Interface mobility from interface random walk

Computational studies aimed at extracting interface mobilities require driving forces orders of magnitude higher than those occurring experimentally. We present a computational methodology that extracts the absolute interface mobility in the zero driving force limit by monitoring the one-dimensional random walk of the mean interface position along the interface normal. The method exploits a fluctuation-dissipation relation similar to the Stokes-Einstein relation, which relates the diffusion coefficient of this Brownian-like random walk to the interface mobility. Atomic-scale simulations of grain boundaries in model crystalline systems validate the theoretical predictions and highlight the profound effect of impurities. The generality of this technique, combined with its inherent spatiotemporal efficiency, should allow computational studies to effectively complement experiments in understanding interface kinetics in diverse material systems.     

Large-area three-dimensional molecular ordering of a polymer brush by one-step processing

Rational molecular design and processing, enabling large-area molecular ordering, are important for creating high-performance organic materials and devices. We show that, upon one-step hot-pressing with uniaxially stretched Teflon sheets, a polymer brush carrying azobenzene-containing mesogenic side chains self-assembles into a freestanding film, where the polymer backbone aligns homeotropically to the film plane and the side chains align horizontally. Such an ordered structure forms through translation of a one-dimensional molecular order of the Teflon sheet and propagates from the interface macroscopically on both sides of the film. The resultant wide-area bimorph configuration allows the polymer film to bend rapidly and reversibly when the azobenzene units are photoisomerized. The combination of polymer brushes with hot-pressing and Teflon sheets provides many possibilities in designing functional soft materials.     

Materials science. Polymers go with the flow

The flow of polymer melts and solutions differs substantially from that of other liquids, because the long polymer chains can become entangled and cannot cross each other freely in their motion. In his Perspective, Marrucci provides an overview of theoretical attempts to explain polymer flow. He highlights the report by Bent et al., who have performed sophisticated experiments on linear polymers that prove key elements of the latest theory. Furthermore, the approach used by the authors will become an important experimental tool for investigating more complex polymer systems.     

Chemistry of the metal-polymer interfacial region

In many polymer-metal systems, chemical bonds are formed that involve metal-oxygen-carbon complexes. Infrared and M?ssbauer spectroscopic studies indicate that carboxylate groups play an important role in some systems. The oxygen sources may be the polymer, the oxygen present in the oxide on the metal surface, or atmospheric oxygen. Diffusion of metal ions from the substrate into the polymer interphase may occur in some systems that are cured at elevated temperatures. It is unclear whether a similar, less extensive diffusion occurs over long time periods in systems maintained at room temperature. The interfacial region is dynamic, and chemical changes occur with aging at room temperature. Positron annihilation spectroscopy may have application to characterizing the voids at the metal-polymer interface.     

Total alignment of calcite at acidic polydiacetylene films: cooperativity at the organic-inorganic interface

Biological matrices can direct the absolute alignment of inorganic crystals such as calcite. Cooperative effects at an organic-inorganic interface resulted in similar co-alignment of calcite at polymeric Langmuir-Schaefer films of 10,12-pentacosadiynoic acid (p-PDA). The films nucleated calcite at the (012) face, and the crystals were co-aligned with respect to the polymer's conjugated backbone. At the same time, the p-PDA alkyl side chains reorganized to optimize the stereochemical fit to the calcite structure, as visualized by changes in the optical spectrum of the polymer. These results indicate the kinds of interactions that may occur in biological systems where large arrays of crystals are co-aligned.     

Moving beyond the opposition of diverse knowledge systems for food security and nutrition

Food Security and Nutrition(FSN) is influenced by diverse and complex factors, and therefore requires a holistic approach to agriculture and food systems plus integration of knowledge from diverse sources in science and society. Using the results of a colloquium held at the University of Hohenheim(Germany) in September 2016 leading up to the recent High Level Panel of Experts(HLPE) Note on Critical and Emerging Issues for Food Security and Nutrition, this article underlines the role of research and innovation as a social and political process and draws attention to neglected types of knowledge. It illustrates the potential of knowledge co-production and co-innovation to transform food systems in order to achieve the Sustainable Development Goals.     

Self-assembly of large and small molecules into hierarchically ordered sacs and membranes

We report here the self-assembly of macroscopic sacs and membranes at the interface between two aqueous solutions, one containing a megadalton polymer and the other, small self-assembling molecules bearing opposite charge. The resulting structures have a highly ordered architecture in which nanofiber bundles align and reorient by nearly 90 degrees as the membrane grows. The formation of a diffusion barrier upon contact between the two liquids prevents their chaotic mixing. We hypothesize that growth of the membrane is then driven by a dynamic synergy between osmotic pressure of ions and static self-assembly. These robust, self-sealing macroscopic structures offer opportunities in many areas, including the formation of privileged environments for cells, immune barriers, new biological assays, and self-assembly of ordered thick membranes for diverse applications.     

