A new formulation is proposed to lubricate tribopairs in extreme conditions where the amount of lubricant is small and the lubricating region highly confined. It is composed of non-magnetic solid lubricants dispersed in an oil-based ferrofluid. When this inverse ferrofluid (IFF) is magnetically activated, the lubricant particles are subjected to magnetophoretic forces. By using appropriate magnetic field gradients, they can be driven to the region of interest and thus control the friction locally. The rheological and tribological performances of three IFF formulations are evaluated in several conditions of applied magnetic field strength and shear flow rates.

This article analyzes the main modernizing projects promoted by the City Council of Granada constituted under the Municipal Statute of 1924. The City Council cleared up the accounts, paid outstanding debts and set the sewerage, water supply, and water treatment project as a priority. For this, it introduced a new tax collection. The water project, however, generated serious internal conflicts into the power bloc that had led the Marquis of Casablanca to be the Mayor of Granada. That project began at the end of 1923 and culminated with the approval by the majority of one of the two solutions proposed by the Jury in the summer of 1927. The decision generated a political crisis that was resolved with the forced resignation of the ten councilors who did not vote with the Mayor and the Duke of San Pedro de Galatino was fined for disseminating chemical and bacteriological analyzes that questioned the potability of the waters of the upper Genil river basin. The acceptance of the project by the Central Board of Health occurred four years after the approval of the Statute. After overcoming all the challenges presented to the project, the Marquis of Casablanca resigned due to poor health in August 1928.

We simulate the thin-film and frictional properties of shear-thinning and shear-thickening fluids in the isoviscous elastohydrodynamic (I-EHL) regime in point contacts. A double-Newtonian Carreau-Yasuda viscosity model is employed to incorporate the shear rate dependence of the viscosity. Regression formulas for the film thickness and friction coefficient are proposed for both non-Newtonian fluids. A master curve is proposed by using the product between the entrainment speed and the effective viscosity in the contact. Numerical results are satisfactorily compared with experimental data in xantham gum solutions and fumed silica suspensions in the form of Stribeck curves.

The behavior of complex fluids in thin films under the isoviscous elastohydrodynamic lubrication regime is a topic of current interest. A vast majority of the materials used to lubricate compliant contacts are shear-thinning fluids and, in comparison, very little is known about the lubrication properties of shear-thickening (ST) fluids. In this work we carried out both rheological and tribological experiments on ST fluids based on cornstarch suspensions. We investigated the influence of the polarity and viscosity of the carrier fluid and the cornstarch particles' concentration. From a rheological perspective, the ST response is diminished (as the shear-thinning region enlarges) when non-polar carriers are used (e.g. silicone oil) while a marked thickening is observed for polar carriers (e.g. glycerol-water mixtures). From a tribological perspective, the friction coefficient at slow sliding speeds is strongly affected by the nature of the carrier fluid, and in particular its wettability in contact with hydrophobic PDMS tribopairs. We demonstrate that experimental data, plotted in terms of Stribeck curves, provide useful information on the entrainment of particles/fluid through the contact. Finally, experimental results are also compared with numerical solutions of the Reynolds equation for inelastic non-Newtonian fluids in the isoviscous elastohydrodynamic regime. A Carreau-Yasuda constitutive equation for the viscosity-shear rate relationship is used.

