María José Pastoriza-Gallego

Thermal conductivity and viscosity measurements of ethylene glycol-based Al2O3 nanofluids

The dispersion and stability of nanofluids obtained by dispersing Al2O3 nanoparticles in ethylene glycol have been analyzed at several concentrations up to 25% in mass fraction. The thermal conductivity and viscosity were experimentally determined at temperatures ranging from 283.15 K to 323.15 K using an apparatus based on the hot-wire method and a rotational viscometer, respectively. It has been found that both thermal conductivity and viscosity increase with the concentration of nanoparticles, whereas when the temperature increases the viscosity diminishes and the thermal conductivity rises. Measured enhancements on thermal conductivity (up to 19%) compare well with literature values when available. New viscosity experimental data yield values more than twice larger than the base fluid. The influence of particle size on viscosity has been also studied, finding large differences that must be taken into account for any practical application. These experimental results were compared with some theoretical models, as those of Maxwell-Hamilton and Crosser for thermal conductivity and Krieger and Dougherty for viscosity.

A study on stability and thermophysical properties (density and viscosity) of Al2O3 in water nanofluid

The dispersion and stability of nanofluids obtained by dispersing Al2O3 nanoparticles (obtained from different sources) in water have been analyzed. The differences arising from different dispersion techniques, the resulting particle size distribution, and time stability among the different samples are evaluated. Then the volumetric behavior up to high pressures (25 MPa) and atmospheric pressure viscosity were experimentally determined. It has been found that the influence of particle size in density is subtle but not negligible, but the differences in viscosity are very large and must be taken into account for any practical application. These viscosity differences can be rationalized by considering a theory describing the aggregation state of the nanofluid.

Enhancement of thermal conductivity and volumetric behavior of FexOy nanofluids

Homogeneous and stable magnetic nanofluids containing iron oxide nanoparticles, α-Fe2O3 (hematite) and Fe3O4 (magnetite) in ethylene glycol, were prepared at concentrations up to 25% in mass fraction. Commercial Hexagonal Scalenohedral-shaped α-Fe2O3 nanoparticles were selected while Fe3O4 nanoparticles were synthesized using a coprecipitation method. The products were characterized by transmission and scanning electron microscopy and x-ray diffraction. The thermal conductivity of both nanofluids was measured as a function of volume fraction and temperature. The results illustrate that the enhanced thermal conductivity of the nanofluids increases with volume fraction but is temperature independent. The experimental results show that both types of nanoparticles in this base fluid present no significant aggregation. These experimental values were also compared with theoretical models. Moreover, the density of these nanofluids was measured as a function of volume fraction, temperature, and pressure. The volu...

Rheological and volumetric properties of TiO2-ethylene glycol nanofluids

Homogeneous stable suspensions obtained by dispersing dry TiO2 nanoparticles in pure ethylene glycol were prepared and studied. Two types of nanocrystalline structure were analyzed, namely anatase and rutile phases, which have been characterized by scanning electron microscopy. The rheological behavior was determined for both nanofluids at nanoparticle mass concentrations up to 25%, including flow curves and frequency-dependent storage and loss moduli, using a cone-plate rotational rheometer. The effect of temperature over these flow curve tests at the highest concentration was also analyzed from 283.15 to 323.15 K. Furthermore, the influence of temperature, pressure, nanocrystalline structure, and concentration on the volumetric properties, including densities and isobaric thermal expansivities, were also analyzed.

On the Formation of a Third, Nanostructured Domain in Ionic Liquids

The study of solid-fluid transitions in fluorinated ionic liquids using differential scanning calorimetry, rheology, and molecular modeling techniques is an essential step toward the understanding of their dynamics and the thermodynamics and the development of potential applications. Two fluorinated ionic liquids were studied: 1-hexyl-3-methylimidazolium perfluorobutanesulfonate, HMIm(PFBu)SO3, and tetrabutylammonium perfluorobutanesulfonate, NB4(PFBu)SO3. The experimental calorimetric and rheological data were analyzed taking into account the possible mesoscale structure of the two fluorinated ionic liquids. The simulation results indicate the possible formation of three nanosegregated domains-polar, nonpolar, and fluorous-that may have a profound impact on ionic liquid research. In the case of HMIm (PFBu)SO3 the three types of mesoscopic domains can act as interchangeable jigsaw pieces enabling the formation of multiple types of crystals and inducing the observed calorimetry and rheological trends.

