Interplay of electro thermo solutal advection and internal electrohydrodynamics governed enhanced evaporation of droplets

The article experimentally reveals and theoretically establishes the influence of electric fields on the evaporation kinetics of pendant droplets. It is shown that the evaporation kinetics of saline pendant droplets can be augmented by the application of an external alternating electric field. The evaporation behaviour is modulated by an increase in the field strength and frequency. The classical diffusion driven evaporation model is found insufficient in predicting the improved evaporation rates. The change in surface tension due to field constraint is insufficient for explaining the observed physics. Consequently, the internal hydrodynamics of the droplet is probed employing particle image velocimetry. It is revealed that the electric field induces enhanced internal advection, which improves the evaporation rates. A scaled analytical model is proposed to understand the role of internal electrohydrodynamics, electrothermal and the electrosolutal effects. Stability maps reveal that the advection is caused nearly equally by the electrosolutal and electrothermal effects within the droplet. The model is able to illustrate the influence played by the governing thermal and solutal Marangoni number, the electro Prandtl and electro Schmidt number, and the associated Electrohydrodynamic number. The magnitude of the internal circulation can be well predicted by the proposed model, which validates the proposed mechanism

Electromagnetic field orientation and dynamics governs advection characteristics within pendent droplets

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

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

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

Internal advection dynamics in sessile droplets depend on the curvature of superhydrophobic surfaces

The article demonstrates that the internal circulation velocity and patterns in sessile droplets on superhydrophobic surfaces is governed by the surface curvature. Particle Image Velocimetry reveals that increasing convexity deteriorates the advection velocity whereas concavity augments it. A scaling model based on the effective curvature modulated change in wettability can predict the phenomenon, but weakly. Potential flow theory is appealed to and the curvatures are approximated as wedges with the rested droplet engulfing them partly. The spatially averaged experimental velocities are found to conform to predictions. The study may have strong implications in thermofluidics transport phenomena at the microscale

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 {\pi} 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

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.Comment: Manuscript under advanced stage of peer review in Numerical Heat Transfer A. 37 pages, 12 figure

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

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.Comment: 25 pages 11 figure

Elemental substitution tuned magneto elastoviscous behavior of nanoscale ferrite MFe2O4 M = Mn, Fe, Co, Ni based complex fluids

The present article reports the governing influence of substituting the M2 site in nanoscale MFe2O4 spinel ferrites by different magnetic metals Fe,Mn,Co,Ni on magnetorheological and magneto elastoviscous behaviors of the corresponding magnetorheological fluids MRFs. Different doped MFe2O4 nanoparticles have been synthesized using the polyol assisted hydrothermal method. Detailed steady and oscillatory shear rheology have been performed on the MRFs to determine the magneto-viscoelastic responses. The MRFs exhibit shear thinning behavior and augmented yield characteristics under influence of magnetic field. The steady state magnetoviscous behaviors are scaled against the governing Mason number and self similar response from all the MRFs have been noted. The MRFs conform to an extended Bingham plastic model under field effect. Transient magnetoviscous responses show distinct hysteresis behaviors when the MRFs are exposed to time varying magnetic fields. Oscillatory shear studies using frequency and strain amplitude sweeps exhibit predominant solid like behaviors under field environment. However, the relaxation behaviors and strain amplitude sweep tests of the MRFs reveal that while the fluids show solid like behaviors under field effect, they cannot be termed as typical elastic fluids. Comparisons show that the MnFe2O4 MRFs have superior yield performance among all. However, in case of dynamic and oscillatory systems, CoFe2O4 MRFs show the best performance. The viscoelastic responses of the MRFs are noted to correspond to a three element viscoelastic model. The study may find importance in design and development strategies of nano MRFs for different applications