Cooperativity flows and shear-bandings: a statistical field theory approach.

Cooperativity effects have been proposed to explain the non-local rheology in the dynamics of soft jammed systems. Based on the analysis of the free-energy model proposed by L. Bocquet, A. Colin and A. Ajdari, Phys. Rev. Lett., 2009, 103, 036001, we show that cooperativity effects resulting from the non-local nature of the fluidity (inverse viscosity) are intimately related to the emergence of shear-banding configurations. This connection materializes through the onset of inhomogeneous compact solutions (compactons), wherein the fluidity is confined to finite-support subregions of the flow and strictly zero elsewhere. The compacton coexistence with regions of zero fluidity ("non-flowing vacuum") is shown to be stabilized by the presence of mechanical noise, which ultimately shapes up the equilibrium distribution of the fluidity field, the latter acting as an order parameter for the flow-noflow transitions occurring in the material.

Internal dynamics and activated processes in soft-glassy materials.

Plastic rearrangements play a crucial role in the characterization of soft-glassy materials, such as emulsions and foams. Based on numerical simulations of soft-glassy systems, we study the dynamics of plastic rearrangements at the hydrodynamic scales where thermal fluctuations can be neglected. Plastic rearrangements require an energy input, which can be either provided by external sources, or made available through time evolution in the coarsening dynamics, in which the total interfacial area decreases as a consequence of the slow evolution of the dispersed phase from smaller to large droplets/bubbles. We first demonstrate that our hydrodynamic model can quantitatively reproduce such coarsening dynamics. Then, considering periodically oscillating strains, we characterize the number of plastic rearrangements as a function of the external energy-supply, and show that they can be regarded as activated processes induced by a suitable "noise" effect. Here we use the word noise in a broad sense, referring to the internal non-equilibrium dynamics triggered by spatial random heterogeneities and coarsening. Finally, by exploring the interplay between the internal characteristic time-scale of the coarsening dynamics and the external time-scale associated with the imposed oscillating strain, we show that the system exhibits the phenomenon of stochastic resonance, thereby providing further credit to the mechanical activation scenario.

Direct evidence of plastic events and dynamic heterogeneities in soft-glasses.

By using fluid-kinetic simulations of confined and concentrated emulsion droplets, we investigate the nature of space non-homogeneity in soft-glassy dynamics and provide quantitative measurements of the statistical features of plastic events in the proximity of the yield-stress threshold. Above the yield stress, our results show the existence of a finite stress correlation scale, which can be mapped directly onto the cooperativity scale, recently introduced in the literature to capture non-local effects in the soft-glassy dynamics. In this regime, the emergence of a separate boundary (wall) rheology with higher fluidity than the bulk is highlighted in terms of near-wall spontaneous segregation of plastic events. Near the yield stress, where the cooperativity scale cannot be estimated with sufficient accuracy, the system shows a clear increase of the stress correlation scale, whereas plastic events exhibit intermittent clustering in time, with no preferential spatial location. A quantitative measurement of the space-time correlation associated with the motion of the interface of the droplets is key to spot the elastic rigidity of the system.

Growth, competition and cooperation in spatial population genetics.

We study an individual based model describing competition in space between two different alleles. Although the model is similar in spirit to classic models of spatial population genetics such as the stepping stone model, here however space is continuous and the total density of competing individuals fluctuates due to demographic stochasticity. By means of analytics and numerical simulations, we study the behavior of fixation probabilities, fixation times, and heterozygosity, in a neutral setting and in cases where the two species can compete or cooperate. By concluding with examples in which individuals are transported by fluid flows, we argue that this model is a natural choice to describe competition in marine environments.

Phase-field model of long-time glasslike relaxation in binary fluid mixtures.

We present a new phase-field model for binary fluids, exhibiting typical signatures of soft-glassy behavior, such as long-time relaxation, aging, and long-term dynamical arrest. The present model allows the cost of building an interface to vanish locally within the interface, while preserving positivity of the overall surface tension. A crucial consequence of this property, which we prove analytically, is the emergence of free-energy minimizing density configurations, hereafter named "compactons," to denote their property of being localized to a finite-size region of space and strictly zero elsewhere (no tails). Thanks to compactness, any arbitrary superposition of compactons still is a free-energy minimizer, which provides a direct link between the complexity of the free-energy landscape and the morphological complexity of configurational space.

Heterogeneous diffuse interfaces: a new mechanism for arrested coarsening in binary mixtures. Heterogeneous diffuse interfaces.