Long-term studies of vegetation dynamics

By integrating a wide range of experimental, comparative, and theoretical approaches, ecologists are starting to gain a detailed understanding of the long-term dynamics of vegetation. We explore how patterns of variation in demographic traits among species have provided insight into the processes that structure plant communities. We find a common set of mechanisms, derived from ecological and evolutionary principles, that underlie the main forces shaping systems as diverse as annual plant communities and tropical forests. Trait variation between species maintains diversity and has important implications for ecosystem processes. Hence, greater understanding of how Earth's vegetation functions will likely require integration of ecosystem science with ideas from plant evolutionary, population, and community ecology.     

秦岭太白山高山灌丛的生物量及其营养元素含量

选取１２个样方,调查了高山灌丛生物量和营养元素含量随海拔、坡向和坡度的变化状况。头花杜鹃、高山柳和草本植物分别在海拔３５００ｍ、３６１０ｍ和３６２０ｍ达到其生物量的最大值;高山植物的根／茎比随海拔升高而增大;西坡高山灌丛的生物量大于南坡;坡度对高山灌丛生物量在各器官中的分配影响较大;海拔高度、坡向和坡度对高山植物营养元素含量的影响不明显。     

Diffusion of small solutes in polymer-containing solutions

Diffusion processes involving polymers are common in scientific and engineering separations and are a major component of biological functions. Analyses of these systems are usually based on versions of the Stokes-Einstein equation, although order of magnitude deviations have been observed. Presented here is a theoretical correction to the Stokes-Einstein equation containing a "local viscosity" function that combines diffusional hydrodynamics with Maxwell's treatment of electrical resistance in inhomogeneous regions. The resulting equation accurately predicts experimental diffusion data within tight bounds for polymer concentrations from 0 to 9 percent. It requires knowledge only of thermodynamics and of pure solvent and solution viscosities.     

DESIGNING DIVERSIFIED CROPPING SYSTEMS IN CHINA: THEORY,APPROACHES AND IMPLEMENTATION

•Agricultural green transformation of China requires restructuring of cropping systems.•Ecosystem services enhanced by crop diversification is key to sustainable agriculture.•Crop diversification improve ecosystem services at field, farm and landscape scales.•Cropping system design should meet regional characteristics and socio-economic demand.Intensive agriculture in China over recent decades has successfully realized food security but at the expense of negative environmental impacts. Achieving green transformation of agriculture in China requires fundamental restructuring of cropping systems. This paper presents a theoretical framework of theory, approaches and implementation of crop diversification schemes in China. Initially, crop diversification schemes require identifying multiple objectives by simultaneously considering natural resources, limiting factors/constraints, and social and economic demands of different stakeholders. Then, it is necessary to optimize existing and/or design novel cropping systems based upon farming practices and ecological principles, and to strengthen targeted ecosystem services to achieve the identified objectives. Next, the resulting diversified cropping systems need to be evaluated and examined by employing experimental and modeling approaches. Finally, a strategic plan, as presented in this paper, is needed for implementing an optimized crop diversification in China based upon regional characteristics with the concurrent objectives of safe, nutritious food production and environmental protection. The North China Plain is used as an example to illustrate the strategic plan to optimize and design diversified cropping systems. The implementation of crop diversification in China will set an example for other countries undergoing agricultural transition, and contribute to global sustainable development.     

Phonon dispersions of fcc delta-plutonium-gallium by inelastic x-ray scattering

We report an experimental determination of the phonon dispersion curves in a face-centered cubic (fcc) delta-plutonium-0.6 weight % gallium alloy. Several unusual features, including a large elastic anisotropy, a small-shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes, are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. Our results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory calculations for delta-plutonium.     

羧酸酯淀粉取代度的检测方法研究

羧酸酯淀粉是一类重要的淀粉衍生物,已广泛应用于多个工业领域。在众多领域中,准确测量酯化淀粉取代度,对于建立淀粉改性程度与聚合物结构功能关系具有重要意义。介绍了近年来淀粉羧酸酯取代度的检测方法,并对各检测方法的特点进行了比较。     

Chain pullout and mobility effects in friction and lubrication

The interfacial shear stress that occurs when a network of a polymer that is highly mobile at the segment level (an elastomer) is slid over a smooth surface of an immobile (glassy) polymer has been measured. The glassy material is covered by a thin layer of end-attached chains of the mobile material. The experiment was designed so that there were no free chains at the interface; the slip occurred between network chains on the one side and rigid material plus end-attached mobile chains on the other side. Two main results were obtained. (i) The interfacial shear stress is strongly affected by the segment mobility of the materials on both sides of the slip plane, and considerably lower stress is observed when the materials on both sides of the interface are highly mobile. (ii) Very thin layers of tethered chains can increase the interfacial friction. Both results are relevant to the understanding of a number of practical situations that range from the operation of thin layers of lubricants, such as those found in magnetic storage devices, to the problem of wall slip and melt fracture in polymer processing.     

