Dynamical transitions and sliding friction in the two-dimensional Frenkel-Kontorova model

The nonlinear response of an adsorbed layer on a periodic substrate to an external force is studied via a two dimensional uniaxial Frenkel-Kontorova model. The nonequlibrium properties of the model are simulated by Brownian molecular dynamics. Dynamical phase transitions between pinned solid, sliding commensurate and incommensurate solids and hysteresis effects are found that are qualitatively similar to the results for a Lennard-Jones model, thus demonstrating the universal nature of these features.Comment: 11 pages, 12 figures, to appear in Phys. Rev.

Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer

We study the response of an adsorbed monolayer under a driving force as a model of sliding friction phenomena between two crystalline surfaces with a boundary lubrication layer. Using Langevin-dynamics simulation, we determine the nonlinear response in the direction transverse to a high symmetry direction along which the layer is already sliding. We find that below a finite transition temperature, there exist a critical depinning force and hysteresis effects in the transverse response in the dynamical state when the adlayer is sliding smoothly along the longitudinal direction.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let

Rubber friction and the effect of shape

Contrary to the classic laws of friction, rubber friction is not independent of shape. The friction of three shapes of the same rubber compound sliding over a dry-rough surface was measured. The three shapes had the same nominal contact area but different sliding direction-lengths and widths. Frictional differences were found between all three shapes at sliding speeds of 10 mm/s and 0.5 mm/s. The effect of frictional heating and other friction mechanisms that cause these differences are evaluated and discussed

Modelling of intermediate-age stellar populations: III Effects of dust-shells around AGB stars

In this paper,we present single stellar population models of intermediate age stellar populations where dust-enshrouded AGB stars are introduced. The formation of carbon stars is also accounted for, and is taken to be a function of both initial mass and metallicity. The effect of the dusty envelopes around AGB stars on the optical/near-infrared spectral energy distribution were introduced using semi-emipirical models where the mass-loss and the photospheric chemistry determine the spectral properties of a star along the AGB sequence. The spectral dichotomy between O-rich stars and C-rich stars is taken into account in the modelling. We have investigated the AGB sequence morphology in he near-infrared CMD as a function of time and metallicity. We show that this diaggram is characterized by three morphological features, occupied by optically bright O-rich stars, optically bright C-rich stars, and dust-enshrouded O-rich and C-rich stars respectively. Our models are able to reproduce the distribution of the three AGB subtype stellar populations in colour-colour diagrams. Effects of dusty envelopes on the luminosity function are also investigated (Abriged).Comment: 11 pages, 9 figures, accepted for publication in A&

Nonlinear sliding friction of adsorbed overlayers on disordered substrates

We study the response of an adsorbed monolayer on a disordered substrate under a driving force using Brownian molecular-dynamics simulation. We find that the sharp longitudinal and transverse depinning transitions with hysteresis still persist in the presence of weak disorder. However, the transitions are smeared out in the strong disorder limit. The theoretical results here provide a natural explanation for the recent data for the depinning transition of Kr films on gold substrate.Comment: 8 pages, 8 figs, to appear in Phys. Rev.

New search strategy for high z intervening absorbers: GRB021004, a pilot study

We present near-infrared narrow- and broad-band imaging of the field of GRB021004, performed with ISAAC on the UT1 of the ESO Very Large Telescope. The narrow-band filters were chosen to match prominent emission lines at the redshift of the absorption-line systems found against the early-time afterglow of GRB021004: [OIII] at z=1.38 and Halpha at z=1.60, respectively. For the z=1.38 system we find an emission-line source at an impact parameter of 16", which is somewhat larger than the typical impact parameters of a sample of MgII absorbers at redshifts around unity. Assuming that this tentative redshift-identification is correct, the star formation rate of the galaxy is 13 +- 2 Msun/year. Our study reaches star-formation rate limits (5 sigma) of 5.7 Msun/year at z=1.38, and 7.7 Msun/year at z=1.60. These limits correspond to a depth of roughly 0.13 L*. Any galaxy counterpart of the absorbers nearer to the line of sight either has to be fainter than this limit or not be an emission-line source.Comment: 4 pages, 3 figures, accepted for publication in A&A letter

Friction, order, and transverse pinning of a two-dimensional elastic lattice under periodic and impurity potentials

Frictional phenomena of two-dimensional elastic lattices are studied numerically based on a two-dimensional Frenkel-Kontorova model with impurities. It is shown that impurities can assist the depinning. We also investigate anisotropic ordering and transverse pinning effects of sliding lattices, which are characteristic of the moving Bragg glass state and/or transverse glass state. Peculiar velocity dependence of the transverse pinning is observed in the presence of both periodic and random potentials and discussed in the relation with growing order and discommensurate structures.Comment: RevTeX, 4 pages, 5 figures. to appear in Phys. Rev. B Rapid Commu

Dynamics of stick-slip in peeling of an adhesive tape

We investigate the dynamics of peeling of an adhesive tape subjected to a constant pull speed. We derive the equations of motion for the angular speed of the roller tape, the peel angle and the pull force used in earlier investigations using a Lagrangian. Due to the constraint between the pull force, peel angle and the peel force, it falls into the category of differential-algebraic equations requiring an appropriate algorithm for its numerical solution. Using such a scheme, we show that stick-slip jumps emerge in a purely dynamical manner. Our detailed numerical study shows that these set of equations exhibit rich dynamics hitherto not reported. In particular, our analysis shows that inertia has considerable influence on the nature of the dynamics. Following studies in the Portevin-Le Chatelier effect, we suggest a phenomenological peel force function which includes the influence of the pull speed. This reproduces the decreasing nature of the rupture force with the pull speed observed in experiments. This rich dynamics is made transparent by using a set of approximations valid in different regimes of the parameter space. The approximate solutions capture major features of the exact numerical solutions and also produce reasonably accurate values for the various quantities of interest.Comment: 12 pages, 9 figures. Minor modifications as suggested by refere

Global agricultural trade and land system sustainability: Implications for ecosystem carbon storage, biodiversity, and human nutrition

Global land systems are increasingly shaped by international trade of agricultural products. An increasing number of studies have quantified the implications of agricultural trade for single different aspects of land system sustainability. Bringing together studies across different sustainability dimensions, this review inves-tigates how global agricultural trade flows have affected land systems and resulting impacts on food and nutrient availability, natural habitat conversion, biodiversity loss, and ecosystem carbon storage. We show that the effects of trade on land systems are highly heterogeneous across regions and commodities, revealing both synergies and trade-offs between improved nutrition and environmental conservation. For instance, we find that while the concentration of cereal production in North America has spared land, the increased demand for tropical products induced by trade has negatively impacted tropical ecosystems. Based on the current state of knowledge, we identify six pathways for how future research can contribute to a more comprehensive understanding of how agricultural trade can positively contribute to meeting global sustainability goals