Out-of-plane and in-plane actuation effects on atomic-scale friction

The influence of out-of-plane and in-plane contact vibrations and temperature on the friction force acting on a sharp tip elastically pulled on a crystal surface is studied using a generalized Prandtl- Tomlinson model. The average friction force is significantly lowered in a frequency range determined by the "washboard" frequency of the stick-slip motion and the viscous damping accompanying the tip motion. An approximately linear relation between the actuation amplitude and the effective corrugation of the surface potential is derived in the case of in-plane actuation, extending a similar conclusion for out-of-plane actuation. Temperature causes an additional friction reduction with a scaling relation in formal agreement with the predictions of reaction rate theory in absence of contact vibrations. In this case the actuation effects can be described by the effective energy or, more accurately, by introducing an effective temperature.Comment: To appear in Physical Review

Lateral vibration effects in atomic-scale friction

The influence of lateral vibrations on the stick-slip motion of a nanotip elastically pulled on a flat crystal surface is studied by atomic force microscopy (AFM) measurements on a NaCl(001) surface in ultra-high vacuum. The slippage of the nanotip across the crystal lattice is anticipated at increasing driving amplitude, similarly to what is observed in presence of normal vibrations. This lowers the average friction force, as explained by the Prandtl-Tomlinson model with lateral vibrations superimposed at finite temperature. Nevertheless, the peak values of the lateral force, and the total energy losses, are expected to increase with the excitation amplitude, which may limit the practical relevance of this effect.Comment: To appear in Applied Physics Letter

Anisotropy effects and friction maps in the framework of the 2d PT model

We present a series of numerical simulations on the friction-anisotropy behavior and stick-slip dynamics of a point mass in the framework of a 2d Prandtl-Tomlinson model. Results for three representative surface lattice are shown: square, hexagonal and honeycomb. Curves for scan angle dependence of static friction force, and kinetic one at T=0 K and T=300 K are shown. Friction force maps are computed at different directions.The authors acknowledge financial support from Spanish MINECO through Project no. FIS2011-25167, cofinanced by FEDER funds and Project no. MAT2012-34487. O.Y.F. acknowledges financial support from FPU grant by Ministerio de Ciencia e Inovación of Spain.Peer Reviewe

Co supported on N and S dual-doped reduced graphene oxide as highly active oxygen-reduction catalyst for direct ethanol fuel cells

Oxygen reduction reaction (ORR) is one of the key features for the efficient functioning of several energy conversion devices such as fuel cells, appearing the necessity of development of new low-cost catalyst materials. Heteroatom-doped carbon materials have attracted attention in this field due to its physicochemical and electronic properties. In this work, a nitrogen and sulfur doped material with anchored Co3O4 nanoparticles (Co/SN-rGO) is proposed as cathode catalyst for direct ethanol fuel cells (DEFCs) and results are compared with different doped graphene nanomaterials (GMs). The effect of the heteroatoms and cobalt oxide nanoparticles in the final efficiency was studied. Synthesized materials were characterized and the activity of Co/SN-rGO and GMs for the ORR was studied. Co/SN-rGO presents high ORR performance in terms of onset potential (Eonset), 0.86 V (vs RHE) and half-wave potential (E1/2) 0.72 V (vs RHE). Tafel analysis shows 60 mV dec-1 at low overpotential for potential dependent ORR mechanism. Besides, when Co/SN-rGO performance is evaluated in a DEFC using a fuel cell test station, main results indicate higher catalytic activity, stability, and ethanol tolerance of Co/SN-rGO in comparison to a carbon-supported Pt catalystThis work has been developed in the framework of the projects PID2020-117586RB-100, PID2020-112594RB-C33, PID2020- 116712RBC21 funded by MCIN/AEI/10.13039/501100011033, and ProID2021010098 funded by the Gobierno de Canarias (FEDER). S. Fajardo acknowledge the MCIN for the pre-doctoral grant (PRE2018- 085718). The authors thank SEGAI-ULL for the collaboratio

Effects of direction-specific training interventions on physical performance and inter-limb asymmetries

This study analyzed the effects of two different training programs on functional performance and inter-limb asymmetries in basketball players. Twenty-four elite youth basketball players were randomly assigned to a training program including variable unilateral horizontal movements (VUH, n = 12) or unilateral lateral movements (VUL, n = 12). Eccentric-overload training (EOT) was performed twice a week for a six-week period. Functional performance assessment included a countermovement jump test, unilateral multidirectional jumping tests (i.e., lateral, horizontal, and vertical), a rebound jump test, a limb symmetry index, a 25 m linear sprint test, and several change of direction (COD) tests. Within-group analysis showed substantial improvements in almost all functional tests in both groups (ES = 0.35−0.89). Furthermore, almost all jumping asymmetries were improved in both groups (ES = 0.38−0.69) except for vertical jumping asymmetry in VUL (ES = −0.04). Between-group analyses showed a substantial and possibly better performance in vertical jumping asymmetry and 5 m in VUH compared to that of VUL, respectively. In contrast, lateral jumping with left (ES = 1.22) and right leg (ES = 0.49) were substantially greater in VUL than in VUH. Specific force-vector training programs induced substantial improvements in both functional performance tests and inter-limb asymmetries, although greater improvements of lateral and horizontal variables may depend on the specific force vector targeted

Plant height and hydraulic vulnerability to drought and cold

Understanding how plants survive drought and cold is increasingly important as plants worldwide experience dieback with drought in moist places and grow taller with warming in cold ones. Crucial in plant climate adaptation are the diameters of water-transporting conduits. Sampling 537 species across climate zones dominated by angiosperms, we find that plant size is unambiguously the main driver of conduit diameter variation. And because taller plants have wider conduits, and wider conduits within species are more vulnerable to conduction-blocking embolisms, taller conspecifics should be more vulnerable than shorter ones, a prediction we confirm with a plantation experiment. As a result, maximum plant size should be short under drought and cold, which cause embolism, or increase if these pressures relax. That conduit diameter and embolism vulnerability are inseparably related to plant size helps explain why factors that interact with conduit diameter, such as drought or warming, are altering plant heights worldwide

Characterization and genetic diversity between guarana progenies.

