Metastability at the Yield-Stress Transition in Soft Glasses

We study the solid-to-liquid transition in a two-dimensional fully periodic soft-glassy model with an imposed spatially heterogeneous stress. The model we consider consists of droplets of a dispersed phase jammed together in a continuous phase. When the peak value of the stress gets close to the yield stress of the material, we find that the whole system intermittently tunnels to a metastable "fluidized" state, which relaxes back to a metastable "solid" state by means of an elastic-wave dissipation. This macroscopic scenario is studied through the microscopic displacement field of the droplets, whose time statistics displays a remarkable bimodality. Metastability is rooted in the existence, in a given stress range, of two distinct stable rheological branches as well as long-range correlations (e.g., large dynamic heterogeneity) developed in the system. Finally, we show that a similar behavior holds for a pressure-driven flow, thus suggesting possible experimental tests.Comment: 13 pages, 11 figure

Fluidisation and plastic activity in a model soft-glassy material flowing in micro-channels with rough walls

By means of mesoscopic numerical simulations of a model soft-glassy material, we investigate the role of boundary roughness on the flow behaviour of the material, probing the bulk/wall and global/local rheologies. We show that the roughness reduces the wall slip induced by wettability properties and acts as a source of fluidisation for the material. A direct inspection of the plastic events suggests that their rate of occurrence grows with the fluidity field, reconciling our simulations with kinetic elasto-plastic descriptions of jammed materials. Notwithstanding, we observe qualitative and quantitative differences in the scaling, depending on the distance from the rough wall and on the imposed shear. The impact of roughness on the orientational statistics is also studied

Strongly-Driven One-Atom Laser and Decoherence Monitoring

We propose the implementation of a strongly-driven one-atom laser, based on the off-resonant interaction of a three-level atom in $\Lambda$-configuration with a single cavity mode and three laser fields. We show that the system can be described equivalently by a two-level atom resonantly coupled to the cavity and driven by a strong effective coherent field. The effective dynamics can be solved exactly, including a thermal field bath, allowing an analytical description of field statistics and entanglement properties. We also show the possible generation of Schr\"odinger cat states for the whole atom-field system and for the field alone after atomic measurement. We propose a way to monitor the system decoherence by measuring atomic population. Finally, we confirm the validity of our model through numerical solutions.Comment: 9 pages, 7 figures Accepted in Phys. Rev.

Tissue biomechanical strength, wear resistance and recovery in C4 turfgrass species: physiological and morphological factors and innovative evaluation techniques.

