关键词： Anelastic deformation   Cell adhesion   Cell mechanics   Viscoelasticity  

摘要： Cell adhesion is an essential biological process. However, previous theoretical and experimental studies ignore a key variable, the changes of cellular volume and pressure, during the dynamic adhesion process. Here, we treat cells as open systems and propose a theoretical framework to investigate how the exchange of water and ions with the environment affects the shape and dynamics of cells adhered between two adhesive surfaces. We show that adherent cells can be either stable (convex or concave) or unstable (spontaneous rupture or collapse) depending on the adhesion energy density, the cell size, the separation of two adhesive surfaces, and the stiffness of the flexible surface. Strikingly, we find that the unstable states vanish when cellular volume and pressure are constant. We further show that the detachments of convex and concave cells are very different. The mechanical response of adherent cells is mainly determined by the competition between the loading rate and the regulation of the cellular volume and pressure. Finally, we show that as an open system the detachment of adherent cells is also significantly influenced by the loading history. Thus, our findings reveal a major difference between living cells and nonliving materials.

关键词： Crystallization kinetics   Crystalline morphology   Mechanical property  

摘要： Distribution of nanoparticles in nanocomposites is one of the most important roles to design unique morphologies and properties;however, up to now, it is still very difficult to control distribution of nanoparticles in polymer matrix. Here we show a simple approach to fabricate ferroferric oxide (Fe3O4) nanoparticles with controllable chain-like distribution in poly(e-caprolactone) (PCL) by a magnetic self organization. Randomly distributed Fe3O4 nanoparticles can be polarized and the interparticle dipole dipole attraction drives their assembly into linear chains in an external magnetic field. The accelerated PCL crystallization happens at low Fe3O4 loadings, while the retardation to crystallization happens at high Fe3O4 loadings, especially for the samples with chain-like structures. Furthermore, PCL spherulites are also arranged into chain-like structures by the Fe3O4 nanoparticle chains. As a result, the PCL/Fe3O4 nanocomposites with chain-like structures exhibit higher storage modulus, Young's modulus and tensile strength than that of the corresponding nanocomposites with random distribution of Fe3O4 nano particles, indicating mechanical enhancements. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： ferroelectrics   metamaterials   nonlinear waves   phase field   phase transitions  

摘要： A mechanical metamaterial, a simple, periodic mechanical structure, is reported, which reproduces the nonlinear dynamic behavior of materials undergoing phase transitions and domain switching at the structural level. Tunable multistability is exploited to produce switching and transition phenomena whose kinetics are governed by the same Allen-Cahn law commonly used to describe material-level, structural-transition processes. The reported purely elastic mechanical system displays several key features commonly found in atomic- or mesoscale physics of solids. The rotating-mass network shows qualitatively analogous features as, e.g., ferroic ceramics or phase-transforming solids, and the discrete governing equation is shown to approach the phase field equation commonly used to simulate the above processes. This offers untapped opportunities for reproducing material-level, dissipative and diffusive kinetic phenomena at the structural level, which, in turn, invites experimental realization and paves the road for new active, intelligent, or phase-transforming mechanical metamaterials bringing small-scale processes to the macroscopically observable scale.

关键词： boronic acids   dynamic covalent bonds   Maxwell elements   self-healing   stress relaxation  

摘要： Hydrogels play a central role in a number of medical applications and new research aims to engineer their mechanical properties to improve their capacity to mimic the functional dynamics of native tissues. This study shows hierarchical mechanical tuning of hydrogel networks by utilizing mixtures of kinetically distinct reversible covalent crosslinks. A methodology is described to precisely tune stress relaxation in PEG networks formed from mixtures of two different phenylboronic acid derivatives with unique diol complexation rates, 4-carboxyphenylboronic acid, and o-aminomethylphenylboronic acid. Gel relaxation time and the mechanical response to dynamic shear are exquisitely controlled by the relative concentrations of the phenylboronic acid derivatives. The differences observed in the crossover frequencies corresponding to pK(a) differences in the phenylboronic acid derivatives directly connect the molecular kinetics of the reversible crosslinks to the macroscopic dynamic mechanical behavior. Mechanical tuning by mixing reversible covalent crosslinking kinetics is found to be independent of other attributes of network architecture, such as molecular weight between crosslinks.

