Tribological variable-friction coefficient models for the simulation of dense suspensions of rough polydisperse particles

The rheology of concentrated suspensions of particles is complex and typically exhibits a shear-thickening behavior in the case of repulsive interactions. Despite the recent interest arisen, the causes of the shear-thickening remain unclear. Frictional contacts have been able to explain the discontinuous shear thickening in simulations. However, the interparticle friction coefficient is considered to be constant in most simulations and theoretical works reported to date despite the fact that tribological experiments demonstrate that the friction coefficient can not only be constant (boundary regime) but also decrease (mixed regime) or even increase (full-film lubrication regime), depending on the normal force and the relative velocity between the particles and the interstitial liquid between them. Interestingly, the transition between the boundary regime and the full-lubrication regime is governed by the particle average roughness. Particle-level simulations of suspensions of hard spheres were carried out using short-range lubrication and roughness-dependent frictional forces describing the full Stribeck curve. Suspensions with different particle's roughness were simulated to show that the particle roughness is a key factor in the shear-thickening behavior; for sufficiently rough particles, the suspension exhibits a remarkable shear-thickening, while for sufficiently smooth particles, the discontinuous shear-thickening disappears

Observation of a topologically protected state in a magnetic domain wall stabilized by a ferromagnetic chemical barrier

The precise control and stabilization of magnetic domain walls is key for the development of the next generation magnetic nano-devices. Among the multitude of magnetic configurations of a magnetic domain wall, topologically protected states are of particular interest due to their intrinsic stability. In this work, using XMCD-PEEM, we have observed a topologically protected magnetic domain wall in a ferromagnetic cylindrical nanowire. Its structure is stabilized by periodic sharp alterations of the chemical composition in the nanowire. The large stability of this topologically protected domain wall contrasts with the mobility of other non-protected and non-chiral states also present in the same nanowire. The micromagnetic simulations show the structure and the conditions required to find the topologically protected state. These results are relevant for the design of future spintronic devices such as domain wall based RF oscillators or magnetic memories

Caracterización morfológica por microscopía electrónica de barrido de nanocelulosas de cáscara de sandía (Citrullus lanatus)

La cáscara de sandía (Citrullus lanatus) es un residuo agrícola abundante, disponible y muy poco utilizable. Los objetivos fueron aislar la nanocelulosa a partir de la corteza de sandía y obtener sus imágenes de morfología a través de microscopía electrónica de barrido (SEM). Primero, se realizó un tratamiento alcalino con hidróxido de potasio al 5% (p/v); segundo, las fibras fueron blanqueadas con tampón de acetato glacial y clorito de sodio a 80 °C durante 4 horas; y finalmente se realizó la hidrólisis con ácido sulfúrico a diferentes concentraciones (50%, 60% y 70%, v/v) y tiempos de hidrólisis (20, 30 y 40 minutos), a 45 °C en agitación constante. Por último, las nanocelulosas obtenidas fueron observadas a través del SEM. El contenido de lignina, celulosa, hemicelulosa, ceniza y humedad de la harina de cáscara de sandía fueron 10,00 ± 0,48%; 46,00 ± 0,15%; 23,00 ± 0,85%; 15,00 ± 0,26% y 10,30 ± 0,70%, respectivamente. El máximo rendimiento obtenido de nanocelulosa fue 64,96%, a 60% de ácido sulfúrico y 30 minutos de hidrólisis. La morfología de las nanocelulosas se observó mejor a 200 μm que a 100 μm. Las fibras de nanocelulosa estuvieron más expuestas y mejor distribuidas a 200 μm

Effective interactions of colloids on nematic films

The elastic and capillary interactions between a pair of colloidal particles trapped on top of a nematic film are studied theoretically for large separations $d$. The elastic interaction is repulsive and of quadrupolar type, varying as $d^{-5}$. For macroscopically thick films, the capillary interaction is likewise repulsive and proportional to $d^{-5}$ as a consequence of mechanical isolation of the system comprised of the colloids and the interface. A finite film thickness introduces a nonvanishing force on the system (exerted by the substrate supporting the film) leading to logarithmically varying capillary attractions. However, their strength turns out to be too small to be of importance for the recently observed pattern formation of colloidal droplets on nematic films.Comment: 13 pages, accepted by EPJ

Magnetic domains on magnetite islands: from XMCD-PEEM to micromagnetism

Oral presentation given at the 13th European Conference on Surface Crystallography and Dynamics, held in Donostia-San Sebastián, Spain, on June 19-21th, 2017.Magnetite nanostructures and thin films have been grown in spintronic devices such as spin valves in order to take advantage of the high Curie temperature, stability, and predicted half-metal character. However, thin films present magnetic properties which are rather different from the properties of bulk magnetite: high coercive fields, high saturation fields, out-of-plane magnetization, superparamagnetism in ultrathin films, or unexpected easy-axes. An explanation for these effects are growth defects, among which antiphase domain boundaries (APBs) are the best example. In the present work, we study the magnetic domains on flat single-crystal magnetite and other mixed spinels grown on Ru(0001) by molecular beam epitaxy [1,2]. As each island grows from a single nucleus, there are expected to be free of APBs. We have measured with nanometer-resolution the 3D magnetization of the islands by combining x-ray magnetic circular dichroism images acquired in a photoemission electron microscope at different azimuthal angles. The 3D magnetization maps have been used as the initial magnetization configuration for micromagnetic simulations of islands with the same lateral and vertical dimensions as the experimental ones. The Mumax3 software has been used to perform the micromagnetic simulations. By comparing the evolution of the micromagnetic simulations with the experimental behavior of the islands after annealing, we seek to validate the material parameters that define their magnetic behavior and to identify cases where defects or other effects play a role

