Irreversible Rearrangements Research Articles

Cluster-Related Phenomena in the Properties and Transformations of Transition Metal-Based Glassy Alloys

A survey of the cluster formation tendency and mechanism in transition metal-based glassy alloys is made with an emphasis on their manifestation in various physical properties. The cluster formation is partially inherited from the supercooling of the melt. However, it also develops due to the interaction between dissolved hydrogen and the frozen glassy structure. The glassy state as “cluster assembly” is regarded as a structural background for the interpretation of several anomalous concentration dependences of thermal and magnetic properties in these glasses. We will focus on the manifestation of alloying effects, the relation between irreversible and reversible structural relaxations both in the high, and low temperature range (observed near to the glass transition or after low temperature storage). The development of the cluster assembly is the consequence of the co-existence of various bonding types between the alloy components. These are brought together in the melt, ensuring sufficient glass-forming ability. The nucleation mechanism of the amorphous-nanocrystalline transformation is also explained as a cluster phenomenon, which significantly contributes to the evolution of magnetic ultra-softness in FINEMET-type alloys. Finally, the role of the quenched-in cluster structure in the mechanism of reversible and irreversible H-absorption is discussed. Irreversible H-induced structural rearrangements can appear as microphase separation in multicomponent systems, governed by the affinity difference between the metallic components and the absorbed hydrogen. This kind of H-induced reordering is responsible for the “volume activation” of amorphous H-storage alloys and it also causes the gradual breakdown of storage capacity during cyclic absorption–desorption steps. This article mainly focuses on the cluster phenomena in Fe-based glasses because of its unique combination of high mechanical strength, strong corrosion resistance, good thermal stability and excellent magnetic properties.

Jamming and irreversibility

We investigate irreversibility in soft frictionless disk packings on approach to the unjamming transition. Using simulations of shear reversal tests, we study the relationship between plastic work and irreversible rearrangements of the contact network. Infinitesimal strains are reversible, while any finite strain generates plastic work and contact changes in a sufficiently large packing. The number of irreversible contact changes grows with strain, and the stress–strain curve displays a crossover from linear to increasingly nonlinear response when the fraction of irreversible contact changes approaches unity.

Structural evolution in deformation-induced rejuvenation in metallic glasses: A cavity perspective**Project supported by the National Natural Science Foundation of China (Grant Nos. 51631003 and 51271197), the Ministry of Science and Technology (MOST) 973 Program, China (Grant No. 2015CB856800), the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University of China (Grant No. 16XNLQ01).

Classical molecular dynamics simulations have been performed to investigate the structural evolution in deformation-induced rejuvenation in Cu80Zr20 metallic glass. Metallic glasses obtained by different cooling rates can be rejuvenated into the glassy state with almost the same potential energy by compressive deformation. The aging effect in different metallic glasses in cooling process can be completely erased by the deformation-induced rejuvenation. The evolution of cavities has been analyzed to understand the structural evolution in rejuvenation. It is found that as metallic glasses are rejuvenated by mechanical deformation, a lot of cavities are created. The lower the potential energy is, the more the cavities are created. The cavities are mainly created in the regions without cavities or with small cavities populated, indicating that the irreversible rearrangements induced by deformation are accompanied by the creation of cavity. This finding elucidates the underlying structural basis for rejuvenation and aging in metallic glasses from the cavity perspective.

Exciton‐ and Polaron‐Induced Reversible Dipole Reorientation in Amorphous Organic Semiconductor Films

AbstractThe quantum efficiency of organic light‐emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) highly depends on the S1−T1 energy gaps (ΔEST) of the emitters. However, the ΔEST values determined in solution or even in organic thin films through a static approach continuously fail in predicting device performance. Herein, by systematically investigating the time‐resolved emission spectra of several TADF emitters in various matrixes and the dielectric spectra of the matrixes, it is demonstrated that molecular organic semiconductors can undergo reversible and irreversible spatial rearrangements within a few dozen nanoseconds in an electric field created by a polarized charge‐transfer state and can in turn lower the excited‐state energy. In contrast to solution solvation, the polarized microenvironment in films can last for tens of microseconds or more after removing the electric field, leading to a narrowing and a broader distribution of ΔEST. In TADF OLEDs, the large dipole moment of emitter‐based polarons is key for reducing the ΔEST of subsequently generated excitons by inducing the dipole orientation of the surrounding molecules. Understanding the so‐called solid‐state solvation with a dynamic approach can provide deeper insight into the working mechanism in organic semiconductor devices.

