arXiv subjects

Purbarun Dhar

Publications and source records attributed to Purbarun Dhar.

53 records · Page 3Linked to original sources

arXiv2016

Smart viscoelastic and self-healing characteristics of graphene nano-gels

Readily synthesizable nano-graphene and poly ethylene glycol based stable gels have been synthesized employing an easy refluxing method and exhaustive rheological and viscoelastic characterizations have been performed to understand the nature of such complex gel systems. The gels exhibit shear thinning response with pronounced yield stress values which is indicative of a microstructure where the graphene nanoflakes intercalate with the polymer chains and form a pseudo spring damper network. Experimentations on the thixotropic behavior of the gels indicate that the presence of the G nanoflakes leads to immensely augmented structural stability capable of withstanding severe impact shears. Further information about the localized interactions of the G nanoflakes with the polymer chains is revealed from the amplitude and frequency sweep analyses in both linear and nonlinear viscoelastic regimes. Massively enhanced cross over amplitude values are recorded and several smart effects such as enhanced elastic behavior at increasing forcing frequencies are registered. Structural resonance induced disruption of the elastic behavior is observed for the gels for a given range of frequency and the proposition of resonance has been justified mathematically. It is observed that post this resonance bandwidth, the gels are able to self-heal and regain their original elastic behavior back without any external intervention. More detailed information on the viscoelastic nature of the gels has been obtained from creep and recovery compliance tests and justifications for the spring damper microstructure has been obtained. Smart features such as enhanced stress relaxation behavior with increasing strain have been observed and the same explained based on the proposed microstructure.

physics.flu-dyn↗

arXiv2016

Selecting optimal parallel microchannel configurations for active hot spot mitigation of multicore microprocessors in real time

Design of effective micro cooling systems to address the challenges of ever increasing heat flux from microdevices requires deep examination of real time problems and has been tackled in depth. The most common and apparently misleading assumption while designing micro cooling systems is that the heat flux generated by the device is uniform, but the reality is far from this. Detailed simulations have been performed by considering non uniform heat load employing the configurations U, I, Z for parallel microchannel systems with water and nanofluids as the coolants. An Intel Core i7 4770 3.40 GHz quad core processor has been mimicked using heat load data retrieved from a real microprocessor with non-uniform core activity. The study clearly demonstrates that there is a non-uniform thermal load induced temperature maldistribution along with the already existent flow maldistribution induced temperature maldistribution. The suitable configuration(s) for maximum possible overall heat removal for a hot zone while maximizing the uniformity of cooling have been tabulated. An Eulerian Lagrangian model of the nanofluids show that such smart coolants not only reduce the hot spot core temperature, but also the hot spot core region and thermal slip mechanisms of Brownian diffusion and thermophoresis are at the crux of this. The present work conclusively shows that high flow maldistribution leads to high thermal maldistribution, as the common prevalent notion, is no longer valid and existing maldistribution can be effectively utilized to tackle specific hot spot location, making the present study important to the field.

physics.flu-dyn↗

arXiv2016

Mega electrorheological phenomena in graphene nanogels

Unprecedentedly massive electrorheology has been reported for dilute graphene nanoflakes based ER fluids that have been engineered as novel, readily synthesizable polymeric gels. Polyethylene glycol based graphene gels have been synthesized and very high ER response, 25,000 percent enhancement in viscosity under influence of electric field, has been observed for low concentration systems 2 wt. percent. The gels overcome several drawbacks innate to ER fluids. The gels exhibit long term stability, high graphene packing ratio which ensures very high ER response and the microstructure of the gels ensure that fibrillation of the graphene nanoflakes under field is undisturbed by thermal fluctuations, further leading to mega ER. The gels exhibit large yield stress handling caliber with yield stress observed as high as 13 kPa at 2 wt. perc. graphene. Detailed investigations on the effects of graphene concentration, electric field strength, imposed shear resistance and transients of electric field actuation on the ER response of the gels have been performed. The gels show nearly negligible hysteresis with respect to electro viscous effects which speak of the stability and responsiveness required in transient situations. Indepth analyses with explanations have been provided for the observations and effects, such as sheet over-crowding induced loss of structural integrity at high concentrations and inter flake lubrication or slip induced augmented ER response. The present gels show great promise as potential ER gels for variant smart applications, such as damping and vibration control in active aircraft and vehicle structures, micronanoscale electroactuation, in suspensions or braking clutching in high performance specialized vehicular systems, etc.

cond-mat.mes-hall↗

arXiv2016

Thermally smart characteristics of nanofluids in parallel microchannel systems to mitigate hot spots in MEMS

