Surface defects and temperature on atomic friction

We present a theoretical study of the effect of surface defects on atomic friction in the stick-slip dynamical regime of a minimalistic model. We focus on how the presence of defects and temperature change the average properties of the system. We have identified two main mechanisms which modify the mean friction force of the system when defects are considered. As expected, defects change locally the potential profile and thus affect the friction force. But the presence of defects also changes the probability distribution function of the tip slip length and thus the mean friction force. We corroborated both effects for different values of temperature, external load, dragging velocity and damping. We show also a comparison of the effects of surface defects and surface disorder on the dynamics of the system

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

Cernuda in Current Spanish Poetry

The poet Luis Cernuda (Spain, 1902-Mexico, 1963) has left his mark on much of the poetry written in Spain since the sixties. First rediscovered in the Peninsula in the late fifties and early sixties by, among others, Francisco Brines, José Angel Valente, and Jaime Gil de Biedma, his influence became pervasive both through the work of these poets, and, through the reading of Cernuda’s poetry itself, available since 1975 in Harris and Maristany edition. Referring in particular to Biedma, whose impact on younger poets has been significant, this paper examines the presence of Cernuda in certain approaches to language and reality in the poetry of several “poetas de la experiencia” ‘poets of experience,’ such as Jesús García Montero, Felipe Benítez Reyes, and Álvaro García. Centering mainly on the simplification of language and the search for a non-rhetorical rhythm, developing in Cernuda from Invocaciones ‘Invocations,’ to Desolación de la Quimera ‘The Disconsolate Chimera,’ this article examines the same traits in Biedma. Thereafter it traces their incorporation in the poetry of García Montero, Benítez Reyes, and García. These readings offer an occasion to reflect on some of the strengths of the “poesía de la experiencia” that underlie its apparent straightforwardness and simplicity

Molecular Mechanisms Involved in Neural Substructure Development during Phosphodiesterase Inhibitor Treatment of Mesenchymal Stem Cells.

Stem cells are highly important in biology due to their unique innate ability to self-renew and differentiate into other specialised cells. In a neurological context, treating major injuries such as traumatic brain injury, spinal cord injury and stroke is a strong basis for research in this area. Mesenchymal stem cells (MSC) are a strong candidate because of their accessibility, compatibility if autologous, high yield and multipotency with a potential to generate neural cells. With the use of small-molecule chemicals, the neural induction of stem cells may occur within minutes or hours. Isobutylmethyl xanthine (IBMX) has been widely used in cocktails to induce neural differentiation. However, the key molecular mechanisms it instigates in the process are largely unknown. In this study we showed that IBMX-treated mesenchymal stem cells induced differentiation within 24 h with the unique expression of several key proteins such as Adapter protein crk, hypoxanthine-guanine phosphoribosyltransferase, DNA topoisomerase 2-beta and Cell division protein kinase 5 (CDK5), vital in linking signalling pathways. Furthermore, the increased expression of basic fibroblast growth factor in treated cells promotes phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase (MAPK) cascades and GTPase-Hras interactions. Bioinformatic and pathway analyses revealed upregulation in expression and an increase in the number of proteins with biological ontologies related to neural development and substructure formation. These findings enhance the understanding of the utility of IBMX in MSC neural differentiation and its involvement in neurite substructure development

Determination of the optimal range of the compressor inlet air temperature in a power plant with stig cycle through of advanced exergetic analysis

Conventional exergy analysis identifies the more inefficient components; however, this doesn’t regard interaction between components, neither real improvement potential to each component of the system, this information is providing for the advanced exergy analysis. In this paper was developed an advanced exergy analysis to determine the optimal range of the compressor inlet air temperature, to compensate the power loss in a power plant with Stig cycle and an air cooling system. This plant without cooling system at ISO conditions produce 52 MW, while in local conditions (32 °C, 80%RH) its productions decreases to 44.3MW. The results showed that for every degree centigrade that the air temperature decreases at inlet compressor the power output increases in 0.17 MW and total destroyed exergy increases 0.23 MW. It was determined that for the optimal range of compressor inlet air temperature is between 10 and 12°C; at this range were obtained the highest power output values, and the values of the avoidable and endogenous exergy destroyed are diminished in 0.28 MW and 0.20 MW respectively compared to those given in local operating conditions. Copyright © 2019 ASME

Producing ultrashort, ultraintense plasma-based soft-x-ray laser pulses by high-harmonic seeding.

Simulations show that intense plasma-amplified pulses of 100 fs duration and below are feasible by seeding specifically tailored plasma with an ultrashort pulse of high harmonic radiation. Seeding overcomes gain narrowing by driving amplifying media into saturation earlier, and compensates for reduced gain resulting from boosting the lasing transition linewidth. We conclude that ultrahigh intensities (above 1016 W cm-2) could be reached