We discuss the dynamics of binary fluid mixtures in which surface tension density is allowed to become locally negative within the interface, while still preserving positivity of the overall surface tension (heterogeneous diffuse interface). Numerical simulations of two-dimensional Ginzburg-Landau phase field equations implementing such mechanism and including hydrodynamic motion, show evidence of dynamically arrested domain coarsening. Under specific conditions on the functional form of the surface tension density, dynamical arrest can be interpreted in terms of the collective dynamics of metastable, non-linear excitations of the density field, named compactons, as they are localized to finite-size regions of configuration space and strictly zero elsewhere. Aside from compactons, the heterogeneous diffuse interface scenario appears to provide a robust mechanism for the interpretation of many aspects of soft-glassy behaviour in binary fluid mixtures.

Generalized lattice Boltzmann method with multirange pseudopotential.

The physical behavior of a class of mesoscopic models for multiphase flows is analyzed in details near interfaces. In particular, an extended pseudopotential method is developed, which permits to tune the equation of state and surface tension independently of each other. The spurious velocity contributions of this extended model are shown to vanish in the limit of high grid refinement and/or high order isotropy. Higher order schemes to implement self-consistent forcings are rigorously computed for 2d and 3d models. The extended scenario developed in this work clarifies the theoretical foundations of the Shan-Chen methodology for the lattice Boltzmann method and enhances its applicability and flexibility to the simulation of multiphase flows to density ratios up to O(100).

Mesoscopic modeling of a two-phase flow in the presence of boundaries: The contact angle.

We present a mesoscopic model, based on the Boltzmann equation, for the interaction between a solid wall and a nonideal fluid. We present an analytic derivation of the contact angle in terms of the surface tension between the liquid-gas, the liquid-solid, and the gas-solid phases. We study the dependency of the contact angle on the two free parameters of the model, which determine the interaction between the fluid and the boundaries, i.e. the equivalent of the wall density and of the wall-fluid potential in molecular dynamics studies. We compare the analytical results obtained in the hydrodynamical limit for the density profile and for the surface tension expression with the numerical simulations. We compare also our two-phase approach with some exact results obtained by E. Lauga and H. Stone [J. Fluid. Mech. 489, 55 (2003)] and J. Philip [Z. Angew. Math. Phys. 23, 960 (1972)] for a pure hydrodynamical incompressible fluid based on Navier-Stokes equations with boundary conditions made up of alternating slip and no-slip strips. Finally, we show how to overcome some theoretical limitations connected with the discretized Boltzmann scheme proposed by X. Shan and H. Chen [Phys. Rev. E 49, 2941 (1994)] and we discuss the equivalence between the surface tension defined in terms of the mechanical equilibrium and in terms of the Maxwell construction.

Increased insulin responsiveness in vivo and in vitro consequent to induced hyperinsulinemia in the rat.

The effect of constant, controlled hyperinsulinemia on in vivo and in vitro insulin responsiveness has been investigated in rats that have received insulin infusions through chronically implanted jugular vein catheters. Constant rates of insulin infusion during days 1-4 resulted in stable plasma insulin concentrations. The plasma glucose initially fell from 122 +/- 3 to 53 +/- 4 mg/dl. While the infusion rate was maintained constant, the plasma glucose continued to fall over the subsequent days so that on day 4 the plasma glucose, 40 +/- 2 mg/dl, was significantly lower than that in the same animals on day 1 (P less than 0.02). Subsequently, the rate of insulin infusion was decreased to maintain the plasma glucose level in the 35-40 mg/dl range. Plasma catecholamine levels were high in insulin-infused rats. On the eighth day an in vivo insulin tolerance test (0.5 U/kg) was performed. Insulin-infused rats responded with a hypoglycemia that was both more pronounced and longer sustained than in saline-infused controls. Insulin responsiveness in vitro has been measured in isolated adipocytes. Adipocytes from epididymal fat pads were of similar size in the two groups of animals. Glucose uptake by adipocytes from insulin-infused rats was similar to that of controls under basal (zero insulin) conditions, but showed an increase in the maximum response to insulin. Glucose incorporation into total lipid and fatty acid was greater in adipocytes from insulin-infused rats than in controls under both basal (zero insulin) and insulin-stimulated conditions. Activities of the lipogenic enzymes acetyl CoA carboxylase and fatty acid synthetase were markedly increased in epididymal fat pads of insulin-infused rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperthermia induced by pre-pontine knife-cut: evidence for a tonic inhibition of non-shivering thermogenesis in anaesthetized rat.

