关键词： Molecular dynamics simulation   mechanical properties   Mg2Si  

摘要： This paper reports a study of the mechanical behavior of single-crystal bulk Mg2Si by the molecular dynamics method in view of the effect of temperature and strain rate. A modified Morse potential energy function in which the bond-angle deformation has been taken into account is developed and employed to describe the atomic interactions to shed light on the mechanical properties. The stress-strain response of single-crystal bulk Mg2Si under uniaxial tensile loading is investigated at different temperatures (300 K to 1200 K) and different strain rates, respectively. Through comparison of the mechanical properties of single-crystal bulk Mg2Si at different temperatures and strain rates, it is found that the elastic modulus gradually decreases with increasing temperature, while the strain rate seems to have little effect on the mechanical properties of single-crystal bulk Mg2Si. Moreover, in contrast to conventional polycrystalline bulk Mg2Si, single-crystal bulk Mg2Si exhibits much better mechanical performance.

关键词： Allometric Trophic Network model   community ecology   food web   multi-trophic dynamics   seasonal plankton succession  

摘要： Mechanistic understanding of consumer-resource dynamics is critical to predicting the effects of global change on ecosystem structure, function and services. Such understanding is severely limited by mechanistic models inability to reproduce the dynamics of multiple populations interacting in the field. We surpass this limitation here by extending general consumer-resource network theory to the complex dynamics of a specific ecosystem comprised by the seasonal biomass and production patterns in a pelagic food web of a large, well-studied lake. We parameterised our allometric trophic network model of 24 guilds and 107 feeding relationships using the lakes food web structure, initial spring biomasses and body-masses. Adding activity respiration, the detrital loop, minimal abiotic forcing, prey resistance and several empirically observed rates substantially increased the model's fit to the observed seasonal dynamics and the size-abundance distribution. This process illuminates a promising approach towards improving food-web theory and dynamic models of specific habitats.

关键词： Molecular dynamics   mechanical properties   vacancy effect   temperature effect   beta-Zn4Sb3  

摘要： The molecular dynamics method is employed to study the mechanical properties of single-crystal bulk -Zn4Sb3. According to the interatomic potential obtained from first-principles calculation and fitting to the ground-state physical equations, the stress-strain curves of single-crystal bulk -Zn4Sb3 under different conditions, which include Zn atom vacancy and temperature effects, are presented. From the stress-strain responses, single-crystal bulk -Zn4Sb3 exhibits typical nonlinear elastic brittleness of thermoelectric materials. With increasing Zn atom vacancy proportion, the mechanical properties of single-crystal bulk -Zn4Sb3 gradually degrade. When the Zn atom vacancy proportion reaches 10% in the vacancy model, the elastic modulus and the ultimate stress are found to decrease by 30% and 50% compared with the full-occupancy model. With increasing temperature from 300 K to 700 K, the crystal structure of the vacancy model of -Zn4Sb3 maintains stability, and the mechanical properties are degraded slowly. The mechanical properties along the [001] axis are better than along the [010] axis.

关键词： Cell mechanics   Subcellular   Live-imaging   Vesicle tracking   Drosophila   Aplysia  

摘要： Growing experimental evidence suggests that mechanical tension plays a significant role in determining the growth, guidance, and function of neurons. Mechanical tension in axons contributes to neurotransmitter clustering at the neuromuscular junction (NMJ) and is actively regulated by neurons both and . In this work, we applied mechanical strain on neurons and neurons and studied their vesicle dynamics by live-imaging. Our experiments show that mechanical stretch modulates the dynamics of vesicles in two different model systems: (1) The global accumulation of synaptic vesicles (SV) at the NMJ and (2) the local motion of individual large dense core vesicles (LDCV) in neurites. Specifically, a sustained stretch results in enhanced SV accumulation in the NMJ. This increased SV accumulation occurs in the absence of extracellular Ca2+, plateaus after approximately 50 min, and persists for at least 30 min after stretch is reduced. On the other hand, mechanical compression in neurites immediately disrupts LDCV motion, leading to decreased range and processivity. This impairment of LDCV motion persists for at least 15 min after tension is restored. These results show that mechanical stretch modulates both local and global vesicle dynamics and strengthens the notion that tension serves a role in regulating neuronal function.

关键词： MESENCHYMAL stem cells   BONE marrow cells   FOCAL adhesion kinase   CELLS -- Mechanical properties   GUANOSINE triphosphatase   PHOSPHORYLATION   CYTOCHALASINS   CELL differentiation  

摘要： Human bone marrow mesenchymal stem cells (hMSCs) have the potential to differentiate into tendon/ligament-like lineages when they are subjected to mechanical stretching. However, the means through which mechanical stretch regulates the tenogenic differentiation of hMSCs remains unclear. This study examined the role of RhoA/ROCK, cytoskeletal organization, and focal adhesion kinase (FAK) in mechanical stretch-induced tenogenic differentiation characterized by the up-regulation of tendon-related marker gene expression. Our findings showed that RhoA/ROCK and FAK regulated mechanical stretch-induced realignment of hMSCs by regulating cytoskeletal organization and that RhoA/ROCK and cytoskeletal organization were essential to mechanical stretch-activated FAK phosphorylation at Tyr397. We also demonstrated that this process can be blocked by Y-27632 (a specific inhibitor of RhoA/ROCK), cytochalasin D (an inhibitor of cytoskeletal organization) or PF 573228 (a specific inhibitor of FAK). The results of this study suggest that RhoA/ROCK, cytoskeletal organization, and FAK compose a signaling network that senses mechanical stretching and drives mechanical stretch-induced tenogenic differentiation of hMSCs. This work provides novel insights regarding the mechanisms of tenogenesis in a stretch-induced environment and supports the therapeutic potential of hMSCs. J. Cell. Physiol. 227: 27222729, 2012. (c) 2011 Wiley Periodicals, Inc.

