This study demonstrates that the free-surface flow dynamics of dense piles of contactless silica microparticles depend on the resting period prior to flow. Microfluidic rotating drum experiments reveal that longer resting times lead to delayed flow onsets and reduced flow velocities, both evolving logarithmically with the resting time. These aging effects are more pronounced for thermally driven creep flows in piles with initial tilting angle below the athermal angle of repose, in contrast to piles initially tilted above this repose angle, where gravity-driven flows tend to gradually erase aging effects. Moreover, we show that the packing fraction does not change during the resting period, and that aging occurs in both monodisperse and polydisperse piles, indicating that crystallization is not required for the time-dependent behavior to appear. Remarkably, vigorous agitation that redisperses the particles fully restores the piles to their initial state, demonstrating that the observed effects are not due to sample degradation. These findings evidence a form of aging in quiescent suspensions intermediate between colloidal and granular media, where thermal fluctuations, still significant relative to particle weight, progressively stabilize the system, making it more resistant to flow and deformation.

We investigate the angle of repose θ_r of dense suspensions of colloidal silica particles (d = 2 μm to 7 μm) in water-filled microfluidic rotating drums experiments, to probe the crossover between the thermal (colloidal) and athermal (granular) regimes. For the smallest particles, thermal agitation promotes slow creep flows, and piles always flatten completely regardless of their initial inclination angle, resulting in θ_r = 0. Above a critical particle size, piles of colloids stop flowing at a finite angle of repose, which increases with particle size but remains below the minimal value expected for athermal frictionless granular materials: 0 < θ_r < θ_ath≈ 5.8°. We quantify the arrest dynamics as a function of the gravitational Péclet number Pe_g, which characterizes the competition between particle weight and thermal agitation. Our measurements are consistent with a recent rheological model [Billon et al., Phys. Rev. Fluids 8, 034302, 2023], in which the arrested state stems from a crossover between glass-like and jamming-dominated regimes as the granular pressure in the pile increases relative to the thermal pressure.

Creep tests on heterogeneous materials under subcritical loading typically show a power-law decay in strain rate before failure, with the exponent often considered material dependent but independent of applied stress. By imposing successive small stress relaxations through a displacement feedback loop, we probe creep dynamics and show experimentally that this exponent varies with both applied load and loading direction. Simulations of a disordered fiber bundle model reproduce this load dependence, demonstrating that such models capture essential features of delayed rupture dynamics.

Copper-thiolate coordination polymers (CPs) are photoluminescent compounds displaying good thermal stability and photoemission up to 120 °C, offering a high potential for optical temperature sensor applications. However, achieving composite films with stable RGB (red, green, and blue) emission remains a significant challenge in terms of material fabrication and photophysical efficiency. This study presents a process to disperse homogeneously insoluble red- and green-emissive CPs in PDMS (polydimethylsiloxane) matrices to fabricate environmentally friendly, transparent, flexible, stretchable, and large-scale composite films. Depending on the ratio of the mixture of three primary colors, red- and green-emissive CPs mixed with a blue-emissive organic polymer, a broad array of colored emissive composite films, including white, can be produced. Moreover, a composite film made of red- and green-emissive CPs has been studied for ratiometric temperature sensing. The temperature of the film can be directly read with a unique dual-camera setup, providing good spatial and temporal resolutions and thereby paving the way for optical temperature sensors complementary to traditional infrared cameras that enable the measurement of the temperature of surfaces.

Hydrogen induced single crack propagation is studied in an embrittled aluminum alloy. Hydrogen is introduced into the system by electrochemical reactions in an acidic aqueous medium. After hydrogen charging, tensile tests are performed in air, on notched samples, with a microtensile machine under an optical microscope. A high magnification of x 2000 is used to follow the single crack initiation and propagation. Digital Image Correlation gives the displacement field on the surface with a spatial resolution of approximately 1 mm. It enables the determination of the position of the crack tip and the local velocity at a sub grain scale. The von Mises strain is calculated and provides a precise measure of the local plastic field that accompanies crack propagation. In addition to the primary plasticity which is emitted from the crack tip or its immediate neighborhood in the form of two intense slip bands, a secondary plastic zone that spreads over several microns ahead of the tip is sytematically found. The characteristics of the plastic zone are measured, together with the velocity and the applied stress intensity factor. In addition, different fracture mechanisms are found on the fracture surface. In particular there are transitions in the fracture mode from intergranular smooth to transgranular parallel to the grain boundary plane. The local fracture mechanisms, in the vicinity of the surface, are linked to the local velocities and plastic deformations. Surprisingly no strong velocity/plasticity correlations are found while the velocities are scattered over a wide range, which is interpreted as a strong polycrystalline effect.& COPY; 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The d10 coinage metal coordination polymers (CPs) are known to display photophysical properties which can be tuned depending on the functionality of the ligand. Three new CPs made of d10 coinage metals and methyl thiosalicylate, [M(o-SPhCO2Me)]n (M = Cu, Ag, Au), are reported. They are all constructed from one-dimensional metal-sulfur networks, in which Cu and Ag are three-coordinated to sulfur atoms, whereas Au is only two-coordinated. It results that both Cu(I) and Ag(I) CPs show orange photoemission at room temperature, and the Au(I) one exhibits near-infrared emission at low temperatures. The intense orange-emissive Ag(I) CP and the blue-emissive coumarin 120 have been mixed in an organic matrix, the polyvinylidene fluoride (PVDF), to form a dual luminescent flexible composite film. This film, evaluated for thermometry, shows great sensitivity for temperatures up to 100°C, a temperature never reached with non-lanthanide-based CPs.

