Negative incremental bulk modulus in foams

Negative incremental stiffness is known to occur in structures such as post-buckled flexible tubes and single-cell models. A single foam cell under uniaxial loading buckles and exhibits a non-monotonic S-shaped deformation curve, which is indicative of negative incremental stiffness. Negative stiffness is not observed in bulk materials. For example, individual foam cells display negative stiffness but foams tested in uniaxial compression exhibit a plateau in the stress–strain curve because the buckled cells localize in bands. This behaviour is consistent with the continuum view in which strong ellipticity and, hence, a positive shear modulus G and positive C ₁₁ modulus are required for stability, even for a constrained object. It is hypothesized that a solid with negative bulk modulus can be stabilized by control of the surface displacement. Experimentally, foams were hydrostatically compressed by controlled injections of small volumes of water into a plastic chamber, causing volumetric deformation. A negative incremental bulk modulus was observed in a foam with 0.4-mm cell size beyond about 20% volumetric strain. A foam with large cells, 2.5–4?mm in size, was anisotropic and did not exhibit the cell buckling required for negative modulus.     

Juggling with bubbles in cylindrical ferrofluid foams

Monodisperse foams in long cylinders exhibit ordered spiral structures. We have made ferrofluid foams of this kind and find that they can be manipulated in a variety of ways by an external magnetic field. Effects include morphological changes, twisting of structures and size control in the bubble formation process. This offers a promising technique for the efficient transport, switching and combining of samples in fluidic networks, possibly on the microfluidic scale.     

Dynamic instabilities as mechanisms for emergence

That competences may emerge given appropriate environmental and behavioral context is a long-standing theme in developmental research. Work in the motor domain, but also in cognitive development, has made it possible to transform this idea into a mechanistic account closely linked to empirical evidence. In dynamic systems thinking, such capacities as keeping a motor goal in mind, remembering a location, or resisting a motor habit, are all understood in terms of the generation of stable patterns of neuronal activation. These may be input-driven, but also be stabilized by interactions within neuronal representations. A key theoretical insight is that whether a particular pattern of activation is stable or not is not determined by any single factor, learning process, or structural parameter. Instead, ongoing activity, recent activation history, current input, all may affect when a particular dynamic regime is reachable. In spite of such broad interdependence, sharp transitions may characterize the onset of a skill in any given context. Dynamic instabilities are the mechanistic basis for this phenomenon and thus form the basis for understanding development in terms of emergence. We exemplify the concepts of instability and emergence around the phenomenon of infant perseverative reaching and discuss implications for identifying key markers of development and their link to neuronal processes.     

The relative energy of fcc and hcp foams

The energies of face-centred cubic (fcc) and hexagonal close-packed (hcp) monodisperse foams, associated with their total surface area, are equal in the wet and dry limits, in the usual model. We prove that for all intermediate values of liquid fraction, hcp has lower energy. Energy considerations are thus not sufficient to explain the observed preference for crystallization into fcc over hcp in experiments using monodisperse bubbles.     

Nonlocal effects in the continuum theory of shear localisation in 2d foams

Goyon et al. [J. Goyon, A. Colin, G. Ovarlez, A. Ajdari and L. Bocquet, Nature 454 (2008) p. 84] have shown that nonlocal effects in the rheology of foams may be accounted for by a modification of the standard (Herschel–Bulkley) model. Here we consider the effects of this modification on the continuum theory of 2d shear localisation. We compute results for various examples, showing that the localisation length is increased, and explore the limiting cases of zero and infinite nonlocality length ξ. Velocity profiles are shown to take an exponential form in the case where ξ is large. As the formulation of the nonlocal continuum model presented in this article is general, it may also be directly applicable to other complex fluids.     

A bifurcation theory for the instabilities of optimization and design

The world I grew up in believed that change and development in life are part of a continuous process of cause and effect, minutely and patiently sustained throughout the millenniums. With the exception of the initial act of creation ..., the evolution of life on earth was considered to be a slow, steady and ultimately demonstrable process. No sooner did I begin to read history, however, than I began to have my doubts. Human society and living beings, it seemed to me, ought to be excluded from so calm and rational a view. The whole of human development, far from having been a product of steady evolution, seemed subject to only partially explicable and almost invariably violent mutations. Entire cultures and groups of individuals appeared imprisoned for centuries in a static shape which they endured with long-suffering indifference, and then suddenly, for no demonstrable cause, became susceptible to drastic changes and wild surges of development. It was as if the movement of life throughout the ages was not a Darwinian caterpillar but a startled kangaroo, going out towards the future in a series of unpredictable hops, stops, skips and bounds. Indeed, when I came to study physics I had a feeling that the modern concept of energy could perhaps throw more light on the process than any of the more conventional approaches to the subject. It seemed that species, society and individuals behaved more like thunder-clouds than scrubbed, neatly clothed and well-behaved children of reason. Throughout the ages life appeared to build up great invisible charges, like clouds and earth of electricity, until suddenly in a sultry hour the spirit moved, the wind rose, a drop of rain fell acid in the dust, fire flared in the nerve, and drums rolled to produce what we call thunder and lightening in the heavens and chance and change in human society and personality.LAURENS VAN DER POST, The Lost World of the Kalahari     