Real space observation of the magnetic coupling between a Co film and a barium hexaferrite film

RIVA ONLINE 2021 – IBERIAN VACUUM ONLINE MEETING. The Iberian Vacuum Conference, (Reunión Ibérica de Vacío, RIVA) is a joint meeting of the Portuguese Vacuum Society (SOPORVAC) and the Spanish Vacuum Society (ASEVA), 2021 RIVA will take place ON-LINE from 4-6th October 2021. .-https://aseva.es/conferences/riva-online/Barium ferrite (BaFe12O19, BFO) is a hexagonal ferrite with applications as permanent magnet in many different devices due to its high magnetocrystalline anisotropy, high coercive field and low cost. However, the moderate saturation magnetization of BFO means that the energy product is orders of magnitude smaller than the one that rare-earth-based magnetic materials offer. To overcome this limitation, a commonly proposed strategy to enhance the energy product is exchange-coupling the magnetically hard component (BFO) with a soft phase in order to improve the combined remanent magnetization without a high loss in coercivity. Nonetheless, the results obtained in other hard/soft systems (SFO/Co bilayers) have pointed out the difficulty to take advantage of this rigid coupling magnetic regime1. In this research, we focus on two steps to investigate the Co/BFO coupling in a bilayer system: first, we sought to obtain BFO films with an in- plane magnetic easy axis to avoid shape anisotropy competition, and second, we deposit Co on top of such a BFO film while monitoring both the BFO and Co magnetic domains

FeCo Nanowire-Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products

Due to the issues associated with rare-earth elements, there arises a strong need for magnets with properties between those of ferrites and rare-earth magnets that could substitute the latter in selected applications. Here, we produce a high remanent magnetization composite bonded magnet by mixing FeCo nanowire powders with hexaferrite particles. In the first step, metallic nanowires with diameters between 30 and 100 nm and length of at least 2 {\mu}m are fabricated by electrodeposition. The oriented as-synthesized nanowires show remanence ratios above 0.76 and coercivities above 199 kA/m and resist core oxidation up to 300 {\deg}C due to the existence of a > 8 nm thin oxide passivating shell. In the second step, a composite powder is fabricated by mixing the nanowires with hexaferrite particles. After the optimal nanowire diameter and composite composition are selected, a bonded magnet is produced. The resulting magnet presents a 20% increase in remanence and an enhancement of the energy product of 48% with respect to a pure hexaferrite (strontium ferrite) magnet. These results put nanowire-ferrite composites at the forefront as candidate materials for alternative magnets for substitution of rare earths in applications that operate with moderate magnet performance

Composición proximal y propiedades funcionales del surimi liofilizado de Dosidicus gigas “calamar gigante”

The purpose of the research was to determinate proximal composition and functional properties of the giant squid freeze-dried surimi (Dosidicus gigas). Surimi elaborated from giant squid and it lyophilized until obtaining surimi powder to evaluate its characteristics and functional properties. The protein content was 58.7% and carbohydrate 30.5%. Protein solubility in water and salt (3%) were 21.1% and 40.6% respectively, these values being higher than surimi powder of commercial species. The gelling capacity was 2.4% and the emulsifying capacity was 79.9% at a concentration of 1.0%. The color on the Hunter scale was L*: 91.5; a*: 0.5; b*: 7.0. Giant squid powder surimi considered as a functional protein powder due to its percentage protein content, and had good technological characteristics and great potential in the food industry.El propósito de la investigación fue determinar la composición proximal y propiedades funcionales del surimi liofilizado de calamar gigante (Dosidicus gigas). Se elaboró surimi a partir de calamar gigante y fue liofilizado hasta obtener surimi en polvo para evaluar sus características y propiedades funcionales. El contenido de proteínas fue 58,7% y carbohidratos 30,5%. La solubilidad proteica en agua y sal (3%) fueron 21,1% y 40,6% respectivamente, siendo estos valores superiores a surimi en polvo de especies comerciales. La capacidad de gelificación fue 2,4% y la capacidad emulsificante fue 79,9% a una concentración del 1,0%. El color en la escala de Hunter fue L*: 91,5; a*: 0,5; b*:7,0. El surimi en polvo de calamar gigante fue considerado como un polvo proteico funcional debido a su contenido porcentual proteico, y tuvieron buenas características tecnológicas y de gran potencial en la industria alimentaria