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

Critical behavior of ferromagnetic pure and random diluted nanoparticles with competing interactions: variational and Monte Carlo approaches

The magnetic properties and critical behavior of both ferromagnetic pure and metallic nanoparticles having concurrently atomic disorder, dilution and competing interactions, are studied in the framework of an Ising model. We have used both the free energy variational principle based on the Bogoliubov inequality and Monte Carlo simulation. As a case of study for random diluted nanoparticles we have considered the Fe$_{0.5}$Mn$_{0.1}$Al$_{0.4}$ alloy characterized for exhibiting, under bulk conditions, low temperature reentrant spin glass (RSG) behavior and for which experimental and simulation results are available. Our results allow concluding that the variational model is successful in reproducing features of the particle size dependence of the Curie temperature for both pure and random diluted particles. In this last case, low temperature magnetization reduction was consistent with the same type of RSG behavior observed in bulk in accordance with the Almeida-Thouless line at low fields and a linear dependence of the freezing temperature with the reciprocal of the particle diameter was also obtained. Computation of the correlation length critical exponent yielded the values $\nu=0.926\pm 0.004$ via Bogoliubov and$\nu =0.71\pm 0.04$ via Monte Carlo. This fact indicates that even though thermodynamical models can be indeed used in the study of nanostructures and they can reproduce experimental features, special attention must be paid regarding critical behavior. From both approaches, differences in the $\nu$ exponent with respect to the pure Ising model agree with Harris and Fisher arguments.Comment: 11 pages, 11 figures. Submitted to Phys. Rev.

Role of non-coding RNAs in the transgenerational epigenetic transmission of the effects of reprotoxicants

13 p.-1 fig.Non-coding RNAs (ncRNAs) are regulatory elements of gene expression and chromatin structure. Both long and small ncRNAs can also act as inductors and targets of epigenetic programs. Epigenetic patterns can be transmitted from one cell to the daughter cell, but, importantly, also through generations. Diversity of ncRNAs is emerging with new and surprising roles. Functional interactions among ncRNAs and between specific ncRNAs and structural elements of the chromatin are drawing a complex landscape. In this scenario, epigenetic changes induced by environmental stressors, including reprotoxicants, can explain some transgenerationally-transmitted phenotypes in non-Mendelian ways. In this review, we analyze mechanisms of action of reprotoxicants upon different types of ncRNAs and epigenetic modifications causing transgenerationally transmitted characters through germ cells but affecting germ cells and reproductive systems. A functional model of epigenetic mechanisms of transgenerational transmission ncRNAs-mediated is also proposed.This work was supported by grant from MINECO (BFU2013-42164-R), Spain.We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).Peer reviewe

A model for hand-over-hand motion of molecular motors

A simple flashing ratchet model in two dimensions is proposed to simulate the hand-over-hand motion of two head molecular motors like kinesin. Extensive Langevin simulations of the model are performed. Good qualitative agreement with the expected behavior is observed. We discuss different regimes of motion and efficiency depending of model parameters.Comment: 8 pages, Phys. Rev. E (in press

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

Nucleation and detachment of polystyrene nanoparticles from plowing-induced surface wrinkling

We report the formation of spherical particles (up to 250 nm in diameter) from polystyrene surfaces repeatedly scratched by atomic force microscopy nanotips (nominal radius < 10 nm) along a series of parallel lines. The particles nucleate from the crests of the ripple profiles formed in the beginning of the scratch process. They are subsequently detached and progressively displaced by the tip across the ripples till the edge of the scanned area, where they pile up without coalescing. The detachment occurs smoothly without static friction peaks, suggesting that the particles are torn off as a result of a crazing mechanism induced by the tip when it is pushed against the ripple crests. Considering the negative impact of nanoplastics on the environment and human health, and the absence of established methodologies for a quantitative analysis of these processes at the level of single particles, our results will help to promote systematic characterization of plowing wear on different polymeric materials and different environmental conditions. © 2021 The Author(s

Biased random walks on complex networks: the role of local navigation rules

We study the biased random walk process in random uncorrelated networks with arbitrary degree distributions. In our model, the bias is defined by the preferential transition probability, which, in recent years, has been commonly used to study efficiency of different routing protocols in communication networks. We derive exact expressions for the stationary occupation probability, and for the mean transit time between two nodes. The effect of the cyclic search on transit times is also explored. Results presented in this paper give the basis for theoretical treatment of the transport-related problems on complex networks, including quantitative estimation of the critical value of the packet generation rate.Comment: 5 pages (Phys. Rev style), 3 Figure