espanolEste trabajo se ocupa de la trayectoria politica del catedratico de Quimica General de la Universidad de Granada, D. Jesus Yoldi Bereau, quien desde el 27 de abril hasta el 23 septiembre de 1932 fue alcalde de la ciudad de Granada. Al ser nombrado alcalde sintetizo su programa en el lema: “Dinero, trabajo, sanidad y cultura”. Aqui se describen los logros conseguidos y las dificultades que el alcalde encontro para desarrollar el lema de su programa de gobierno. Como alcalde tuvo que representar la posicion de Granada ante el Estatuto andaluz y la conveniencia de formar parte de la Mancomunidad Andaluza. Formo parte del comite ejecutivo del PRAG y posteriormente fue afiliado de Accion Republicana y Presidente de la junta local de Izquierda Republicana en Granada en 1935. El 23 de octubre de 1936 fue fusilado contra las tapias del cementerio granadino EnglishThis paper deals with the political career of Jesus Yoldi Bereau, professor of Chemistry at the University of Granada, who from April 27 to September 27, 1932 was the Mayor of Granada. When Jesus Yoldi was appointed as mayor of the city, he summed up his political program under the motto: “Money, work, health and culture”. Here we describe his accomplishments as well as the difficulties he had to confront in his attempt to develop the motto of his plan for governing Granada. As mayor, he had to represent Granada´s political position in relation to the Statute of Andalusia and the advantages of belonging to the Andalusian Community. Jesus Yoldi was a member of the executive committee of PRAG and later became affiliated to Republican Action and was elected President of the local board of the Republican Left in Granada in 1935. On October 23, 1936 Jesus Yoldi was executed against the wall of the cemetery of Granada

We demonstrate that inverse ferrofluids behave as model magnetorheological fluids. A universal master curve is proposed, using a reduced Mason number, under the frame of a structural viscosity model where the magnetic field strength dependence is solely contained in the Mason number and the particle concentration is solely contained in the critical Mason number (i.e. the yield stress). A linear dependence of the critical Mason number with the particle concentration is observed that is in good agreement with a mean (average) magnetization approximation, particle level dynamic simulations and micromechanical models available in the literature.

Particles adsorbed at liquid interfaces are commonly used to stabilise water-oil Pickering emulsions and water-air foams. The fundamental understanding of the physics of particles adsorbed at water-air and water-oil interfaces is improving significantly due to novel techniques that enable the measurement of the contact angle of individual particles at a given interface. The case of non-aqueous interfaces and emulsions is less studied in the literature. Non-aqueous liquid-liquid interfaces in which water is replaced by other polar solvents have properties similar to those of water-oil interfaces. Nanocomposites of non-aqueous immiscible polymer blends containing inorganic particles at the interface are of great interest industrially and consequently more work has been devoted to them. By contrast, the behaviour of particles adsorbed at oil-oil interfaces in which both oils are immiscible and of low dielectric constant (ε<3) is scarcely studied. Hydrophobic particles are required to stabilise these oil-oil emulsions due to their irreversible adsorption, high interfacial activity and elastic shell behaviour.

Since de Gennes coined in 1992 the term Janus particle (JP), there has been a continued effort to develop this field. The purpose of this review is to present the most relevant theoretical and experimental results obtained so far on the surface activity of amphiphilic JPs at fluid interfaces. The surface activity of JPs at fluid–fluid interfaces can be experimentally determined using two different methods: the classical Langmuir balance or the pendant drop tensiometry. The second method requires much less amount of sample than the first one, but it has also some experimental limitations. In all cases collected here the JPs exhibited a higher surface or interfacial activity than the corresponding homogeneous particles. This reveals the significant advantage of JPs for the stabilization of emulsions and foams.

Faceted particles have been used to prepare dilute magnetorheological (MR) fluids with enhanced aggregate strength. The measured storage modulus of these suspensions is significantly larger than that of the MR fluids prepared with spherical particles, and comparable to that of the rod-based fluids, whereas no sign of formation of a percolated system was observed at the largest concentration we studied (5 vol. %). Finite element method calculations confirm that the more intimate surface contacts between faceted particles lead to larger magnetic interparticle forces than the point contacts associated with the spherical particles. The contribution of friction is expected to be significant but remains unknown.

Janus gold nanoparticles (JPs) of ∼4 nm-diameter half functionalized with 1-hexanethiol as a hydrophobic capping ligand exhibit significantly higher interfacial activity, reproducibility and rheological response when the other half is functionalized with 1,2-mercaptopropanediol (JPs-MPD) than with 2-(2-mercaptoethoxy)ethanol (JPs-MEE), both acting as hydrophilic capping ligands. The interfacial pressure measured by pendant drop tensiometry reaches 50 mN m(-1) and 35 mN m(-1) for the JPs-MPD at the water/air and water/decane interface, respectively. At the same area per particle, the JPs-MEE reveal significantly lower interfacial pressure: 15 mN m(-1) and 5 mN m(-1) at the water/air and water/decane interface, respectively. Interfacial dilatational rheology measurements also show an elastic shell behaviour at higher compression states for JPs-MPD while the JPs-MEE present near-zero elasticity. The enhanced interfacial activity of JPs-MPD is explained in terms of chemical and hydration differences between the MPD and MEE ligands, where MPD has a shorter hydrocarbon chain and twice as many hydroxyl terminal groups as MEE.