The capillary underfill is the most commonly used method to fill the interconnection gap in a flip chip structure. Its main role is to give a good mechanical strength to the structure and protect the interconnections from the severe environment. With the evolution of electronic devices, and the need for higher resolutions as in the case of the display market, the interconnections are getting smaller and their pitch lower and lower. This causes the underfill operation to be more challenging to avoid voids and defects formation. Specifically, in the micro display screens, when there is lot of defects, it can create a cluster visible when the device is working. The goal of this study is to understand the wetting and adhesion mechanisms between the epoxy resin and the different materials of the flip chip structure, in order to better prevent these defects. We based our research on epoxy based underfill within a flip chip structure with a large array of copper pillar interconnections at 30μm pitch.

Anticipating the overall evolution of an epidemic and estimating the impact of sanitary measures are central to appropriate health crisis management and societal decision-making. However, accurate early projections remain difficult to make. Building on a new analogy with chemical reactions, we show that an epidemic decelerates exponentially since the very first instants of the outbreak. The rise and fall of an epidemic wave directly follows from this exponential relaxation, allowing prediction of the peak time and amplitude of the incoming wave. Furthermore, changes in social behavior trigger epidemic subwaves affecting the relaxation dynamics of the epidemic. The entropy of mixing between noninfected and infected people that drives the relaxation process helps decipher the most likely evolution of an epidemic outbreak.

We present here a novel cantilever based apparatus to perform translational stress or strain controlled rheology in very soft solids, and obtain simultaneous confocal imaging of the 3D microstructure. The stress is measured using eddy based sensors. Both the stress and strain are controlled by applying PID control loops on measured quantities and changing position using a micromanipulator. To get rid of surface tension forces, the sample and cantilever are immersed. This enables stress measurement and control down to 6 mPa. With this apparatus, we can independently apply shear and normal stress, or strain, with same precision. We demonstrate the technical capability of the setup with steady shear strain or stress experiments on a soft protein gel system. The simultaneous confocal imaging offers insight into the macroscopic breaking observed in an increasing shear strain experiment.

The shape of an epidemic wave in simple epidemic models applies to a homogeneous distribution of infected people in the population. In large inhomogeneous systems, at country-scale for instance, the wave shape is similar except for the short time behavior. For such cases, we show that the full wave shape is tied to an exponential decay. Using out-of-equilibrium thermodynamics, we build a model in which this decay results from an increase in entropy until reaching a stable infected population fraction. We find that the elementary probability of being infected determines this fraction, leading to a thermodynamic criterion for herd immunity and epidemic outbreaks.

We analyze the dilation of the system in a cylindrical granular fault consisting of one single layer of disks submitted to both normal pressure and continuous and slow shear, which results in intermittent and sudden energy release events that reproduce the main laws of seismicity. The dilation of the system can be separated into two parts: a smooth increase of dilation, plus sudden changes both contracting and dilating the medium, which are correlated to abrupt jumps -both positive and negative- in the measured resisting torque. We explain the four possible (and existing) general scenarios combining those two variables: dilation jumps and torque jumps, thanks to the assumption of an optimal local angle in the direction of force chains, and each reorganization of the structure as a replacement of the force chain holding most of the applied stress. The average rate of increase of global dilation varies monotonically with the size of the energy release event, making dilation a plausible candidate to predict catastrophic events in such earthquake-like systems.

