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. © 2017, Springer-Verlag GmbH Germany.

A. R. Harikrishnan

Concentration (process)

Electrochemistry

Electrohydrodynamics

Electromagnetic fields

Nanofluidics

Temperature

Charge transport mechanisms

DLVO THEORY

Effect of temperature

ELECTRIC DOUBLE LAYER

ELECTROKINETIC PROPERTIES

Nanofluids

Nanoparticle concentrations

Particle concentrations

Electrodynamics

Aluminum oxide

NANOCOLLOID

nanoparticle

proton

unclassified drug

Acidity

article

colloid

concentration (parameters)

priority journal

Scanning electron microscopy

Surface charge

temperature sensitivity

Thermal conductivity

zeta potential

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Colloid and Polymer Science

Optofluidic manipulation using lasers and colloidal systems have immense applications in microscale thermofluidic devices. The present study analyses the effect of laser as an external optical source in modulating the evaporation characteristics of pendent nanofluid microdroplets (which are free from substrate effects) so as to capture the physics behind the interfacial mass transport. The study analyses the effect of the power of laser, nature and concentration of the particle on evaporation rate of the complex fluids. Evidence of internal circulation was observed with Particle Image Velocimetry (PIV) technique in colloidal systems which together with the volumetric heat generation due to laser irradiation can be attributed to be the cause behind the enhanced evaporation rate. The nanoparticles are observed to efficiently convert the energy of radiant photons to heat while water is observed to show poor evaporative performance under laser irradiation. Theoretical analysis of the evaporation rate with the classical diffusion driven evaporation model is found to fall short in predicting the evaporation rate in colloidal systems, even in the absence of laser irradiation. Thermal Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found not potent enough to induce the circulation in such systems which could improve evaporation. Hence the observed weak internal circulation can be attributed to the solutal Marangoni arising out of the local concentration gradients of nanoparticles in the droplet and the enhanced Thermophoretic drift and Brownian dynamics of the nanoparticles. Also the enhancement in the diffusion coefficient and its strong dependence with the particle concentration is also contributing to the enhancement in enhanced evaporation rate in nanocolloidal systems. The augmentation in the evaporative process under laser radiation is found to be governed majorly by the optical heat generation with weak solutal Marangoni and a scaling model is able to accurately predict the experimental observations. The present findings could have implications in modulation of thermofluidic and species transport phenomena in microscale devices. © 2017 Elsevier Ltd

International Journal of Heat and Mass Transfer

Optical thermogeneration induced enhanced evaporation kinetics in pendant nanofluid droplets

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. © 2016 IOP Publishing Ltd.

Fulltext

Nanotechnology

Large electrorheological phenomena in graphene nano-gels

Magnetic nanofluids have enormous potential to improve thermal conductivity under the influence of magnetic fields. Magnetic field induced thermal transport capabilities of metallic magnetic nanoparticles viz. Fe, Ni and Co based stable magnetic colloids under the influence of magnetic field has been reported for the first time (detailed survey of literature supports the claim). Experimental investigations reveal highly enhanced thermal conductivity of such colloids under the influence of external magnetic field. The highest magnitude of thermal conductivity enhancement ∼106% and 284% is achieved for the Fe/HTO magnetic-nanofluids w.r.to the base nanofluid and pristine base fluid (in the absence of magnetic field) respectively at 0.05&nbsp;T magnetic fields and 7.0&nbsp;vol.% particle concentration. Ni and Co based nanofluids demonstrate less enhancement in the thermal conductivity magnitude compared to Fe based nanofluids due to lower values of saturation magnetic moments. However, the reduction in performance is not as drastic as expected since Ni and Co possesses better thermal conductivities than Fe, leading to compensation of the reduced field response. The underlying mechanism of enhanced conduction has been explained based on the formation of stable nanoparticle chains along the magnetic field lines which act as ‘short circuits’ for the thermal waves to travel faster. The enhancement in the thermal conductivity drops gradually beyond a critical magnetic field due to zippering/self-aggregation of the chained structure. The magnetic fluids have also been observed to be reversible with low thermal hysteresis and prove to be potential candidates as smart fluids in micro-scale devices. © 2016 Elsevier Inc.

Experimental Thermal and Fluid Science

Enhanced heat conduction characteristics of Fe, Ni and Co nanofluids influenced by magnetic field

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. © 1994-2012 IEEE.

IEEE Transactions on Dielectrics and Electrical Insulation

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

Magnetic nanocolloids consisting of synthesized superparamagnetic iron(ii,iii) oxide nanoparticles (SPION) (5-15 nm) dispersed in poly(ethylene glycol) (PEG) and a nano-silica complex have been synthesized. The PEG-nano-silica complex physically encapsulates the SPIONs, ensuring that there is no phase separation under high magnetic fields (∼1.2 T). Exhaustive magneto-rheological investigations have been performed to understand the structural behavior and response of the ferrocolloids. Remarkable stability and reversibility have been observed under magnetic field for concentrated systems. The results show the impact of particle concentration, size and encapsulation efficiency on parameters such as shear viscosity, yield stress, viscoelastic moduli, magneto-viscous hysteresis, and so on. Analytical models to reveal the system mechanism and mathematically predict the magneto-viscosity and magneto-yield stress have been developed. The mechanistic approach based on near-field magnetostatics and Néel-Brownian interactivities could predict the colloidal properties under the effect of the magnetic field accurately. The colloid exhibits amplified storage and loss moduli together with a highly augmented linear viscoelastic region under magnetic stimuli. The transition of the colloidal state from the fluidic phase to the soft condensed phase and its viscoelastic stimuli under the influence of a magnetic field has been explained based on the mathematical analysis. The remarkable stability, magnetic properties and accurate physical models reveal promise for the colloids in transient situations, namely, magneto-microelectromechanical/nanoelectromechanical devices, anti-seismic damping, biomedical invasive treatments, and so on. © 2015 The Royal Society of Chemistry.

Soft Matter

Near-field magnetostatics and Néel-Brownian interactions mediated magneto-rheological characteristics of highly stable nano-ferrocolloids

Journal of Applied Physics

Effects of alignment, pH, surfactant, and solvent on heat transfer nanofluids containing Fe2O3 and CuO nanoparticles

Influence of temperature and particle concentration on the pH of complex nanocolloids

Journal	Data powered by TypesetColloid and Polymer Science
Publisher	Data powered by TypesetSpringer Verlag
ISSN	0303402X
Open Access	No