Mitigation of hot spots in MEMS employing in situ microchannel systems requires a comprehensive picture of the maldistribution of the working fluid and uniformity of cooling within the same. In this article, detailed simulations employing parallel micro channel systems with specialized manifold-channel configurations i.e. U, I and Z have been performed. Eulerian Lagrangian Discrete Phase Model and Effective Property Model with water and alumina water nanofluid as working fluids have been employed. The distributions of the dispersed particulate phase and continuous phase have been observed to be, in general, different from the flow distribution and this has been found to be strongly dependent on the flow configuration. Particle maldistribution has been conclusively shown to be influenced by various migration and diffusive phenomena like Stokesian drag, Brownian motion, thermophoretic drift, etc. To understand the uniformity of cooling within the device, which is of importance in real time scenario, an appropriate figure of merit has been proposed. It has been observed that uniformity of cooling improved using nanofluid as working fluid as well as enhanced relative cooling in hot zones, providing evidence of the smart nature of such dispersions. To further quantify this smart effect, real time mimicking hot-spot scenarios have been computationally probed with nanofluid as the coolant. A silicon-based microchip emitting non-uniform heat flux (gathered from real-time monitoring of an Intel Core i7 4770 3.40 GHz quad core processor) under various processor load conditions has been studied and evidence of enhanced cooling of hot spots has been obtained from DPM analysis.

physics.flu-dyn↗

arXiv2015

Analytical prediction of sub surface thermal history in translucent tissue phantoms during plasmonic photo thermotherapy

Knowledge of thermal history in biological tissues during laser based hyperthermia is essential to achieve necrosis of tumour orcarcinoma cells. A semi analytical model to predict sub surface thermal history in translucent, soft, bio tissue mimics has been proposed. The model can accurately predict the spatio temporal temperature variations along depth and the anomalous thermal behaviour in such media, viz. occurrence of sub surface temperature peaks. Based on opto thermal properties, the augmented temperature and shift of the peak positions in case of gold nanostructure mediated tissue phantom hyperthermia can be predicted. Employing inverse approach, the absorbance coefficient of nano graphene infused tissue mimics is determined from the peak temperature and found to provide appreciably accurate predictions along depth. Furthermore, a simplistic, dimensionally consistent correlation to theoretically determine the position of the peak in such media is proposed and found to be consistent with experiments and computations. The model shows promise in predicting thermal history in optically active soft matter and deduction of therapeutic hyperthermia parameters, thereby providing a priori estimates for clinical trials.

physics.med-ph↗

arXiv2015

Particle and thermo-hydraulic maldistribution of nanofluids in parallel microchannel systems

Fluidic maldistribution in microscale multichannel devices requires deep understanding to achieve optimized flow and heat transfer characteristics. A thorough computational study has been performed to understand the concentration and thermohydraulic maldistribution of nanofluids in parallel microchannel systems using an Eulerian Lagrangian twin phase model. The study reveals that nanofluids cannot be treated as homogeneous single phase fluids in such complex flow domains and effective property models fail drastically to predict the performance parameters. To comprehend the distribution of the particulate phase, a novel concentration maldistribution factor has been proposed. It has been observed that distribution of particles need not essentially follow the flow pattern, leading to higher thermal performance than expected from homogeneous models. Particle maldistribution has been conclusively shown to be due to various migration and diffusive phenomena like Stokesian drag, Brownian motion, thermophoretic drift, etc. The implications of particle distribution on the cooling performance have been illustrated and smart fluid effects (reduced magnitude of maximum temperature) have been observed and a mathematical model to predict the enhanced cooling performance in such flow geometries has been proposed. The article presents lucidly the effectiveness of discrete phase approach in modelling nanofluid thermohydraulics and sheds insight on behavior of nanofluids in complex flow domains.

physics.flu-dyn↗

arXiv2015

Near field magnetostatics and Neel Brownian interactions mediated magnetorheological characteristics of highly stable nano ferrocolloids