关键词： Kaolinite   Alumina   Mullite   Mechanical activation   Non-isothermal kinetics  

摘要： The non-isothermal kinetics of mullite formation from both non-activated and mechanically activated kaolinite + alumina ceramic system have been studied by differential thermal analysis (DTA). The mixture of kaolinite and alumina was activated mechanically in a planetary mill, while amorphization in the kaolinite and alumina structure was studied by X-ray diffraction analysis. The activation energies depending on the conversion for mullite formation have been calculated from the DTA curves by using the non-isothermal method of Coats and Redfern at heating rates of 5, 10, 15, and 20 A degrees C min(-1). The mechanical activation of the kaolinite and alumina mixture resulted in the decrease in activation energy values for mullite formation.

关键词： Kinetics   Reaction mechanism   Oxidations   Cerium(IV) sulphate   Fumaric acid   Thermodynamic parameters  

摘要： Kinetics of ruthenium(III) catalyzed oxidation of fumaric acid by acidic cerium(IV) has been studied by the rate of disappearance of [Ce(IV)]. The reaction is zero order with respect to [Ce(IV)] and unity with respect to both [fumaric acid] and [catalyst]. Negative effect of [H+] and inverse first order with respect to [Cl-] was observed while negligible effect of variation of [HSO4-] and ionic strength was observed. The result suggest the formation of a complex between substrate and hydroxylated species of ruthenium(III) in a fast step. This complex. yields the product in a rate determining step. The complex reacts further with the acidic cerium(IV) species in a rate determining step to yield the products. The reaction constants involved in the mechanism and the activation parameters have been calculated. There is a good agreement between observed and calculated rate constants under different experimental conditions. Formic acid was detected as end product.

关键词： Quantum dynamics   Mechanical resonator   Optomechanical systems  

摘要： We explore the quantum dynamics of a mechanical resonator whose position is coupled to the frequency of an optical (or microwave) cavity mode. When the cavity is driven at a frequency above resonance the mechanical resonator can gain energy and for sufficiently strong coupling this results in limit-cycle oscillations. Using a truncated Wigner function approach, which captures the zero-point fluctuations in the system, we develop an approximate analytic treatment of the resonator dynamics in the limit-cycle regime. We find that the limit-cycle oscillations produced by the cavity are associated with rather low levels of energy fluctuations in the resonator. Compared to a resonator at the same temperature which is driven by a pure harmonic drive to a given average energy, the cavity-driven oscillations can have much lower energy fluctuations. Furthermore, at sufficiently low temperatures, the cavity can drive the resonator into a non-classical state which is number-squeezed. (C) 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

关键词： Condensed matter: structural   mechanical & thermal Nanoscale science and low-D systems  

摘要： Single-crystal aluminium nanowires under torsion are studied using molecular dynamics simulations based on the many-body tight-binding potential. The effects of temperature, loading rate and nanowire length are evaluated in terms of atomic trajectories, potential energy, von Mises stress, a centrosymmetry parameter, torque, shear modulus and radial distribution function. Simulation results clearly show that torsional deformation begins at the surface, extends close to the two ends and finally diffuses to the middle part. The critical torsional angle which represents the beginning of plastic deformation varies with different conditions. Before the critical torsional angle is reached, the potential energy and the torque required for the deformation of a nanowire significantly increase with the torsional angle. The critical torsional angle increases with increasing nanowire length and loading rate and decreasing temperature. The torque required for the deformation decreases and the shear modulus increases with increasing nanowire length. For higher temperatures and higher loading rates, torsional buckling more easily occurs at the two ends of a nanowire, whereas it occurs towards the middle part at or below room temperature with lower loading rates. Geometry instability occurs before material instability ( buckling) for a long nanowire.

关键词： Soft Matter: Polymers and Polyelectrolytes  

摘要： The aim of this study is to analyze the mobility of polymer chains in semicrystalline poly(vinylidene fluoride) (PVDF). PVDF crystallizes from the melt in the α crystalline phase. The transformation from the α phase to the electroactive β phase can be induced by stretching at temperatures in the range between 80 and 140 °C. The spherulitic structure of the crystalline phase is deformed during stretching to form fibrils oriented in the direction of the strain. The amorphous phase confined among the crystalline lamellae is distorted as well and some degree of orientation of the polymer chains is expected. Dynamic-mechanical and dielectric spectroscopy measurements were performed in PVDF films stretched to strain ratios up to 5 at temperatures between 80 and 140 °C. Dynamic-mechanical measurements were conducted between −60 °C and melting and in this temperature range the relaxation spectra show the main relaxation of the amorphous phase (called β-relaxation) and at higher temperatures a relaxation related to crystallites motions (αc-relaxation). Although the mean relaxation times of the β-relaxation are nearly equal in PVDF before and after crystal phase transformation, a significant change of shape of the relaxation spectrum proves the effect of chain distortion due to crystal reorganization. In stretched PVDF the elastic modulus of the polymer in the direction of deformation is significantly higher than in the transversal one, as expected by chain and crystals fibril orientation. The recovery of the deformation when the sample is heated is related with the appearance of the αc-relaxation. Dielectric spectroscopy spectrum shows the main relaxation of the amorphous phase and a secondary process (γ-relaxation) at lower temperatures. Stretching produces significant changes in the relaxation processes, mainly in the strength and shape of the main relaxation β. The Havriliak-Negami function has been applied to analyze the dielectric response.