关键词： 汽车机械   变速器   动力性能   试验台  

摘要： 目前,在汽车发达国家自动变速器占据了主流地位,但在不同国家和地区,由于技术水平、道路状况、交通法规、认知也理等诸多因素,自动变速器的市场占有率所有不同。在我国,由于自动变速器的部分核屯、技术储备不足,相当一部分自动变速器依靠爱信、采埃孚、艾利逊、博格华纳等公司引进装车。在此背景之下,本文将着重分析探讨汽车机械变速器动力性能试验台,以期能为以后的实际工作起到一定的借鉴作用。

关键词： 铁路机务系统   机械动力设备   优化管理  

摘要： 随着铁路运用技术与设备的不断发展与进步,当前铁路机务系统对于机械动力设备的管理方式已经不能满足时代发展的需求,因此,需要对机械动力设备的使用、维修以及保养等各方面的管理工作进行优化,才能满足当前铁路事业的发展需要。

关键词： colloids   small-angle X-ray scattering   drying   solidification   fracture   shear bands  

摘要： Charged colloidal dispersions make up the basis of a broad range of industrial and commercial products, from paints to coatings and additives in cosmetics. During drying, an initially liquid dispersion of such particles is slowly concentrated into a solid, displaying a range of mechanical instabilities in response to highly variable internal pressures. Here we summarize the current appreciation of this process by pairing an advection-diffusion model of particle motion with a Poisson-Boltzmann cell model of inter-particle interactions, to predict the concentration gradients in a drying colloidal film. We then test these predictions with osmotic compression experiments on colloidal silica, and small-angle X-ray scattering experiments on silica dispersions drying in Hele-Shaw cells. Finally, we use the details of the microscopic physics at play in these dispersions to explore how two macroscopic mechanical instabilities-shear-banding and fracture-can be controlled. This article is part of the themed issue 'Patterning through instabilities in complex media: theory and applications'.

关键词： Phenolic resins   Thermosetting polymer   Molecular dynamics simulation   Mechanical property  

摘要： An atomistic molecular dynamics simulation was performed for crosslinked phenolic resins which were constructed from phenols and crosslinkers using a pseudo-reaction algorithm, in order to understand the structure-mechanical property relationships therein from an atomistic perspective. The tensile modulus was characterized from the linear elastic region of the stress-strain curves under uniaxial tensile deformation. Analysis of the relationships between the moduli and interatomic interactions indicated that bond interaction, especially bond orientation for the elongation axis, dominantly affects the tensile modulus in a range of strain 0 <= epsilon <= 0.05, while stress-concentration in specific domains and long-range interactions including hydrogen bonding do not. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： Mechanical response   Steered molecular dynamics   Type IV pilus   Coarse-grained model  

摘要： Type IV pili are long filamentous structures on the surface of bacteria, which can be rapidly assembled or disassembled with pilin subunits by molecular motors. They can generate force during retraction and are involved in many bacterial functions. Steered molecular dynamics simulations with coarse-grained MARTINI models are carried out to investigate the mechanical behaviors of pili under tension. Our study is the first to report a Young's modulus of 0.80 +/- 0.07 GPa and a spring constant of 1294.6 +/- 116.5 kJ mol(-1) nm(-2) for pilus. Our results show the mechanical responses of pili are different from those described by the worm-like chain model and the van der Waal's interactions play a critical role in the mechanical responses. Moreover, the effects of pulling rates and virtual spring constants of pilus on Young's modulus are studied and two distinct morphological stages with the conformational changes appear during the extension of pilus are observed. This work provide insight into the mechanics and the deformation mechanism of pilus assembly. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： 机械能守恒定律   动力学问题   巧用   高中物理   知识点   金钥匙   简单化  

摘要： 机械能守恒定律是高中物理的重要知识点之一，也是解决一些物理难题的金钥匙。在动力学问题中，运用机械守恒定律往往可以使过程分析简单化。