Effective medium theory of semiflexible filamentous networks

We develop an effective medium approach to the mechanics of disordered, semiflexible polymer networks and study the response of such networks to uniform and nonuniform strain. We identify distinct elastic regimes in which the contributions of either filament bending or stretching to the macroscopic modulus vanish. We also show that our effective medium theory predicts a crossover between affine and nonaffine strain, consistent with both prior numerical studies and scaling theory.     

一种新型光纤法布里-珀罗应变干涉仪

论述了一种非本征法布里 珀罗光纤应变传感器。采用透明弹性聚合物薄膜作为法布里 珀罗干涉腔,聚合物固定在多膜光纤末端作为应变敏感元件。传感器的分辨率为2μm,测量精度在0～5000μm范围内为5μm。     

Influence of surface strain on the MOVPE growth of InGaP epitaxial layers

In this paper, we present a study of the influence of strain on the composition uniformity, phase separation and ordering of compressive and tensile strained InGaP epitaxial layers grown on GaP and GaAs substrates. The paper demonstrates how strain, caused by lattice mismatch, influences the spinodal-like decomposition and ordering in InGaP layers. We concentrated our efforts on an alloy with the lattice mismatch close to a value of Δa/a=10^-2. Transmission electron microscopy (TEM), low temperature photoluminescence and atomic force microscopy (AFM) were used to demonstrate that epitaxial layers grown under compressive strain have additional properties in comparison to those ones grown under tensile strain. The presence of ordering, spinodal-like decomposition and compositional fluctuations was studied in relation to the sign of strain. PACS 61.72.Lk; 68.35.Ct; 68.37.Lp; 68.35.Dv; 78.30.Fs     

Filamin cross-linked semiflexible networks: fragility under strain

The semiflexible F-actin network of the cytoskeleton is cross-linked by a variety of proteins including filamin, which contains Ig domains that unfold under applied tension. We examine a simple filament network model cross-linked by such unfolding linkers that captures the main mechanical features of F-actin networks cross-linked by filamin proteins and show that, under sufficient strain, the network spontaneously self-organizes so that an appreciable fraction of the filamin cross-linkers are at the threshold of domain unfolding. We propose and test a mean-field model to account for this effect. We also suggest a qualitative experimental signature of this type of network reorganization under applied strain that may be observable in intracellular microrheology experiments of Crocker et al.     

Analysis of spontaneous activity in neuronal cultures through recurrence plots: impact of varying connectivity

We analyzed spontaneous activity of cortical neuronal networks in vitro using recurrence plots (RPs). Our data encompasses fluorescence traces of average network activity from two experimental explorations, namely the development of connections during the maturation of the network and the gradual weakening of connections through chemical action. The dynamical richness of the networks in these connectivity-evolving scenarios was examined through recurrence quantification analysis. Measures such as determinism and laminarity were used to portray the degree of uniformity and periodicity of the spontaneous activity patterns. The analysis shows that RPs are a powerful tool to visualize and interpret neuronal networks dynamics, and pinpoint its hallmarks.     

Strain-dependent localization, microscopic deformations, and macroscopic normal tensions in model polymer networks

We use molecular dynamics simulations to investigate the microscopic and macroscopic response of model polymer networks to uniaxial elongations. By studying networks with strand lengths ranging from N(s)=20 to 200 we cover the full crossover from cross-link to entanglement dominated behavior. Our results support a recent version of the tube model which accounts for the different strain dependence of chain localization due to chemical cross-links and entanglements.     

Effect of barrier thickness on strain uniformity of wire in laterally aligned InGaAs/GaAs quantum wire nanostructures

The effect of barrier thickness on strain uniformity of a laterally aligned array of InGaAs quantum wire in GaAs matrix has been investigated with the finite elements method. A decrease in GaAs barrier thickness was predicted to assist the InGaAs wire to maintain its strain state in the central region up to a longer distance towards the edge of the wire along the width direction. It is suggested that, by reducing the spacing between the quantum wires, it is possible to improve uniformity of strains within the wire, thereby yielding more uniform opto-electronic properties such as sharp and narrow peaks in photoluminescence spectra.     

One-dimensional single (In,Ga)As quantum dot arrays formed by self-organized anisotropic strain engineering

Well-defined one-dimensional single (In,Ga)As quantum dot (QD) arrays have been successfully formed on planar singular GaAs (1 0 0) in molecular beam epitaxy by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs quantum wire (QWR) superlattice (SL) template. The distinct stages of template formation, which govern the uniformity of the QD arrays, are directly imaged by atomic force microscopy (AFM). The AFM results reveal that excess strain accumulation causes fluctuations of the QWR template and the QD arrays. By reducing the amount of (In,Ga)As and increasing the GaAs separation layer thickness in each SL period, the uniformity of the QD arrays dramatically improves. The single QD arrays are straight over more than 1 μm and extended to 10 μm length. Capped QD arrays show clear photoluminescence emission up to room temperature.     