Irreversible hardening of a colloidal gel under shear: The smart response of natural rubber latex gels

Natural rubber is obtained by processing natural rubber latex, a liquid colloidal suspension that rapidly gels after exudation from the tree. We prepared such gels by acidification, in a large range of particle volume fractions, and investigated their rheological properties. We show that natural rubber latex gels exhibit a unique behavior of irreversible strain hardening: when subjected to a large enough strain, the elastic modulus increases irreversibly. Hardening proceeds over a large range of deformations in such a way that the material maintains an elastic modulus close to, or slightly higher than the imposed shear stress. Local displacements inside the gel are investigated by ultrasound imaging coupled to oscillatory rheometry, together with a Fourier decomposition of the oscillatory response of the material during hardening. Our observations suggest that hardening is associated with irreversible local rearrangements of the fractal structure, which occur homogeneously throughout the sample.

Investigating ceramic breeder pebble beds thermomechanics evolution using novel pressure mapping technology

Ceramic breeder pebble beds undergo complex thermomechanical interactions during blanket operation due to stress build-up and relaxation under the effects of confined thermal expansion, thermal cycling, and creep. Understanding the evolution of such processes can aid in guiding blanket design/assembly, breeder materials developments, predicting performance and possible failure modes. This study introduces experimental techniques that enable us to create an environment in which the pebble bed stresses are organically generated as a result of relevant temperature gradients and magnitudes that allows us to observe the combined thermomechanical interaction effects. A novel non-intrusive in-situ tactile pressure sensing technology is used to generate real-time contact pressure maps revealing the spatial and temporal stress evolution with emphasis on understanding the roles that each of the thermo-mechanical forces play in dictating the pebble bed’s equilibrium conditions. Two types of bed-wall contact pressure drop were recorded: (1) within the subsequent cycles due to pebbles irreversible rearrangements, and (2) within the cycle itself as a result of creep/stress-relaxation. Consequently, by the fourth cycle, the contact pressure drops to negligible values and remains stable. This stability is attributed to two modes of self-regulation: (1) stress self-regulation as a result of pressure rise and fall due to thermal expansion and creep/thermal cycling, respectively, and (2) temperature self-regulation due to the locally enhanced thermal conductivity in the core region of the bed accompanied by deteriorated interface conductance. These two mechanisms are desirable as they lower the probability of the events of pebbles crushing and thermal runaways under high temperatures/stresses and poor heat extraction.

Microscopic signatures of yielding in concentrated nanoemulsions under large-amplitude oscillatory shear

We report x-ray photon correlation spectroscopy (XPCS) experiments on a series of concentrated oil-in-water nanoemulsions with varying droplet volume fraction subjected to in situ steady-state large-amplitude oscillatory shear (LAOS). The shear strain causes periodic echoes in the x-ray speckle patterns that lead to peaks in the intensity autocorrelation function. Above an onset strain amplitude that depends on nanoemulsion concentration, the peaks become attenuated, signaling spatially heterogeneous, shear-induced droplet dynamics. These dynamics include irreversible rearrangements among the droplets that occur in some regions of the nanoemulsions during a given shear cycle and residual strain-like displacements in those regions that do not rearrange. The wave-vector dependence of the peak attenuation indicates a power-law distribution in the size of regions undergoing shear-induced rearrangement that is similar to that observed previously in LAOS-XPCS measurements on concentrated nanocolloidal gels. The values of the onset strains for rearrangement correlate with the concentration-dependent macroscopic yielding behavior of the nanoemulsions. Specifically, they occur below the strains at which the nanoemulsions become effectively fluidized and, except for the lowest-concentration nanoemulsion in the study, significantly above the threshold strain for nonlinear rheological response.