We propose a micromechanical model for the behavior of dilute magnetorheological fluids under unidirectional slow-compression, constant-volume squeeze flow mode. In the linear magnetization regime, the model predicts a power law scaling of the normal stress with the particle volume fraction and magnetic field strength squared at low fields. The predictions are satisfactorily compared with experimental measurements for different particle loadings, sample volume, surface roughness, and initial gap distance.

We propose a new versatile approach for the fabrication of flexible magnetic microfibers having tunable size and mechanical properties. The methodology is based on the magnetic field-driven self-assembly of agarose-loaded kinetically unstable aqueous ferrofluid-in-oil emulsions. Agarose is dissolved at a temperature above its gelling point in an aqueous ferrofluid and the resulting suspension is then emulsified in oil and later exposed to a DC uniaxial magnetic field. Once the fibers have formed, the emulsion is quenched to force the gelification of the agarose, and a second addition of surfactant is carried out to prevent lateral aggregation of the fibers. Resulting fibers have diameters of the order of a few to tens of micrometers. A phase diagram is constructed and the influence of surfactant concentration and residence time under field in the synthesis is discussed.

We report a finite element method study on the effect of surface roughness on the field-induced magnetization of micrometric iron particles and on the interparticle magnetostatic forces between them. Calculations were carried out for two-dimensional geometries in which particles were modelled as discs. Roughness was introduced as semicircular protrusions or as triangular- or square-wave profiles. Interestingly, we found that increasing amplitudes of the triangular- or square-wave profiles facilitated the magnetization of the particles, resulting in larger interparticle forces at fields below saturation. The effect of the semicircular protrusions and of the spatial frequency of the wave profiles was comparatively small, suggesting that in real systems the effect of particle roughness on magnetic properties may depend on the specific surface morphology. The permeability of the particles also influenced the extent to which roughness facilitated the magnetization process: a larger permeability resulted in larger differences between the magnetization curves of the smooth and the rough particles. Results are relevant to magnetorheological fluids, since we show that surface roughness can affect the magnetic interactions between particles.

An extensive experimental and simulation study is carried out in conventional magnetorheological fluids formulated by dispersion of mixtures of carbonyl iron particles having different sizes in Newtonian carriers. Apparent yield stress data are reported for a wide range of polydispersity indexes (PDI) from PDI = 1.63 to PDI = 3.31, which for a log-normal distribution corresponds to the standard deviation ranging from ν = 0.38 to ν = 0.76. These results demonstrate that the effect of polydispersity is negligible in this range in spite of exhibiting very different microstructures. Experimental data in the magnetic saturation regime are in quantitative good agreement with particle-level simulations under the assumption of dipolar magnetostatic forces. The insensitivity of the yield stresses to the polydispersity can be understood from the interplay between the particle cluster size distribution and the packing density of particles inside the clusters.

Polymethylmethacrylate/poly-tert-butylmethacrylate Janus nanoparticles were synthesized by the electrohydrodynamic co-jetting method. The Janus character of these nanoparticles was visualized through super-resolution imaging with Structured Illumination Microscopy. The Janus nanoparticles, and the corresponding homogeneous sets, were then morphologically characterized to assess their size, distribution, and concentrations. All nanoparticles presented high interfacial activity as measured by pendant drop tensiometry at water/decane interfaces. At high concentrations and compression states, the Janus nanoparticles exhibited higher interfacial activity than the homogeneous nanoparticles. This is in agreement with theoretical and experimental works in which Janus nanoparticles present higher interfacial activity than homogeneous nanoparticles. (C) 2016 Elsevier B.V. All rights reserved.