Characteristics of strain transfer and the reflected spectrum of a metal-coated fiber Bragg grating sensor

Previous researchers have simulated strain transfer and spectrum of normal fiber Bragg grating (FBG) sensors with a polymer coating bonded on the structure. They only considered the shear stress in a polymer coating for the simulation. However, for metal-coated FBG sensors, not only shear stress but also axial stress in the metal coating should be reflected into the calculation because its axial stiffness is no longer negligible. Thus, the author investigated the strain transfer and reflected spectra of metal-coated FBG sensors by considering both shear stress and axial stress. The strain transfer analysis involved evaluating the strain profiles along the sensor by plotting an analytical solution, and validating the evaluated profiles with the results obtained by a finite element analysis (FEA). The solution was also verified by the experiments that used aluminum-coated FBG sensors bonded on a carbon fiber reinforced polymer (CFRP) composite specimen. A transfer-matrix (T-matrix) formulation and coupled mode theory were used to simulate the reflected spectra of metal-coated FBG sensors for the evaluated strain profile. In addition, the effect of mechanical and geometric parameters of the sensor was examined. The findings revealed that the strain transfer characteristics and reflected spectra deteriorated with increases in the thickness and Young's modulus of the metal coating due to the consideration of axial stress. It is the opposite results for the normal FBG sensor with a polymer coating. Furthermore, the results also indicated that the decrease in bonding thickness resulted in improved strain transfer and signal characteristics. Moreover, a bonding length of 14 mm was suitable in suppressing an asymmetric shape of the reflected spectrum and in achieving an accurate measurement. The results of the parametric study are expected to contribute to improve the measurement accuracy of metal-coated FBG sensors in actual applications. The analytical methodology can be usefully employed in the design of a metal-coated FBG sensor system.     

Quasistatic rheology and the origins of strain

Features of rheological laws applied to solid-like granular materials are recalled and confronted to microscopic approaches via discrete numerical simulations. We give examples of model systems with very similar equilibrium stress transport properties—the much-studied force chains and force distribution—but qualitatively different strain responses to stress increments. Results on the stability of elastoplastic contact networks lead to the definition of two different rheological regimes, according to whether a macroscopic fragility property (propensity to rearrange under arbitrary small stress increments in the thermodynamic limit) applies. Possible consequences are discussed. To cite this article: J.-N. Roux, G. Combe, C. R. Physique 3 (2002) 131–140.     

Tuning strain relaxation by surface morphology: Surfactant-mediated epitaxy of germanium on silicon

In this paper, we summarize the results on the surfactant-mediated epitaxy (SME) of germanium on (0 0 1) and (1 1 1) silicon substrates. Then, we discuss, how the surfactant-controlled development of micro-facets determines the strain relaxation process. We place particular emphasis on the different types of strain-compensating dislocation networks that form at the Ge/Si(0 0 1) interface in epitaxy with and without Sb as a surfactant. At elevated temperatures, high Sb-coverage promotes the generation of a regular array of edge type misfit dislocations, which allows for abrupt instead of gradual strain relaxation in the initial stage of growth.     

超薄GaMnAs外延膜空穴浓度和应变弛豫研究

稀磁GaMnAs外延膜中的Mn含量会影响外延膜的空穴浓度和应变弛豫.Raman散射研究表明,Mn含量为3%的超薄GaMnAs样品的空穴浓度大于2%样品,4%样品的空穴浓度小于3%样品.应变弛豫理论和高分辨X射线衍射研究表明,Mn含量为2%和3%的超薄GaMnAs外延层分别处于准共格或低弛豫状态,Mn含量为4%的GaMnAs外延层的弛豫度明显大于3%样品的弛豫度.我们认为,准共格或低弛豫度状态对空穴浓度随Mn含量的变化趋势几乎没有影响,较大弛豫度的应变状态将导致样品外延层产生较多缺陷,影响能带结构和能级,引起空穴浓度异常减小. 关键词： 空穴浓度 应变弛豫 倒易空间图 准共格     

Order and strain in main-chain smectic liquid-crystalline polymers and elastomers

The layer correlations in main-chain smectic liquid-crystal polymer and elastomer systems have been studied using high-resolution X-ray scattering. In contrast to side-chain smectic polymers, in main-chain systems the polymer chains are oriented parallel to the layer normal. As a result they couple directly to the lamellar structure and any polymer defect is translated into layer distortions. For the homopolymers the resulting X-ray lineshapes are well described by Lorentzians. This is interpreted as an average of algebraically decaying order in domains with dimensions of hundreds of nm and a wide dispersion of sizes. The elastomers show much broader peaks than the correponding polymers. This is attributed to strong non-uniform strain within the finite-size domains due to defects of the layer structure. An erratum to this article is available at .     