Temperature of colon, interscapular brown adipose tissue (IBAT) and paw skin (index of vasomotor activity) were monitored before and after microwire knife lesions at the pre-pontine or/and the post-mammillary levels in the urethane-anaesthetized rats at room temperature of 23-24 degrees C. Following the pre-pontine, but not the post-mammillary cut, colonic and IBAT temperatures increased by 3-4 degrees C within 90-240 min. IBAT temperature rose faster with a shorter latency and attained a higher steady-state value than colonic temperature; skin temperature, however rose by only 0.8 degrees C. A procaine microinjection into the pre-pontine area transiently increased by more than 1 degree C both colonic and IBAT temperatures, with similar kinetics as for the knife cut. Cardiac output distribution was measured using radiolabelled microspheres. Brown adipose tissue (BAT) was found to be the only organ to which the fractional blood flow increased dramatically (12 times over baseline value) during the development of hyperthermia. Propanolol, injected after the hyperthermia had fully developed, decreased IBAT and then colonic temperatures. Hexamethonium decreased both colonic and IBAT temperatures with a concomitant rise in skin temperature while tubocurarine was without effect. It is concluded that the hyperthermia observed after the pre-pontine lesion results from an increased sympathetic stimulation of BAT thermogenesis triggered by the release of a tonic inhibitory control on its heat production. Such an inhibitory system would be located somewhere between the lower midbrain and the upper pons.

Double scaling and intermittency in shear dominated flows.

The nature of intermittency in shear dominated flows changes with respect to homogeneous and isotropic conditions since the process of energy transfer is affected by the turbulent kinetic energy production associated with the Reynolds stresses. For these flows, a new form of refined similarity law is able to describe the increased level of intermittency. Ideally a length scale associated with the mean shear separates the two ranges, i.e., the classical Kolmogorov-like inertial range, below, and the shear dominated range, above. In the present paper we give evidence of the coexistence of the two regimes and we support the conjecture that the statistical properties of the dissipation field are practically insensible to the mean shear. This allows for a theoretical prediction of the scaling exponents of structure functions in the shear dominated range based on the known intermittency corrections for isotropic flows. The prediction is found to closely match the available numerical and experimental data. The analysis shows that the larger anisotropic scales of shear turbulence display universality, and determines the modality by which the dissipation field fixes the properties of turbulent fluctuations in the shear dominated range.

Universal intermittent properties of particle trajectories in highly turbulent flows.

We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers in the range R{lambda}in[120:740]. Lagrangian structure functions from all data sets are found to collapse onto each other on a wide range of time lags, pointing towards the existence of a universal behavior, within present statistical convergence, and calling for a unified theoretical description. Parisi-Frisch multifractal theory, suitably extended to the dissipative scales and to the Lagrangian domain, is found to capture the intermittency of velocity statistics over the whole three decades of temporal scales investigated here.

Surface roughness-hydrophobicity coupling in microchannel and nanochannel flows.

An approach based on a lattice version of the Boltzmann kinetic equation for describing multiphase flows in nano- and microcorrugated devices is proposed. We specialize it to describe the wetting-dewetting transition of fluids in the presence of nanoscopic grooves etched on the boundaries. This approach permits us to retain the essential supramolecular details of fluid-solid interactions without surrendering--actually boosting--the computational efficiency of continuum methods. The method is used to analyze the importance of conspiring effects between hydrophobicity and roughness on the global mass flow rate of the microchannel. In particular we show that smart surfaces can be tailored to yield very different mass throughput by changing the bulk pressure. The mesoscopic method is also validated quantitatively against the molecular dynamics results of [Cottin-Bizonne, Nat. Mater. 2, 237 (2003)].

[Physiopathological basis of protection of the ischemic myocardium]. / Les bases physiopathologiques de la protection du myocarde ischémique.

Ischemia initiates a cascade of intracellular events that culminates in the death of the myocyte. Reperfusion of ischemic myocardium is the most efficient way of salvaging jeopardized cells. Additional measures referred to as "myocardial protection" may further reduce ischemic injury, primarily by delaying the progression of cellular damage during ischemia. The pathophysiological aspects of ischemic injury and the possibilities for protection of ischemic myocardium are reviewed. In addition to the "established" methods of myocardial protection, the concept of prevention of "reperfusion injury", presently under experimental investigation, is discussed.