Quantitative determination of α-tocopherol distribution in a tributyrin/Brij 30/water model food emulsion

Until recently, determining the distribution of antioxidants, AOs, between the oil, interfacial and aqueous regions of opaque emulsions has not worked well because the concentrations of AOs in interfacial regions cannot be determined separately from their concentrations in the oil and water phases. However, our novel kinetic method based on the reaction between an arenediazonium ion and vitamin E, or alpha-tocopherol, provides the first good estimates for the two partition constants that describe alpha-tocopherol distribution between the oil/interfacial and water/interfacial regions of tributyrin/Brij 30/water emulsions without physical isolation of any phase. The reaction is monitored by a new derivatization method based on trapping unreacted arenediazonium ion as an azo dye and confirmed by linear sweep voltammetry, LSV. The results by both derivatization and LSV methods are in good agreement and show that alpha-tocopherol distributes strongly in favor of the interfacial region when the oil is tributyrin, e.g., ca. 90% when the surfactant volume fraction is Phi I=0.01. The second-order rate constant for reaction in the interfacial region is also obtained from the results. Our kinetic method provides a robust approach for determining antioxidant distributions in emulsions and should help develop a quantitative interpretation of antioxidant efficiency in emulsions.

To Model Chemical Reactivity in Heterogeneous Emulsions, Think Homogeneous Microemulsions.

Two important and unsolved problems in the food industry and also fundamental questions in colloid chemistry are how to measure molecular distributions, especially antioxidants (AOs), and how to model chemical reactivity, including AO efficiency in opaque emulsions. The key to understanding reactivity in organized surfactant media is that reaction mechanisms are consistent with a discrete structures-separate continuous regions duality. Aggregate structures in emulsions are determined by highly cooperative but weak organizing forces that allow reactants to diffuse at rates approaching their diffusion-controlled limit. Reactant distributions for slow thermal bimolecular reactions are in dynamic equilibrium, and their distributions are proportional to their relative solubilities in the oil, interfacial, and aqueous regions. Our chemical kinetic method is grounded in thermodynamics and combines a pseudophase model with methods for monitoring the reactions of AOs with a hydrophobic arenediazonium ion probe in opaque emulsions. We introduce (a) the logic and basic assumptions of the pseudophase model used to define the distributions of AOs among the oil, interfacial, and aqueous regions in microemulsions and emulsions and (b) the dye derivatization and linear sweep voltammetry methods for monitoring the rates of reaction in opaque emulsions. Our results show that this approach provides a unique, versatile, and robust method for obtaining quantitative estimates of AO partition coefficients or partition constants and distributions and interfacial rate constants in emulsions. The examples provided illustrate the effects of various emulsion properties on AO distributions such as oil hydrophobicity, emulsifier structure and HLB, temperature, droplet size, surfactant charge, and acidity on reactant distributions. Finally, we show that the chemical kinetic method provides a natural explanation for the cut-off effect, a maximum followed by a sharp reduction in AO efficiency with increasing alkyl chain length of a particular AO. We conclude with perspectives and prospects.

Study of viscoelastic properties of magnetic nanofluids: an insight into their internal structure

Three magnetic nanofluids were prepared as colloidal suspensions in ethylene glycol using two types of Fe3O4 ferrimagnetic nanoparticles (MGMFs)—one of them commercial (CMGMFs) and the other synthesized in our laboratory (SMGMFs)—and from one type of Fe2O3 nanoparticle (HMF). Using a shear stress controlling rheometer, they were characterized in order to comparatively analyze their viscoelastic behavior and structure. Effects of magnetic particle size and nature on elastic and viscous moduli were determined. The static yield stress and the percolation concentration were also obtained. The fractal dimension (Df) was estimated from the suspension static yield-stress and volume fraction (ϕ) dependence, and was determined to be Df ≈ 2.3 for SMGMFs and CMGMFs, and Df ≈ 1.5 for HMFs. The corresponding aggregation models and structures were discussed. Finally, master curves were obtained for the magnetic fluids under study, which provide valuable information to estimate the material response for any concentration.

Tailoring Nanofluid Thermophysical Profile through Graphene Nanoplatelets Surface Functionalization

In this study, the effect of chemical surface functionalization through oxidation of exfoliated graphite nanoplatelets in the transport properties of their aqueous nanofluids has been analyzed. With this objective, thermal conductivity and viscoelastic properties have been determined for original and oxidized nanoplatelets. The results show that the functionalization completely changes the internal structure of the suspension, which is reflected in shifts of even orders of magnitude on viscosity, yield stress, or storage or loss moduli. It is evident that this influences thermal conduction properties as well, as it has been also demonstrated. This shows that nanostructure surface functionalization can be a useful strategy to tune nanofluid thermophysical properties.