Effect of crystallization on the surface area of porous Ni-based metallic glass foams

The change of the specific surface area in porous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass (MG) upon partial crystallization was investigated. The observed increase in the surface area of the annealed Ni-based MG foams is due to the formation of homogeneously distributed Ni₁₀(Zr,Ti)₇ rod-shape intermetallic phases with nominal diameters around 250?nm and ～800?nm length on the surface of MG struts during the crystallization. For longer annealing, the specific surface area decreases again due to a change of the morphology of the crystals from rod-like to disc-like appearance, thus suggesting an optimum regime for increasing the specific surface area upon isothermal annealing at a given temperature.     

自伤青少年的冲动性

以自我报告、行为学和脑电为指标, 检验自伤青少年的冲动性。研究1, 对820名普通中学生和72名工读生进行问卷调查, 探讨自伤行为与情绪调节困难、冲动性的关系。结果表明, 冲动性能够预测自伤行为, 且预测效应量大于情绪调节困难。研究2, 采用Go/Nogo范式的ERPs实验, 检验自伤组与对照组冲动控制的行为学与脑电差异。结果表明, 自伤组Nogo正确反应的N2波幅显著高于对照组, N2潜伏期在部分电极点处高于对照组。脑电地形图显示两者的脑电差异主要体现在前额叶区。结论：自伤青少年的冲动性高于同龄普通青少年。     

Certification in Testing in Canada

This article presents a brief outline of a program developed by the Canada Employment and Immigration Commission (CEIC) for certifying employment counselors to use tests in Canada Employment Centres (CECs).     

儿童Graves'病研究进展

Grave's病(GD)是导致儿童甲亢最常见的病因,其易感性80%决定于基因因素.ATD治疗是最常推荐的初始治疗手段,但长期缓解率仅为30%.ATD诱导儿童GD缓解的合适用药时间尚不明确.最佳治疗方式(ATD、RAI或手术)的选择还存在重大争议.药物治疗复发的预测非常重要,最近建立了一种复发风险评分,通过种族、年龄、诊断时FT4、TRAb水平、ATD预期治疗持续时间将GD儿童分成低中高三类风险人群,对其复发风险作出评估,对于确定GD儿童在初始治疗时选择何种方案更为适合意义重大.     

Sex differences in relational aggression in preschool children in Iran

The purpose of this study was to compare scores on a Relational Aggression Questionnaire for preschool boys and girls in Iran. The questionnaire was developed here, and its reliability and validity were assessed. Teachers' reports of relational aggression in 258 children ages 3 to 7 years was studied. Sex differences in relational aggression were not observed and was not significantly different across age groups.     

Occupational Stress in Workers and Managers in Steelworks in China

Occupational stressors and strains of 121 Chinese steelwork employees and 122 managers were measured using the Chinese version of the Occupational Stress Indicator (OSI). It was found that factors intrinsic to the job, Type A behavior, logic, and organizational structure and climate were the main predictors of mental ill-health and physical ill-health in managers. Organizational structure and climate and relationships with other people were important predictors for workers. Management processes and organizational forces were the strongest predicting factors of job satisfaction in both samples. On certain OSI scales and subscales, managers scored statistically higher than workers. It is proposed that these results reflect the enormous economic and social changes currently taking place in China, together with certain features inherent in Chinese organizational and managerial processes.     

Ethnic variation in depressive symptoms in a community sample in Hawaii

A modified CES-D was administered to a community sample of 176 European Americans (EA), 209 Native Hawaiians (NH), and 357 Japanese Americans (JA), yielding measures of depression, positive affect, depressed affect, somatic disturbance, and disturbed interpersonal relations. Positive affect was lower in JA relative to EA, consistent with findings among Native Japanese, a pattern attributed to cultural variation in emotion regulation. NH reported lower positive affect than EA, accompanied by elevated negative affect and somatic disturbance, suggesting generally higher levels of depressive symptoms. The three ethnic groups varied in mental health care usage with differing associations between depressive symptoms and experiences of stressful life events. Taken together, these results suggest ethnic variation in depressive symptoms may arise from differing cultural beliefs.     

Ethics in Research in Mathematics Education

Most scientific domains have guidelines concerning ethics in research. These guidelines are designed to enable researchers to conduct good research while avoiding potential harm to research participants. This paper examines the bases for ethical codes in research and discusses how these codes are operationalized in research conducted in the field of mathematics education.     

Competitiveness in males and in females

Male and female undergraduates performed a task at which they either succeeded or failed in competition with either a male or female partner who subsequently either accepted or rejected them. Subjects who were rejected by a male partner following success or accepted following failure performed less well on a subsequent anagram task than did those who were accepted following success or rejected following failure. These results suggest the importance of situational factors as determinants of fear of success behavior.