A multiscale shear-transformation-zone (STZ) model and simulation of plasticity in amorphous solids

In this work, we have developed a multiscale shear-transformation-zone (STZ) model to simulate inelastic deformations in amorphous materials at macroscale without using phenomenological constitutive modeling. The main novelties of the multiscale method are two: (1) the multiscale method employs an atomistic-based representative sampling cell (RS-cell) that is embedded in every quadrature points of macroscale finite elements, and it generically contains potential shear-transformation-zones. Thus the evolution of RS-cells can naturally allow molecular clusters having irreversible microstructure rearrangements at microscale in response to applied loads, and subsequently translate the effects of the irreversible rearrangements of molecular clusters in each RS-cell to inelastic responses at the continuum scale, i.e. delineating the defect-mediated amorphous plasticity, and (2) the method uses a Parrinello-Rahman molecular mechanics based the Cauchy-Born rule to construct an atomistically-informed constitutive model at continuum level, which is able to quantitatively measure amorphous plasticity behaviors, including plastic flow stress, inelastic hysteresis loops under cyclic loading, and strain localizations at macroscale. By doing so, we have successfully simulated plastic deformation in the Lennard-Jones binary glass (LJBG) at macroscale level. Moreover, we have shown that the multiscale shear-transformation-zone can capture cyclic plasticity and the shear band formations in LJBG material.

Microscopic dynamics and failure precursors of a gel under mechanical load

Material failure is ubiquitous, with implications from geology to everyday life and material science. It often involves sudden, unpredictable events, with little or no macroscopically detectable precursors. A deeper understanding of the microscopic mechanisms eventually leading to failure is clearly required, but experiments remain scarce. Here, we show that the microscopic dynamics of a colloidal gel, a model network-forming system, exhibit dramatic changes that precede its macroscopic failure by thousands of seconds. Using an original setup coupling light scattering and rheology, we simultaneously measure the macroscopic deformation and the microscopic dynamics of the gel, while applying a constant shear stress. We show that the network failure is preceded by qualitative and quantitative changes of the dynamics, from reversible particle displacements to a burst of irreversible plastic rearrangements.

Nuclear magnetic resonance analysis and activation energy spectrum of the irreversible structural relaxation of amorphous zirconium tungstate

Zirconium tungstate undergoes a sequence of phase transitions from cubic ($\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) to orthorhombic ($\ensuremath{\gamma}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) to amorphous ($a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) upon increasing pressure at room temperature. The amorphous phase is known to undergo anomalous endothermic recrystallization into a high-temperature $\ensuremath{\beta}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ phase above ${600}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}\mathrm{C}$ at ambient pressure (and back to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ when brought to room temperature). The endothermic recrystallization of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is preceded by an irreversible exothermic structural relaxation. New W-O bonds are formed upon amorphization, continuing a tendency of increasing W coordination number in going from $\ensuremath{\alpha}$ to $\ensuremath{\gamma}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$. In fact, contrarily to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$, in which one-quarter of the oxygen atoms are bonded only to one W (terminal oxygens), previous works found no evidence of single-bonded oxygen atoms in $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$. It thus could be argued that the irreversible character of the structural relaxation of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is due to W-O bond breaking upon annealing of the amorphous phase. To test this hypothesis, x-ray diffraction, $^{17}\mathrm{O}$ magic-angle spinning NMR, Raman, and far-infrared analyses were performed on samples of amorphous zirconium tungstate previously annealed to increasingly higher temperatures, looking for any evidence of features that could be assigned to the presence of terminal oxygen atoms. No evidence of single-bonded oxygen was found before the onset of recrystallization. Furthermore, the kinetics of the structural relaxation of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is consistent with a continuous spectrum of activation energy, spanning all the range from 1 to $2.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. These findings suggest that the structural relaxation of amorphous zirconium tungstate, however irreversible, is not accompanied by W-O bond breaking, but most probably characterized by a succession of (mostly) irreversible local atomic rearrangements.

A preliminary study on the formation pathways of glycated phosphatidylethanolamine of food rich in phospholipid during the heat-processing.

The formation of food-derived glycated phosphatidylethanolamine (PE) in thermal process was investigated by designing a 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE)-glucose model system heated from 40 to 100 °C for 8 h. The main products of glycated PE were determined by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Results showed that the glycation of DPPE formed three major glycated compounds: amadori-glycated-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (Amadori-DPPE), carboxymethyl-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (CM-DPPE), and carboxyethyl-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (CE-DPPE). Amadori-DPPE was identified to generate CM-DPPE through oxidative cleavage of glycated polar head group under high temperature and extended incubation time. Additionally, during thermal processing, retro-aldol reactions of glucose led to the formation of two reactive dicarbonyl intermediates: glyoxal (GO) and methylglyoxal (MGO), both of them reacted with amino group of DPPE to form CM-DPPE and CE-DPPE, respectively. Thus, the formation pathways of CM-PE might involve the irreversible rearrangements of Amadori-PE following oxidative cleavage, as well as the glycation of amino group of PE with GO. CE-PE could only be formed by reaction of PE with MGO. Moreover, the content of CM-DPPE was higher than that of CE-DPPE in the same incubation conditions, which indicated that CM-PE might be a more useful predictive marker for food-derived glycated amino-phospholipid, rather than Amadori-PE, particularly in thermal processed foodstuffs.