We investigate model magnetorheological (MR) fluids (inverse ferrofluids) under both steady and dynamic oscillatory shear. Analytical theories, particle-level simulations, and magnetorheometry are used in an attempt to obtain universal master curves. Steady shear flow data can be collapsed when plotted as a function of a dimensionless Mason number. The critical Mason number associated with the transition from magnetostatic to hydrodynamic control of the suspension structure is demonstrated to linearly increase with particle concentration which is in good agreement with theories and our simulations. Experimental linear viscoelastic moduli are in good agreement with micromechanical and macroscopic models in the dilute regime. However, upon increasing particle concentration, theoretical predictions underestimate experimental data while particle-level simulations are in good agreement. The accordance with particle-level simulations suggests that the mean (average) magnetization approximation gives a good prediction and multibody and hydrodynamic forces are not expected to play a crucial role in the shear flow behavior of model MR fluids. (C) 2016 The Society of Rheology.

Gold patchy nanoparticles (PPs) were prepared under surfactant-free conditions by functionalization with a binary ligand mixture of polystyrene and poly(ethylene glycol) (PEG) as hydrophobic and hydrophilic ligands, respectively. The interfacial activity of PPs was compared to that of homogeneous hydrophilic nanoparticles (HPs), fully functionalized with PEG, by means of pendant drop tensiometry at water/air and water/decane interfaces. We compared interfacial activities in three different spreading agents: water, water/chloroform, and pure chloroform. We found that the interfacial activity of PPs was close to zero (∼2 mN/m) when the spreading agent was water and increased to ∼14 mN/m when the spreading agent was water/chloroform. When the nanoparticles were deposited with pure chloroform, the interfacial activity reached up to 60 mN/m by compression. In all cases, PPs exhibited higher interfacial activity than HPs, which were not interfacially active, regardless of the spreading agent. The interfacial activity at the water/decane interface was found to be significantly lower than that at the water/air interface because PPs aggregate in decane. Interfacial dilatational rheology showed that PPs form a stronger elastic shell at the pendant drop interface, compared to HPs. The significantly high interfacial activity obtained with PPs in this study highlights the importance of the polymeric patchy shell and the spreading agent.

Irreversible adsorption of particles onto a flat surface as a consequence of sedimentation colloidal suspension has been studied by two simulation techniques: Brownian dynamics (BD) and stochastic rotation dynamics (SRD). The purpose of using both methods is to investigate the effect of hydrodynamic interactions on adsorption kinetics and structure of the first monolayer of sediment obtained from the sedimentation of a concentrated and monodisperse colloidal suspension. Three systems were studied, characterised by the Péclet numbers: 0.1, 1.0 and 10. To physically understand the kinetic behaviour, simulation results were analysed using a kinetic model based on chemical reactions. High values of jamming limit (θ∞ > 0.61) were obtained for both simulation techniques, with the SRD showing the highest figures (0.631) due to the hydrodynamics effect that takes into account the fluid backflow produced on particle sedimentation. A two-step adsorption mechanism was proposed based on the observed kinetic behaviour.

A simulation method is proposed to explore the effect of particle size polydispersity in magnetorheology including Brownian motion. The method aims to extend the classical particle-level simulation methodology developed by Klingenberg et al. [J. Chem. Phys. 91, 7888–7895 (1989)] for the case of polydisperse magnetorheological (MR) fluids. The simulation study concerns the aggregation kinetics at rest as well as the rheological behavior under start-up of steady shear and dynamic oscillatory shear tests at increasing strain amplitudes. Results demonstrate that the effect of polydispersity is only relevant at the transition regime between magnetostatic to hydrodynamic control of the suspension structure. The yielding behavior is correlated to the structural characteristics (radial distribution functions, pair correlation functions, and angular connectivities) of the MR fluids before the onset of flow. A more abrupt transition is observed for polydisperse MR fluids because interparticle links are weaker in this case if compared to monodisperse suspensions in spite of the fact that polydisperse MR fluids exhibit a larger connectivity.