Magnetic nanocolloids with synthesized super paramagnetic Fe3O4 nanoparticles (SPION) (5 to 15 nm) dispersed in insitu developed Polyethylene Glycol (PEG 400) and nano silica complex have been synthesized. The PEG nano Silica complex physically encapsulates the SPIONs, ensuring no phase separation under high magnetic fields (1.2 T). Exhaustive magnetorheological investigations have been performed to comprehend the structural behavior and response of the ferrocolloids. Remarkable stability and reversibility have been observed under magnetic field for concentrated systems. The results exhibit the impact of particle concentration, size and encapsulation efficiency on parameters such as shear viscosity, yield stress, viscoelastic moduli, magnetoviscous hysteresis etc. Analytical models to reveal the system mechanism and mathematically predict the magnetoviscosity and magneto yield stress has been theorized. The mechanistic approach based on near field magnetostatics and Neel Brownian interactivities can predict the colloidal properties under the effect of field accurately. The colloid exhibits amplified storage and loss moduli alongside highly augmented linear viscoelastic region under magnetic stimuli. The transition of the colloidal state from fluidic phase to soft condensed phase and its viscoelastic stimuli under the influence of magnetic field has been explained based on the mathematical analysis. The remarkable stability, magnetic properties and accurate physical models reveal good promise for the colloids in transient situations viz. magneto MEMS/ NEMS devices, antiseismic damping, biomedical invasive treatments etc.

physics.flu-dyn↗

arXiv2015

Enhanced dielectric breakdown performance of anatase and rutile titania based nano-oils

Nano oils synthesized by dispersing dielectric nanostructures counter common intuition as such nano oils possess substantially higher positive dielectric breakdown voltage with reduced streamer velocities than the base oils. Nano oils comprising stable and dilute homogeneous dispersions of two forms of titanium (IV) oxide (TiO2) nanoparticles (Anatase and Rutile) have been experimentally examined and observed to exhibit highly enhanced dielectric breakdown strength compared to conventional transformer oils. Indepth survey of literature yields that research on enhancing insulation properties of mineral oils by utilizing anatase and rutile titania nanoparticles is nought. The present study involves titania dispersed in two different grades of transformer oils, both with varied levels of thermal treatment, to obtain consistent and high degrees of enhancement in the breakdown strength, as well as high degrees of increment in the survival of the oils at elevated electrical stressing compared to the base oils, as obtained via detailed twin parameter Weibull distribution analysis of the experimental observations. It is also observed that heat treatment of the nano oils further enhances the dielectric breakdown performance. Additionally, further enhancements are observed at elevated operational temperatures. Detailed studies on the performance of the nano oils in presence of variant quantities of moisture have also been put forward. The grossly different charging dynamics of dielectric nanoparticles localized within the oil has been proposed to be responsible for efficient electron trapping, leading to decrease in the positive streamer velocity and resulting in high dielectric breakdown voltage. The differences in the performance of anatase and rutile has been explained based on the electronic structure of the two and the affinity towards electron scavenging.

cond-mat.mes-hall↗

arXiv2015

Colloidal graphite/graphene nanostructures using collagen showing enhanced thermal conductivity

Time kinetics of interaction of natural graphite (GR) to colloidal graphene (G) collagen (C) nanocomposites was studied at ambient conditions, and observed that just one day at ambient conditions is enough to form colloidal graphene directly from graphite using the protein collagen. Neither controlled temperature and pressure ambiance nor sonication was needed for the same; thereby rendering the process biomimetic. Detailed spectroscopy, X ray diffraction, electron microscopy as well as fluorescence and luminescence assisted characterization of the colloidal dispersions on day one and day seven reveals graphene and collagen interaction and subsequent rearrangement to form an open structure. Detailed confocal microscopy, in the liquid state, reveals the initial attack at the zigzag edges of GR, the enhancement of auto fluorescence and finally the opening up of graphitic stacks of GR to form near transparent G. Atomic Force Microscopy studies prove the existence of both collagen and graphene and the disruption of periodicity at the atomic level. Thermal conductivity of the colloid shows a 17% enhancement for a volume fraction of less than 0.00005 of G. Time variant increase in thermal conductivity provides qualitative evidence for the transient exfoliation of GR to G. The composite reveals interesting properties that could propel it as a future material for advanced bio applications including therapeutics.

q-bio.BM↗

arXiv2015

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 behavior 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 interparticle electrostatic interactions acts as another mode of transport. Coupled electrothermal diffusion arising out of Brownian randomization in 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 nanocolloid parameters such as temperature and concentration.

cond-mat.soft↗

arXiv2015

Anomalously augmented charge transport capabilities of biomimetically transformed collagen intercalated nano graphene based biocolloids

Collagen micro fibrils bio mimetically intercalate graphitic structures in aqueous media to form graphene nano platelets collagen complex (G Cl). Synthesized G Cl based stable, aqueous bio nanocolloids exhibit anomalously augmented charge transportation capabilities over simple collagen or graphene based colloids. The concentration tunable electrical transport properties of synthesized aqueous G Cl bio nanocolloids 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 bio nanocolloids based on the physico chemical interactions among the two entities, as revealed from extensive characterizations of the G Cl bio complex, 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 bio nanocolloids with concentration tunability promises find dual utility in novel gel bio electrophoresis based protein separation techniques and advanced surface charge modulated drug delivery using biocolloids.