Reliable Polymer-Stabilized Ferroelectric Liquid Crystal Device Using Thin Plastic Substrates

We developed a rollable polymer-stabilized ferroelectric liquid crystal (FLC) display device using thin plastic substrates. In a device using 200-μm-thick substrates which are fastened by polymer walls and networks made by photopolymerization-induced phase separation, disorder of the FLC alignment was caused by exfoliation of these walls in curvature radii of under 30 mm. Otherwise, the uniformity of the FLC alignment was maintained even after a device using 100-μm-thick substrates was bent at a radius of 7 mm. The enhanced bending tolerance does not depend on the FLC alignment direction, and the device could be bent both convexly and concavely without any FLC alignment change. A rollable device with a size of 100 × 100 mm exhibited uniform grayscale display operation between crossed polarizers when bent with a radius of 15 mm.     

Disorder and optical gaps in strained dense amorphous carbon and diamond nanocomposites

We employ empirical tight-binding simulations on strained tetrahedral amorphous carbon and diamond nanocomposite networks. For each applied strain, the optoelectronic properties are monitored through the absorption coefficient and the dielectric function. These lead to the optical gaps and are able to quantify the amount of disorder in the structures. We compare our results to those of unstrained nanostructured diamond and amorphous carbon (a-C) phases and link the degree of disorder in these materials to their structural details as a function of the external load. The atomic rearrangements and distortions imposed by the external strain in these structures are directly observable in their optoelectronic properties. We thoroughly discuss the interplay between increased external strain, structural and topological disorder, atomic rearrangements and their effect on the calculated optoelectronic properties.     

"Bending to stretching" transition in disordered networks

From polymer gels to cytoskeletal structures, random networks of elastic material are commonly found in both materials science and biology. We present a three-dimensional micromechanical model of these networks and identify a "bending-to-stretching" transition. We characterize this transition in terms of concentration scaling laws, the stored elastic energy, and affinity measurements. Understanding the relationship between microscopic geometry and macroscopic mechanics will elucidate, for example, the mechanical properties of polymer gel networks or the role of semiflexible network mechanics in cells.     

有限元法分析不同形状量子点的应变能及弛豫度变化

利用有限元方法研究了不同形状量子点的应变能量分布和弛豫度随着高宽比变化的规律.分析了量子点间距和量子点形状对量子点应变弛豫的影响,定量地讨论了量子点的弛豫度与量子点形状之间的关系.计算结果表明,在不考虑表面能的情况下,当量子点高宽比增加时,弛豫度上升,并且发现平顶金字塔形量子点最先达到稳定;岛间距增大时,量子点内应变能下降,其中立方体形量子点应变能下降最快.研究表明,量子点的弛豫度可以成为控制量子点成岛形状的重要依据. 关键词： 量子点 弛豫度     

Synthesis of novel promising materials via impregnation of crosslinked polymeric networks with metal complexes in supercritical carbon dioxide

Three types of polymers, phenol-formaldehyde copolymer and polypropyleneimine and polyamidoamine dendrimers, are synthesized using the polycondensation method. Mesoporous polymeric networks are formed by treating the above dendrimers with crosslinking agents. Impregnation of polymeric networks with rhodium acetylacetonate in supercritical carbon dioxide has been carried out for the first time and its optimal conditions have been determined. Using Raman spectroscopy, it has been shown that the metal content in polymer networks falls in the range 0.3–2.0 mass % depending on the type of the polymer and the crosslinking agent and on the crosslinking degree. Polymer samples including rhodium nanoparticles have been prepared by reducing immobilized rhodium acetylacetonate with molecular hydrogen. According to the preliminary data, the resulting samples possess extremely high activity as catalysts for hydrogenation of unsaturated compounds.     

Anomalous strain effect in heteroepitaxial SrRuO3 films on (111) SrTiO3 substrates

We report comprehensive investigations into the structure of high-quality (111)-oriented SrRuO₃ films on SrTiO₃ substrates to elucidate the effect of (111) heteroepitaxial strain. We found that SrRuO₃ film with a thickness of ～ 40 nm is compressively strained in plane on the substrate with full coherency. Nevertheless, the out-of-plane spacing is almost the same as in the bulk, which is at odds with the conventional paradigm. By probing a series of half-order Bragg reflections using synchrotron-based x-ray diffraction combined with analyses of the scanning transmission electron microscopy images, we discovered that the heteroepitaxial strain is accommodated via significant suppression of the degree of c⁺ octahedral tilting and the formation of three equivalent domain structures on the (111) SrTiO₃ substrate. This anomalous effect sheds light on the understanding of an unconventional paradigm of film-substrate coupling for the (111) heteroepitaxial strain.