Experimental clues of soft glassy rheology in strained filled elastomers

International audienceTensile stress-relaxation measurements have been performed on a series of cross-linked filled elastomers. The fillers are chosen in order to investigate the effect of the filler-filler and the filler-matrix interactions on the time dependence of the tensile relaxation modulus E(t) after UP and DOWN jumps. For the carbon black filled sample (strong filler-elastomer interaction) E(t) decreases as log(t) when the strain epsilon is strictly larger than 0.2 and reached by UP jumps. For the silica filled samples in the same conditions, and for all samples after a DOWN jump including epsilon = 0.2, the experimental data can be fitted with a power law equation characterized by the exponent m. Thus, in all cases, |dE(t)⁄dt| scales as t^(-α) with α=m+1. Pertinence of the Soft Glassy Rheology (SGR) model for interpreting the present results is examined. It is shown that α could be equivalent to the effective noise temperature x and related to the polymer chain mobility

Flame propagation in random media

We introduce a phase-field model to describe the dynamics of a self-sustaining propagating combustion front within a medium of randomly distributed reactants. Numerical simulations of this model show that a flame front exists for reactant concentration $c > c^* > 0$, while its vanishing at $c^*$ is consistent with mean-field percolation theory. For $c > c^*$, we find that the interface associated with the diffuse combustion zone exhibits kinetic roughening characteristic of the Kardar-Parisi-Zhang equation.Comment: 4, LR541

Scaling, Propagation, and Kinetic Roughening of Flame Fronts in Random Media

We introduce a model of two coupled reaction-diffusion equations to describe the dynamics and propagation of flame fronts in random media. The model incorporates heat diffusion, its dissipation, and its production through coupling to the background reactant density. We first show analytically and numerically that there is a finite critical value of the background density, below which the front associated with the temperature field stops propagating. The critical exponents associated with this transition are shown to be consistent with mean field theory of percolation. Second, we study the kinetic roughening associated with a moving planar flame front above the critical density. By numerically calculating the time dependent width and equal time height correlation function of the front, we demonstrate that the roughening process belongs to the universality class of the Kardar-Parisi-Zhang interface equation. Finally, we show how this interface equation can be analytically derived from our model in the limit of almost uniform background density.Comment: Standard LaTeX, no figures, 29 pages; (to appear in J. Stat. Phys. vol.81, 1995). Complete file available at http://www.physics.helsinki.fi/tft/tft.html or anonymous ftp at ftp://rock.helsinki.fi/pub/preprints/tft

A united statement of the global chiropractic research community against the pseudoscientific claim that chiropractic care boosts immunity.

BACKGROUND: In the midst of the coronavirus pandemic, the International Chiropractors Association (ICA) posted reports claiming that chiropractic care can impact the immune system. These claims clash with recommendations from the World Health Organization and World Federation of Chiropractic. We discuss the scientific validity of the claims made in these ICA reports. MAIN BODY: We reviewed the two reports posted by the ICA on their website on March 20 and March 28, 2020. We explored the method used to develop the claim that chiropractic adjustments impact the immune system and discuss the scientific merit of that claim. We provide a response to the ICA reports and explain why this claim lacks scientific credibility and is dangerous to the public. More than 150 researchers from 11 countries reviewed and endorsed our response. CONCLUSION: In their reports, the ICA provided no valid clinical scientific evidence that chiropractic care can impact the immune system. We call on regulatory authorities and professional leaders to take robust political and regulatory action against those claiming that chiropractic adjustments have a clinical impact on the immune system

Experimental evidence for localization of acoustic waves in three dimensions

We present experimental evidence for localization of acoustic waves in a novel three-dimensional system. The system consists of a polymer melt solidifying by the growth of spherical semicrystalline nuclei. For sufficiently high excitation frequencies, we find renormalization of sound speed and intense absorption peaks over very narrow ranges of wave number. We generalize theory to include shear modes in the scatterers, and find that localization is consistent with our observations

Model for dynamics of structural glasses

We have studied, using dynamical Monte Carlo methods, a facilitated kinetic Ising model for structural glasses. We observe stretched-exponential decays (_ relaxation) of the equilibrium spin autocorrelation function at late times and are able to estimate accurately the corresponding relaxation times _. These are found to diverge at nonzero temperatures following a Vogel-Fulcher law. In addition, we observe early-time exponential relaxation analogous to _ relaxation of glass formers. We also examined the effective thermodynamics of systems quenched below this divergent temperature and subsequently heated. The result is a peak in the specific heat with properties matching those of the putative glass transition. We also find that these peaks, for different heating rates, can be rescaled to lie on a universal curve. Finally, we studied the evolving structure factor of a frozen glass following a rapid jump in temperature. The results are qualitatively the same as those for x-ray studies of heated glasses. We thus demonstrate that this purely dynamical microscopic model can reproduce much of the phenomenology of the glass transition and near-glass relaxation, with no tunable parameters

Elastic effects in the foaming of thermoplastics

We have investigated shear and bulk elastic stresses in non-Newtonian fluids in which bubble formation due to a blowing agent is taking place. In the absence of shear, these elastic fields manifest themselves strongly during the formation of bubbles, and during coalescence events. At later times, their activity is confined to the surface of the bubbles. These effects cause a significant slow-down in radial growth. In the presence of steady shear, elasticity tends to stabilize bubble shapes, as the maximum number of droplets decreases with shear rate, while their growth rate increases

Theory of melt fracture instabilities in the capillary flow of polymer melts

We present a model for the flow of a polymer melt through a capillary with nonlinear slip boundary conditions at the wall of the capillary. The model consists of the linearized Navier-Stokes equations coupled to a Maxwell constitutive relation for the viscoelasticity and a phase-field model for a first-order transition between stick and slip flow at the boundary. Specializing to the case of a two-dimensional capillary, we perform a linear stability analysis about the steady-state solutions and predict in which parameter regimes the steady-state becomes unstable. A numerical study of the model shows regions of steady flow, as well as regimes with periodic oscillations, spatially uniform but temporally chaotic oscillations, and more complicated spatiotemporal behavior. We show that the oscillations can account for the sharkskin texturing and defect structures seen in the extrusion of polymer melt