Slower icosahedral cluster rejuvenation drives the brittle-to-ductile transition in nanoscale metallic glasses

The brittle-to-ductile transition (BDT) in nanoscale multi-element metallic glasses (MGs) has been shown to strongly depend on the element composition and sample aspect ratio. However, the inherent microstructural evolution at the atomic level that leads to such BDT, in particular, the impact from the population and distribution of the full icosahedral clusters, has been largely unexplored. In this paper, we systematically probe the inherent shear band and icosahedral cluster evolutions of CuxZr100-x (x=25, 50 and 75) MGs under different aspect ratios using molecular dynamics simulations. During tensile loading, the initial full icosahedra with local fivefold high-symmetry in CuxZr100-x MGs gradually transform into low-symmetry ones before reaching the maximum stress, which leads to the stress softening. Upon further tensile loading, high-symmetry full icosahedra gradually rejuvenate from low-symmetry ones far away from the shear bands (SBs), accompanied by irreversible atomic rearrangements near the SBs, both resulting in the limited plasticity of MGs. We attribute the observed BDT behavior upon decreasing the aspect ratio primarily to the slower rejuvenation or reformation rates of major full icosahedra. This study provides fundamental guidance for the optimal design of the composition, aspect ratio, and size of MGs for structural materials with high strength and ductility.

Collective nonaffine displacements in amorphous materials during large-amplitude oscillatory shear.

Using molecular dynamics simulations, we study the transient response of a binary Lennard-Jones glass subjected to periodic shear deformation. The amorphous solid is modeled as a three-dimensional Kob-Andersen binary mixture at a low temperature. The cyclic loading is applied to slowly annealed, quiescent samples, which induces irreversible particle rearrangements at large strain amplitudes, leading to stress-strain hysteresis and a drift of the potential energy towards higher values. We find that the initial response to cyclic shear near the critical strain amplitude involves disconnected clusters of atoms with large nonaffine displacements. In contrast, the amplitude of shear stress oscillations decreases after a certain number of cycles, which is accompanied by the initiation and subsequent growth of a shear band.

Colloidal polycrystalline monolayers under oscillatory shear.

In this paper we probe the structural response to oscillatory shear deformations of polycrystalline monolayers of soft repulsive colloids with varying area fraction over a broad range of frequencies and amplitudes. The particles are confined at a fluid interface, sheared using a magnetic microdisk, and imaged through optical microscopy. The structural and mechanical response of soft materials is highly dependent on their microstructure. If crystals are well understood and deform through the creation and mobilization of specific defects, the situation is much more complex for disordered jammed materials, where identifying structural motifs defining plastically rearranging regions remains an elusive task. Our materials fall between these two classes and allow the identification of clear pathways for structural evolution. In particular, we demonstrate that large enough strains are able to fluidize the system, identifying critical strains that fulfill a local Lindemann criterion. Conversely, smaller strains lead to localized and erratic irreversible particle rearrangements due to the motion of structural defects. In this regime, oscillatory shear promotes defect annealing and leads to the growth of large crystalline domains. Numerical simulations help identify the population of rearranging particles with those exhibiting the largest deviatoric stresses and indicate that structural evolution proceeds towards the minimization of the stress stored in the system. The particles showing high deviatoric stresses are localized around grain boundaries and defects, providing a simple criterion to spot regions likely to rearrange plastically under oscillatory shear.

Adhesion, friction and lubrication of nano- and micro-structured surface coatings