Atomic arrangements at metal surfaces

The termination of a solid induces changes in the locations of the outermost atoms of the solid. The changes can be minor or as dramatic as the rearrangement of the atoms into a different crystallographic group. Surface crystallography studies have determined that all surfaces are altered by forces induced at the solid-vacuum interface. At the least, the outermost atomic layers are displaced away from positions that they would have had in the bulk environment. Results from experimental and theoretical investigations for the Al(110) surface are discussed to illustrate present understanding of the surface atomic displacements. Some effects that the truncation- induced forces have on the surfaces of binary metal alloys are also discussed.     

Modulation instability and pattern formation in spatially incoherent light beams

We report on the experimental observation of modulation instability of partially spatially incoherent light beams in noninstantaneous nonlinear media and show that in such systems patterns can form spontaneously from noise. Incoherent modulation instability occurs above a specific threshold that depends on the coherence properties (correlation distance) of the wave packet and leads to a periodic train of one-dimensional filaments. At a higher value of nonlinearity, the incoherent one-dimensional filaments display a two-dimensional instability and break up into self-ordered arrays of light spots. This discovery of incoherent pattern formation reflects on many other nonlinear systems beyond optics. It implies that patterns can form spontaneously (from noise) in diverse nonlinear many-body systems involving weakly correlated particles, such as atomic gases at (or near) Bose-Einstein condensation temperatures and electrons in semiconductors at the vicinity of the quantum Hall regime.     

钢筋混凝土临界锈胀力预测的双K断裂模型

应用混凝土断裂理论,结合混凝土存在初始裂缝和微缺陷的特性,假定混凝土内部钢筋表面位置沿径向存在两条初始裂纹,导出了应力强度因子的近似解析解。通过应力强度因子与锈胀力的关系建立了基于双K断裂参数的保护层初裂和完全开裂时刻的临界锈胀力预测模型,进而提出了考虑界面间隙和钢筋锈损厚度的混凝土结构锈胀开裂耐久寿命预测方法,为钢筋混凝土结构锈蚀损伤耐久寿命预测提供了一种新的理论方法。对不同的初始裂缝长度将临界锈胀力的预测值与实验结果进行比较,证实了预测模型的有效性,并对临界锈胀力的主要影响因素做了分析。     

Quebrada jaguay: early south american maritime adaptations

Excavations at Quebrada Jaguay 280 (QJ-280) (16 degrees30'S) in south coastal Peru demonstrated that Paleoindian-age people of the Terminal Pleistocene (about 11,100 to 10,000 carbon-14 years before the present or about 13,000 to 11,000 calibrated years before the present) in South America relied on marine resources while resident on the coast, which extends the South American record of maritime exploitation by a millennium. This site supports recent evidence that Paleoindian-age people had diverse subsistence systems. The presence of obsidian at QJ-280 shows that the inhabitants had contact with the adjacent Andean highlands during the Terminal Pleistocene.     

Chemistry. Beyond platonic molecules

Molecular motion is influenced by quantum mechanics, which thus affect molecular properties such as their reactivity and the ease with which they isomerize. In his Perspective, Bowman describes recent theoretical advances that allow accurate quantum mechanical calculation of molecular motions for ever larger systems.     

In situ interfacial mass detection with piezoelectric transducers

The converse piezoelectric effect, in which an electric field applied across a piezoelectric material induces a stress in that material, has spurred many recent developments in mass measurement techniques. These methods commonly rely on the changes in the vibrational resonant frequency of piezoelectric quartz oscillators that result from changes in mass on the surface of the oscillator. The dependence of frequency on mass has been exploited extensively for mass measurements in vacuum or gas phase, for example, thickness monitors for thin-film preparation and sensors for chemical agents. Advances in piezoelectric methodology in the last decade now allow dynamic measurements of minute mass changes (< 10(-9) grams per square centimeter) at surfaces, thin films, and electrode interfaces in liquid media as well. Mass measurements associated with a diverse collection of interfacial processes can be readily performed, including chemical and biological sensors, reactions catalyzed by enzymes immobilized on surfaces, electron transfer at and ion exchange in thin polymer films, and doping reactions of conducting polymers.