first validated for the nonporous regime using limiting conditions against previous finite element model reports. Subsequently, the cascaded collision model is applied to the lid-driven porous-filled cavity to demonstrate the largely augmented numerical stability of the model against the more common Bhatnagar–Gross–Krook and multiple relaxation time collision models. Finally, the cascaded model is applied to an inflow–outflow case of flow and heat transfer over a porous bluff body to showcase its efficiency in capturing the complex fluid and heat transport phenomenon through porous media.

Trimodal charge transport in polar liquid-based dilute nanoparticulate colloidal dispersions

The dominant modes of charge transport in variant polar liquid-based nanoparticulate colloidal dispersions (dilute) have been theorized. Theories formulating electrical characteristics of colloids have often been found to over- or under-predict charge transport in dilute suspensions of nanoparticles in polar fluids owing to grossly different mechanistic behaviors of concentrated systems. Three major interacting modes with independent yet simultaneous existence have been proposed and found to be consistent with analyses of experimental data. Electric double layer (EDL) formation at nanoparticle–fluid interface-conjugated electrophoresis under the influence of the electric field has been determined as one important mode of charge transport. Nanoparticle polarization due to short-range field non-uniformity caused by the EDL with consequent particle motion due to inter-particle electrostatic interactions acts as another mode of transport. Coupled electro-thermal diffusion arising out of Brownian randomization in the presence of the electric field has been determined as the third dominant mode. An analytical model based on discrete interactions of the charged particle–fluid domains explains the various behavioral aspects of such dispersions, as observed and validated from detailed experimental analysis. The analysis is also predictive of the dominance and behavior of the three modes with important nanocolloidal parameters such as temperature and concentration.Graphical Abstract

Role and significance of wetting pressures during droplet impact on structured superhydrophobic surfaces

Abstract.The impact dynamics and spreading behavior of droplets impinging on structured superhydrophobic surfaces are dependent on both the droplet initial conditions and the surface texture. The equivalence of wetting and dewetting pressures is classically known to be a critical factor in determining the state of a droplet during the contact and spreading phases. The present study extensively examines the underlying physics behind this pressure balance during the impact process and its direct role in determining the wetting process. Extensive three-dimensional simulations employing droplet impact on a structured superhydrophobic surface has been performed to reveal the intricacies of the interactivities of the fluid with the microstructure. Insight onto the acute role of wetting pressures and the implications of the same on determining the wetting dynamics, with internal fluidics of the droplet during the impact process, has been discussed. The phenomenon of state transition from the Cassie-Baxter to the Wenzel up on impact is also investigated and the intricate flow mechanics at play within the posts has been presented. Knowledge of pressure distribution and internal flow structures within the droplet during its interaction with the surface at different instances of time reveals the root mechanism behind the impalement of the droplet to a fully wetting state. Analysis of the internal pressure and flow distribution also presents necessary justification for the existence of a partially impaled state. The time evolution of spread for different scenarios is in agreement with experimental results and the article provides insight onto the role of wetting pressure in determining fluidic interactions on such surfaces.Graphical abstract