Event Abstract Back to Event Adhesion, friction and lubrication of nano- and micro-structured surface coatings Suzanne Giasson1, 2 and Lucie Giraud1 1 University of Montreal, Faculty of Pharmacy, Canada 2 University of Montreal, Chemistry Department, Canada Self-lubricating surfaces or water-based lubricants are highly in demand for biomedical applications that require friction and wear control. Numerous tribological studies of lubricant coated-surfaces in aqueous media have been reported. However, the role of surface structure and elasticity on surface properties remains highly contentious because of the lack of consistent control of these parameters. We investigated the effect of both parameters (structure and elasticity) on adhesion and friction properties using soft and hard nano- and micro-particles covalently attached on silica-based substrates. Silica spheres were used as hard particles and pH sensitive polymer particles containing poly(acrylic acid) as soft ones. All particles were self-assembled from solution giving rise to a monolayer of particles with heterogeneous coverage but equivalent surface patterns. The adhesion and friction forces as function of applied load between two identically structured surfaces in aqueous media were determined using the Surface Forces Apparatus (SFA) Our results show that all structured surfaces exhibit friction coefficient similar or even lower than that measured on smooth coated surfaces under significantly higher applied loads. The lowest friction coefficient was measured between hard structured surfaces. This most probably arises from the effect of asperities on reducing the real contact area which is most evidenced with non-deformable hard structures. In addition, the elastic contribution to friction for hard structures is negligible resulting in constant dissipation energy between the surfaces leading to very low friction coefficient (10-3<µ<10-2). The friction coefficient measured with the nanoparticles was larger than that between micron- sized ones. Adhesion was observed only for the first compression of the surfaces during which irreversible rearrangements of hard particles occurred. Subsequent compressions and surface separations were reproducible and purely repulsive. The friction behaviour of the soft patterned surfaces was quite different than that of hard structured ones mainly because the soft polymeric structures provide elastic effects that can be tuned via changes in environmental conditions (pH, ionic strength). Increasing pH of the aqueous media gave rise to a swelling of the immobilized soft particles due to an increase in the degree of ionization of the polymer. The friction coefficient decreased with increasing the pH and swelling (µ varying from 1 to 10-2 for pH 4 to 10). No adhesion was measured between the highly charged and swollen soft structured surfaces. The variety of surface interactions arising from such structured surfaces is promising for the development of new coatings for surfaces immersed in aqueous media and subjected to high loads and shear rates, such as those of artificial implants where friction and wear should be minimized. Keywords: self-assembly, surface property, Microfriction, Nano/micro particle Conference: 10th World Biomaterials Congress, Montréal, Canada, 17 May - 22 May, 2016. Presentation Type: Poster Topic: Interfacial phenomena Citation: Giasson S and Giraud L (2016). Adhesion, friction and lubrication of nano- and micro-structured surface coatings. Front. Bioeng. Biotechnol. Conference Abstract: 10th World Biomaterials Congress. doi: 10.3389/conf.FBIOE.2016.01.02219 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Mar 2016; Published Online: 30 Mar 2016. Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Suzanne Giasson Lucie Giraud Google Suzanne Giasson Lucie Giraud Google Scholar Suzanne Giasson Lucie Giraud PubMed Suzanne Giasson Lucie Giraud Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Understanding the effects of inter-particle contact friction on the elastic moduli of granular materials

Understanding the mechanical stiffness of closely packed, dense granular systems is of interest in many fields, such as soil mechanics, material science and physics. The main difficulty arises due to discreteness and disorder in granular materials at the microscopic scale which requires a multi-scale approach. The Discrete Element Method (DEM) is a powerful tool to inspect the influence of the microscopic contact properties of its individual constituents on the bulk behavior of granular assemblies. In this study, the isotropic deformation mode of polydisperse packings of frictionless and frictional spheres are modeled by using DEM, to investigate the effective stiffness of the granular assembly. At various volume fractions, for every sample, we determine the stress and fabric incremental response that result from the application of strain-probes. As we are interested first in the reversible, elastic response, the amplitude of the applied perturbations has to be small enough to avoid opening and closing of too many contacts, which would lead to irreversible rearrangements in the sample. Counterintuitively, with increasing inter-particle contact friction, the bulk modulus decreases systematically with the coefficient of friction for samples with the same volume fraction. We explain this by the difference in microstructure (isotropic fabric) the samples get when compressed to the same density.

Echoes in x-ray speckles track nanometer-scale plastic events in colloidal gels under shear.

We report x-ray photon correlation spectroscopy experiments on a concentrated nanocolloidal gel subject to in situ oscillatory shear strain. The strain causes periodic echoes in the speckle pattern that lead to peaks in the intensity autocorrelation function. Above a threshold strain that is near the first yield point of the gel, the peak amplitude decays exponentially with the number of shear cycles, signaling irreversible particle rearrangements. The wave-vector dependence of the decay rate reveals a power-law distribution in the size of regions undergoing shear-induced rearrangement. The gel also displays strain softening well below the threshold, indicating a range of strains at which the rheology is nonlinear but the microscopic deformations are reversible.