Caracterização e diversidade genética entre progênies do guaraná: o objetivo do estudo foi estimar a diversidade genética, estimar componentes de variância para caracteres morfoagronômicos, e da produção de progênies de guaranazeiro

Biological and molecular characterization of two Brazilian isolates of Apple stem grooving virus.

Apple stem grooving virus (ASGV, genus Capillovirus) is disseminated worldwide, usually causing a latent infection in most commercial apple cultivars. However, infected scions grafted onto sensitive material display reduction of yield, loss of fruit quality and tree decline. In Brazil ASGV is associated with severe phloem necrosis, xylem pitting and decline of apple trees on Maruba-kaido (Malus prunifolia cv. Ringo) rootstocks usually in a complex with other latent viruses. Two Brazilian ASGV isolates from a mixed infection causing differing reactions on apple cv. Fuji on Maruba-kaido rootstocks, i.e., normal growth vs. tree decline, have been completely sequenced. The differing intensity of symptoms they co-induced on several woody indicator species/cultivars are indicative of biological differences between the two inocula. Their nucleotide sequences are 92.2% identical and display between 79.2% and 97.1% identity with other ASGV isolates available in GenBank. The two isolates (M219-3 and M220) shared 92.3% deduced amino acid (daa) sequence identity for ORF1, 95.3% for ORF2 and 93.3% for the CP gene. A high degree of daa identity, 95.8% and 96.6%, was ob- served between the CPs of M219 ? 3 and M220, respectively, with the Brazilian isolate UV01. Potential recombination events and phy logenetic relationships to major parents were investigated in isolates M219 ? 3and M220. Keywords ASGV . Apple virus . Phylogenetic analysis . Recombinatio

Differences in temperature sensitivity and drought recovery between natural stands and plantations of conifers are species-specific

Forests are being impacted by climate and land-use changes which have altered their productivity and growth. Understanding how tree growth responds to climate in natural and planted stands may provide valuable information to prepare management in sight of climate change. Plantations are expected to show higher sensitivity to climate and lower post-drought resilience than natural stands, due to their lower compositional and structural diversity. We reconstructed and compared the radial growth of six conifers with contrasting ecological and climatic niches (Abies pinsapo, Cedrus atlantica, Pinus sylvestris, Pinus nigra, Pinus pinea, Pinus pinaster) in natural and planted stands subjected to seasonal drought in 40 sites. We quantified the relationships between individual growth variability and climate variables (temperature, precipitation and the SPEI drought index), as well as post-drought resilience. Elevated precipitation during the previous autumn-winter and current spring to early summer enhanced growth in both natural and planted stands of all species. Temperature effects on growth were less consistent: only plantations of A. pinsapo, C. atlantica, P. nigra, P. pinea, P. sylvetris and a natural stand of P. nigra showed negative impacts of summer temperature on growth. Drought reduced growth of all species in both plantations and natural stands, with variations in the temporal scale of the response. Drought constrained growth more severely in natural stands than in plantations of C. atlantica, P. pinaster and P. nigra, whereas the inverse pattern was found for A. pinsapo. Resilience to drought varied between species: natural stands of A. pinsapo, C. atlantica and P. pinaster recovered faster than plantations, while P. pinea plantations recovered faster than natural stands. Overall, plantations did not consistently show a higher sensitivity to climate and a lower capacity to recover after drought. Therefore, plantations are potential tools for mitigating climate warming. © 2021 The AuthorsThis study was supported by project FORMAL ( RTI2018-096884-B-C31 ) from the Spanish Ministry of Science, Innovation and Universities . GS-B was supported by a Spanish Ministry of Economy, Industry and Competitiveness Postdoctoral grant ( IJC2019-040571-I ; FEDER funds)

Propuesta de alternativas para la gestión del suelo de equipamientos educativos, en la localidad de Bosa, Distrito Capital

Propuesta de alternativas que gestionen la consecución del suelo para proyectos urbanísticos y arquitectónicos del nuevo equipamiento educativo en la localidad de Bosa del Distrito Capital.La construcción de nuevos colegios públicos en Bogotá ha sido una preocupación de todas las administraciones. El aumento de la población impone retos como la reducción del déficit de cupos en algunas zonas y acercar las instituciones para evitar la deserción. Todo bajo una nueva necesidad: estandarizar la jornada única. Pasar más tiempo en la escuela trae significativos beneficios de orden social: disminuye el embarazo adolescente y permite que los niños permanezcan más tiempo en un espacio sano y seguro que los protege de riesgos como la criminalidad, el consumo de sustancias psicoactivas y la deserción escolar. También, padres y madres pueden obtener y conservar trabajos estables, con la tranquilidad de que sus hijos se encuentran seguros; esto contribuye a un mejoramiento en el ingreso y la calidad de vida de las familias. Según la última entrega del estudio “Bogotá cómo vamos”, la deserción escolar en la ciudad alcanza el 3 %. Y una de las razones por las que los menores dejan sus estudios es porque la institución queda lejos de su casa