The growth of fractures within mechanically loaded materials often shows two different behaviors. When loaded below a particular threshold in energy release rate, cracks tend indeed to creep at very slow velocities, while the rupture becomes catastrophic beyond this threshold, with propagation velocities approaching that of the material mechanical waves. Understanding according to which of these two behaviors a material is prone to break is of paramount importance, notably in engineering, where the brittle rupture of structures can lead to unpredicted disasters. It is also fundamental in Earth science, as damaging earthquakes are rather generated by abrupt ruptures in the crustal rocks than by their slow deformations. To explain both behaviors, we focus here on the thermal effects which are auto-induced by the growth of cracks. During their propagation, some of the system’s energy is indeed partly dissipated by Joule heating, which is arising from the friction in a damaged zone around the fracture fronts. The heat hence generated can in return have a significant impact on the physics of the propagation. For instance, the stability of faults is believed to be affected by the thermo-pressurization of their in situ fluids. Independently of this effect, we show, how statistical physics, as understood by an Arrhenius law that includes the dissipation and diffusion of heat around the fracture tip, can explain the full dynamics of cracks, from the slow creep to the fast rupture. We indeed show that such a model can successfully describe most of the experimentally reported fracture rheology, quantified in terms of velocity / energy release rate relations, in two different types of polymers, acrylic glasses and pressure sensitive adhesives, over eight decades of crack velocities. In these two cases, it is sufficient to assume that these polymers are homogeneous to model their failure. Yet, we in addition illustrate how the thermal disorder, from both the ambient temperature and the propagation induced heat, should interact with the matter typical quenched disorder in fracture energy. Numerical simulations of planar cracks in heterogeneous media indeed show that such quenched disorder helps to trigger hot avalanches in the propagation of cracks, making the overall toughness of a material highly dependent on both its heterogeneities, as it is often reported in the literature, and its thermal properties.

While of paramount importance in material science, the dynamics of cracks still lacks a complete physical explanation. The transition from their slow creep behavior to a fast propagation regime is a notable key, as it leads to full material failure if the size of a fast avalanche reaches that of the system. We here show that a simple thermodynamics approach can actually account for such complex crack dynamics, and in particular for the non-monotonic force-velocity curves commonly observed in mechanical tests on various materials. We consider a thermally activated failure process that is coupled with the production and the diffusion of heat at the fracture tip. In this framework, the rise in temperature only affects the sub-critical crack dynamics and not the mechanical properties of the material. We show that this description can quantitatively reproduce the rupture of two different polymeric materials (namely, the mode I opening of polymethylmethacrylate (PMMA) plates, and the peeling of pressure sensitive adhesive (PSA) tapes), from the very slow to the very fast fracturing regimes, over seven to nine decades of crack propagation velocities. In particular, the fastest regime is obtained with an increase of temperature of thousands of Kelvins, on the molecular scale around the crack tip. Although surprising, such an extreme temperature is actually consistent with different experimental observations that accompany the fast propagation of cracks, namely, fractoluminescence (i.e., the emission of visible light during rupture) and a complex morphology of post-mortem fracture surfaces, which could be due to the sublimation of bubbles.

We report an extensive experimental study of a detachment front dynamics instability, appearing at microscopic scales during the peeling of adhesive tapes. The amplitude of this instability scales with its period as A_{mss}∝T_{mss}^{1/3}, with a prefactor evolving slightly with the peel angle θ, and increasing systematically with the bending modulus B of the tape backing. Establishing a local energy budget of the detachment process during one period of this microinstability, our theoretical model shows that the elastic bending energy stored in the portion of tape to be peeled is converted into kinetic energy, providing a quantitative description of the experimental scaling law.

While of paramount importance in material science and engineering, the rupture of solids is often described by empirical observations rather than by fully understood physical models. The earliest formalism is probably that by Griffith: the propagation of cracks is described as a threshold phenomenon, only obtained when fractures are loaded above a critical fracture energy. This view matches the behaviour of brittle matter, which suddenly snaps passed a certain elastic deformation. It is however acknowledged that a crack loaded below the mentioned threshold is still growing, but at rates that are orders of magnitude below that of a `dynamic' fracture. One possible approach is to consider that the fracture energy is dependent on the propagation velocity, rather than being a strict medium property. Alternatively, these slow creep regimes are well modelled by thermally activated sub-critical laws such as Arrhenius-like growth rate. It was suggested that the description of both the slow and the fast regimes, as well as that of the threshold phenomena, could be unified if accounting for the plasticity around the crack tip, in particular as the associate induced heat might locally soften the matter. We here propose a unifying model which neglects such an effect of the thermal dissipation, but focuses instead on how temperature affects the front sub-critical growth. We show how this model accounts for experimental data gathered during the failure of two different materials, at all velocities over eight orders of magnitude, thus shedding new light on fracture physics.

An experimental setup is presented. It aims at studying stress corrosion cracking in a U-notch aluminum alloy sample at a small scale, intermediate between the polycrystal (1mm) and the micro beam (10µm). Digital Image Correlation (DIC) is used to determine the amount of plasticity introduced before crack initiation from the notch tip. The elastic displacement field is also measured by DIC, and numerically calculated, in order to obtain the Stress Intensity Factor (SIF) during crack propagation. The plasticity introduced, at the load level necessary for obtaining initiation, is negligible. It is less than 0.5% at the tip of the notch and less than 0.2% for the rest of the sample. First stress corrosion cracking results show that the SIF values during crack propagation are in between 4 and 6 MPa √ m, being consistent with a SCC intergranular brittle fracture according to the literature. The intergranular nature of the crack is confirmed by a SEM observation of the final fracture surface.