physics.bio-ph↗

arXiv2015

Percolation network dynamicity and sheet dynamics governed viscous behavior of poly-dispersed Graphene nano-sheet suspensions

The viscosity of Graphene nano-sheet suspensions (GNS)and its behavior with temperature and concentration have been experimentally determined. A physical mechanism for the enhanced viscosity over the base fluids has been proposed for the poly-dispersed GNSs. Experimental data reveals that enhancement of viscosity for GNSs lie in between that of Carbon Nanotube Suspensions (CNTSs) and nano Alumina suspensions (nAS) , indicating the hybrid mechanism of percolation (like CNTs) and Brownian motion assisted sheet dynamics (like Alumina particles). Sheet dynamics and percolation, along with a proposed percolation Network Dynamicity Factor; have been used to determine a dimensionally consistent analytical model to accurately determine and explain the viscosity of poly-dispersed GNSs. It has been hypothesized that the dynamic sheets behave qualitatively analogous to gas molecules. The model also provides insight into the mechanisms of viscous behavior of different dilute nanoparticle suspensions. The model has been found to be in agreement with the GNS experimental data, and even for CNT and nano Alumina suspensions.

cond-mat.mtrl-sci↗

arXiv2015

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.

cond-mat.mes-hall↗

arXiv2015

Bridging thermal and electrical transport in dielectric nanostructure based polar colloids

Heat and charge transport characteristics of nanocolloids have been bridged from fundamental analysis. The relationship between the two transport phenomena in dielectric nanostructure based polar colloids has been quantitatively presented. An extensional intuitive analogy to the Wiedemann Franz law has been drawn. Derived from the fact that mobile electrons transport both heat and charge within metallic crystal structure, the analogy can be extended to nanocolloids, wherein the dispersed population act as the major transporter. The analogy allows modeling the relationship between the two phenomena and sheds more insight and conclusive evidence that nanoparticle traversal within the fluid domain is the main source of augmented transport phenomena exhibit by nanocolloids. Important factors such as the thermal and dielectric responses of the nanocolloid can be quantified and bridged through the semi analytical formalism. The theoretical analysis has been validated against experimental data and variant scientific literature and good accuracy has been observed.

cond-mat.soft↗

arXiv2015

Particle fluid interactivity deteriorates buoyancy driven thermal transport in nanosuspensions : A multi component lattice Boltzmann approach

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 pin pointed 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 has been 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 have been pictured articulately. A comprehensive study on the effects of Rayleigh number, particle size and concentration reveals that the drag force experienced by the particles is dominantly responsible for deterioration 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 thorough survey of existent 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.

physics.flu-dyn↗

arXiv2015

Magnetic field induced augmented thermal conduction phenomenon in magneto nanocolloids

Magnetic field induced drastically augmented thermal conductivity of magneto nanocolloids involving magnetic oxide nanoparticles, viz. Fe2O3, Fe3O4, Nickel oxide (NiO), Cobalt oxide (Co3O4), dispersed in different base fluids (heat transfer oil, kerosene, and ethylene glycol) have been reported. Experiments reveal the augmented thermal transport under the external applied magnetic field, with kerosene based MNCs showing at relatively low magnetic field intensities as compared to the heat transfer oil and EG based MNCs. A maximum thermal conductivity enhancement of 114 % is attained at 7.0 vol. % concentration and 0.1 T field intensity for Fe3O4/EG magneto nanocolloid. However, a maximum of 82% thermal conductivity enhancement is observed for Fe3O4/Kerosene magneto nanocolloid for the same concentration but relatively at low magnetic field (600 G). Thereby, a strong effect of fluid as well as particle physical properties on the chain formation propensity, leading to enhanced conduction, in such systems is observed. Co3O4 nanoparticles show insignificant effect on the thermal conductivity enhancement of MNCs due to their minimal magnetic moment. An analytical approach has been proposed to understand the mechanism and physics behind the thermal conductivity enhancement under external applied magnetic field, in tune with near field magnetostatic interactions as well as Neel relaxivity of the magnetic nanoparticles. Furthermore, the analytical model is able to predict the phenomenon of enhanced thermal conductivity as a function of physical parameters such as chain length, size and types of nanoparticles, fluid characteristics, magnetic field intensity, saturation magnetic moment, nanoparticle concentration etc. and good agreement with the experimental results has been observed.

cond-mat.mes-hall↗

arXiv2015

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

Nano oils comprising Graphene nanoflakes and CNTs 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, such as, 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.

cond-mat.mes-hall↗