Turfgrass wear effects are known to be the sum of soil compaction and plant tissue injury. As such, tissue intrinsic resistance to several mechanical factors, including traction stress, is a decisive in determining the wear resistance of a turfgrass species. Wear simulation in the field can suffer from climate, soil and machinery operator error, and is always inclusive of the soil compaction factor that is one of the origins of turfgrass wear effects in the field. Lignin, dry matter, starch, sugars and silica are some of the tissue constituents and characteristics that have been associated with leaf and stem mechanical resistance, while little information is to be found concerning stolons and rhizomes. These organs not only enable C4 turfgrass species lateral growth, soil colonization and injury recovery, but are also key constituents of mature swards. A firsts study consisted in an extensive investigation on the effective leaf, rhizome traction resistance of Cynodon dactylon L. Pers. var. dactylon x C. transvaalensis Burt-Davy cv. Tifway 419 (Cdxt), Zoysia matrella (L.) Merr. Cv. Zeon (Zm) and Paspalum vaginatum Swartz. cv. Salam (Pv), as measured with a FIFA-approved dynamometer, and correlating these results with laboratory investigations on key tissue constituents. Several aspects emerged from the present work that can be summarized as follows: 1. Tensile strength tests on leaf, rhizome and stolon tissues of Cdxt, Zm and Pv can supply useful information regarding these species’ starch, sugars, dry matter, lignin and silica content. 2. Tensile strength was more influenced by tissue constituents than by tissue dimension. 3. The results of tensile strength tests are in accordance with these species’ wear resistance as tested in previous work, with Zm stronger than Cdxt and Pv. 3. In rhizomes and stolons, tissue breakage usually occurs in the area at the intercalary meristem at the apical zone in the immediate proximity of a node. 4. Older tissues have higher tensile strength thanks to their higher lignification. 5. Starch and sugars content found in tissues is in accordance with the species’ previously observed linear growth rate, with Cdxt faster than Pv and Zm. 6. Starch content is generally inversely proportional to lignin content. 7. Stolon TSS content, and glucose in general, is a clear marker of tissue mechanical strength. 8. Lignin is the principle constituent in determining tissue tensile strength, and as such it could be used as a turfgrass wear resistance predictor in the cultivar breeding stages. 9. Silica is a constituent undermining tissue tensile strength. 10. Leaves are the plant organs with the highest silicization and the lowest lignification of tissues. A second study consisted of testing slabs of mature canopies of the same species for wear resistance in laboratory with a Lisport machine, as adopted by FIFA for artificial turf testing. Worn slabs of turfgrass were then allowed to recover in greenhouse to fathom out percent recovery of shoots. The results of these investigations were once again plotted again laboratory investigations on key tissue constituents. Several aspects emerged from this second work that can be summarized as follows: 1. The Lisport machine can be successfully used in an effective and reproducible way to fathom out natural turfgrass wear resistance, devoid of soil compaction effects. 2. Wear resistance for C4 species as observed in the field does not necessarily coincide with the relative tissue intrinsic resistance, but rather with the initial canopy density. 3. C4 species show a wear resistance that is much higher than C3 species. 4. C3 species show a virtually nil recuperative capability (mainly due to the lack of vegetative propagation organs). 5. The species with a very high intrinsic (tissue) wear resistance are also the species with the slowest recuperation potential. This seems to be due to lower levels of starch and TSS available for recovery. 6. Starch was a clear marker of wear resistance (negatively correlated) and recovery (positively correlated). 7. Silica was a marker positively correlated with wear resistance. 8. Lignin was the clearest marker found to be positively correlated with wear resistance. 9. A more severe wear induces a higher percentage of shoot recovery, and this particular aspects deserves further investigation

On the statistical description of the inbound air traffic over Heathrow airport

We present a model to describe the inbound air traffic over a congested hub. We show that this model gives a very accurate description of the traffic by the comparison of our theoretical distribution of the queue with the actual distribution observed over Heathrow airport. We discuss also the robustness of our model

On the impact of controlled wall roughness shape on the flow of a soft-material

We explore the impact of geometrical corrugations on the near-wall flow properties of a soft-material driven in a confined rough microchannel. By means of numerical simulations, we perform a quantitative analysis of the relation between the flow rate $\Phi$ and the wall stress $\sigma_w$ for a number of setups, by changing both the roughness values as well as the roughness shape. Roughness suppresses the flow, with the existence of a characteristic value of $\sigma_w$ at which flow sets in. Just above the onset of flow, we quantitatively analyze the relation between $\Phi$ and $\sigma_w$. While for smooth walls a linear dependency is observed, steeper behaviours are found to set in by increasing wall roughness. The variation of the steepness, in turn, depends on the shape of the wall roughness, wherein gentle steepness changes are promoted by a variable space localization of the roughness

A climo and litho soil-sequence on the Vico volcano (Italy)

A la suite d'une cartographie pédologique détaillée de la majeure partie de l'appareil volcanique de Vico (Italie), une séquence topoclimatique a été définie. Cing profils ont été choisis à des altitudes représentatives des principales zones climatiques. Les changements observés dans les sols correspondent bien aux effets de variations significatives des paramètres climatique

The multimode covering location problem

In this paper we introduce the Multimode Covering Location Problem. This is a generalization of the Maximal Covering Location Problem that consists in locating a given number of facilities of different types with a limitation on the number of facilities sharing the same site. The problem is challenging and intrinsically much harder than its basic version. Nevertheless, it admits a constant factor approximation guarantee, which can be achieved combining two greedy algorithms. To improve the greedy solutions, we have developed a Variable Neighborhood Search approach, based on an exponential-size neighborhood. This algorithm computes good quality solutions in short computational time. The viability of the approach here proposed is also corroborated by a comparison with a Heuristic Concentration algorithm, which is presently the most effective approach to solve large instances of the Maximal Covering Location Problem