Dissociation between contractile function and oxidative metabolism in postischemic myocardium. Attenuation by ruthenium red administered during reperfusion.

The oxidative metabolic rate may be disproportionately high compared with contractile function in postischemic reperfused myocardium. To study the potential involvement of intracellular calcium transport in high energy expenditure after reperfusion, we determined in isolated rat hearts the effect of ruthenium red, an inhibitor of mitochondrial calcium transport, on recovery of contractile function and oxidative metabolic rate. Hearts subjected to 60 minutes of no-flow ischemia exhibited, at 15 minutes after the onset of reperfusion, poor recovery of left ventricular pressure development to only 7% of the corresponding value measured in control hearts (p less than 0.01). However, myocardial oxygen consumption was recovered to 84% of control (p = NS). The ratio of isovolumic contractile performance (expressed as the product of heart rate and left ventricular pressure development) to myocardial oxygen consumption was severely depressed to 6% of control (p less than 0.01). Supplementation of the perfusate with 6 microM ruthenium red during the initial 40 minutes of reperfusion resulted in a reduction of myocardial oxygen consumption to 65% of the value measured after 15 minutes of reperfusion in hearts reperfused without ruthenium red (p less than 0.01), despite a threefold increase of left ventricular pressure development (p less than 0.05). Oxidation of both palmitate and glucose was reduced to a comparable extent by ruthenium red. The ratio of contractile performance to myocardial oxygen consumption increased progressively during infusion of ruthenium red and did not differ further from control hearts after 30 minutes of reperfusion. Cumulative myocardial release of creatine kinase was reduced by 47% (p less than 0.05) in hearts reperfused with ruthenium red-containing medium. The results provide circumstantial evidence for the hypothesis suggesting that enhanced energy expenditure by intracellular calcium transport may be involved in the mechanisms underlying the dissociation between left ventricular performance and myocardial oxidative metabolic rate early after postischemic reperfusion.

The response of adipose tissue blood flow to insulin-induced hypoglycemia in conscious dogs and rats.

A comparative study, focusing on the modification of regional blood flow in adipose tissue during insulin-induced hypoglycemia, was performed on dogs and rats at room temperature (22 degrees C) and on rats at thermoneutrality (28-32 degrees C). Insulin dosages of 3 IU/kg in rats and 0.75 IU/kg in dogs were found to cause changes of comparable amplitude and kinetics in plasma glucose and catecholamine levels in both species. At thermoneutrality, hypoglycemia induced an increase in blood supply to adipose tissue in both species: in dogs, blood flow density was markedly increased from the periphery (+75% in subcutaneous region) to the deeper locations (+550% in perirenal region); in rats, the increase of fractional cardiac output was especially pronounced in brown adipose tissue. By contrast, in rats acclimated at room temperature, hypoglycemia induced a decrease of fractional cardiac output to white adipose tissue and even more markedly, to brown adipose tissue (-45%). These results strongly suggest that, in rats at ambient temperature below thermoneutrality, thermoregulatory heat production is shut off during hypoglycemia.

Prepontine knife cut-induced hyperthermia in the rat. Effect of chemical sympathectomy and surgical denervation of brown adipose tissue.

The effect of brown adipose tissue (BAT) denervation on the prepontine knife cut-induced hyperthermia was studied. The knife cut has earlier been shown to induce a steady state hyperthermia of 3 to 4 degrees C, as a result of marked activation of the BAT. Before the lesion, the interscapular BAT (IBAT) temperature was lower than the colonic temperature, but the temperature gradient reversed a few minutes after the lesion and the fractional blood flow increased 12-fold. Bilateral sectioning of the 5 nerves supplying IBAT did not modify either the magnitude or the kinetics of the IBAT hyperthermic response. The IBAT fractional blood flow, which was 15 times higher in denervated than in intact tissue before the lesion, scarcely increased following the lesion despite the sharp increase in the tissue's metabolic activity. Chemical sympathectomy with 6-hydroxydopamine suppressed the hyperthermic response. Propranolol or hexamethonium injected i.v. during the steady state hyperthermia resulted in a rapid drop in IBAT temperature and in a reversal of the gradient between IBAT and colonic temperature both in denervated and in intact IBAT. Injection of desipramine, an inhibitor of noradrenaline reuptake, resulted in itself in an increase of temperature in both intact and denervated tissue, which is circumstantial evidence for the presence of a functional residual innervation in the latter.(ABSTRACT TRUNCATED AT 250 WORDS)