Plastic rearrangements in colloidal gels investigated by LAOS and LS-Echo

We investigate the yielding behavior of colloid-polymer gels with intermediate volume fraction, which are subjected to large amplitude oscillatory shear, using rheology and light scattering echo (LS-Echo). Particular attention is given to the anharmonic contributions to the stress response and the characteristic timescale and extent of plastic rearrangements. Yielding is already observed at small strain amplitudes γ0≈1%, where the network of interconnected clusters starts to break up and irreversible particle rearrangements are first observed. However, only at considerably larger strain amplitudes, γ0≳100%, the network is completely disrupted and small clusters or individual particles flow. This complex yielding behavior is reflected in different regimes of in-cycle yielding, which were extracted from the analysis of the anharmonic contributions to the stress. In the range of strain amplitudes where bond breaking starts, in-cycle strain hardening and shear thickening are observed, with the strain hardening possibly caused by the shear-induced formation of more compact clusters. When the network is broken down, in-cycle shear thinning is observed with increasing strain amplitude, but still together with strain hardening. Both, the elastic and viscous nonlinear contributions, decrease with increasing strain amplitude, indicating the progressive fluidization of the system, consistent with the faster plastic rearrangements observed in LS-Echo. Two-distinct frequency-dependent regimes for the initial yielding at small strains are present: At small to moderate oscillation frequencies, the first yield strain increases with increasing frequency, while it decreases at large frequencies. This might be associated with the timescale for bond breaking, namely, the Brownian diffusion time at small frequencies and the inverse of the oscillation frequency at large frequencies.

Fusion Activation through Attachment Protein Stalk Domains Indicates a Conserved Core Mechanism of Paramyxovirus Entry into Cells

Paramyxoviruses are a large family of membrane-enveloped negative-stranded RNA viruses causing important diseases in humans and animals. Two viral integral membrane glycoproteins (fusion [F] and attachment [HN, H, or G]) mediate a concerted process of host receptor recognition, followed by the fusion of viral and cellular membranes, resulting in viral nucleocapsid entry into the cytoplasm. However, the sequence of events that closely links the timing of receptor recognition by HN, H, or G and the "triggering" interaction of the attachment protein with F is unclear. F activation results in F undergoing a series of irreversible conformational rearrangements to bring about membrane merger and virus entry. By extensive study of properties of multiple paramyxovirus HN proteins, we show that key features of F activation, including the F-activating regions of HN proteins, flexibility within this F-activating region, and changes in globular head-stalk interactions are highly conserved. These results, together with functionally active "headless" mumps and Newcastle disease virus HN proteins, provide insights into the F-triggering process. Based on these data and very recently published data for morbillivirus H and henipavirus G proteins, we extend our recently proposed "stalk exposure model" to other paramyxoviruses and propose an "induced fit" hypothesis for F-HN/H/G interactions as conserved core mechanisms of paramyxovirus-mediated membrane fusion. Paramyxoviruses are a large family of membrane-enveloped negative-stranded RNA viruses causing important diseases in humans and animals. Two viral integral membrane glycoproteins (fusion [F] and attachment [HN, H, or G]) mediate a concerted process of host receptor recognition, followed by the fusion of viral and cellular membranes. We describe here the molecular mechanism by which HN activates the F protein such that virus-cell fusion is controlled and occurs at the right time and the right place. We extend our recently proposed "stalk exposure model" first proposed for parainfluenza virus 5 to other paramyxoviruses and propose an "induced fit" hypothesis for F-HN/H/G interactions as conserved core mechanisms of paramyxovirus-mediated membrane fusion.

Nano-scale mechanics of colloidal C–S–H gels

Gels of calcium-silicate-hydrates (C-S-H) are the glue that is largely responsible for the mechanical properties of cement. Despite their practical relevance, their nano-scale structure and mechanics are still mainly unexplored, because of the difficulties in characterizing them in a complex material like cement. We propose a colloidal model to investigate the gel mechanics emerging in the critical range of length-scales from several tens to hundreds of nanometers. We show that the size polydispersity of the hydrates and size-dependent effective interactions can explain the mechanical heterogeneities detected in nano-indentation experiments. We also show how these features control the arising of irreversible structural rearrangements under deformation, which are good candidates as nano-scale mechanisms underlying mechanical aging and slow structural relaxation in the gels.