Thermal and compositional driven convection in thin reaction fronts.

Chemical reaction fronts separate regions of reacted and unreacted substances as they propagate in liquids. These fronts may induce density gradients due to different chemical compositions and temperatures across the front. In this paper, we investigate buoyancy-induced convection driven by both types of gradients. We consider a thin front approximation where the normal front velocity depends only on the front curvature. This model applies for small curvature fronts independent of the specific type of chemical reaction. For density changes due only to heat variations near the front, we find that convection can take place for either upward or downward propagating fronts if density gradients are above a threshold. Convection can set in even if the fluid with lower density is above the higher density fluid. Our model consists of Navier-Stokes equations coupled to the front propagation equation. We carry out a linear stability analysis to determine the parameters for the onset of convection. We study the nonlinear front propagation for liquids confined in narrow two-dimensional domains. Convection leads to fronts of steady shape, propagating with constant velocities.

Front instabilities in the presence of convection due to thermal and compositional gradients.

Reaction fronts separate fluids of different densities due to thermal and compositional gradients that may lead to convection. The stability of convectionless flat fronts propagating in the vertical direction depends not only on fluid properties but also in the dynamics of a front evolution equation. In this work, we analyze fronts described by the Kuramoto-Sivashinsky (KS) equation coupled to hydrodynamics. Without density gradients, the KS equation has a flat front solution that is unstable to perturbations of long wavelengths. Buoyancy enhances this instability if a fluid of lower density is underneath a denser fluid. In the reverse situation, with the denser fluid underneath, the front can be stabilized with appropriate thermal and compositional gradients. However, in this situation, a different instability develops for large enough thermal gradients. We also solve numerically the nonlinear KS equation coupled to the Navier-Stokes equations to analyze the front propagation in two-dimensional rectangular domains. As convection takes place, the reaction front curves, increasing its velocity.

Molecular basis of ageing in chronic metabolic diseases.

AIM: Over the last decades, the shift in age distribution towards older ages and the progressive ageing which has occurred in most populations have been paralleled by a global epidemic of obesity and its related metabolic disorders, primarily, type 2 diabetes (T2D). Dysfunction of the adipose tissue (AT) is widely recognized as a significant hallmark of the ageing process that, in turn, results in systemic metabolic alterations. These include insulin resistance, accumulation of ectopic lipids and chronic inflammation, which are responsible for an elevated risk of obesity and T2D onset associated to ageing. On the other hand, obesity and T2D, the paradigms of AT dysfunction, share many physiological characteristics with the ageing process, such as an increased burden of senescent cells and epigenetic alterations. Thus, these chronic metabolic disorders may represent a state of accelerated ageing. MATERIALS AND METHODS: A more precise explanation of the fundamental ageing mechanisms that occur in AT and a deeper understanding of their role in the interplay between accelerated ageing and AT dysfunction can be a fundamental leap towards novel therapies that address the causes, not just the symptoms, of obesity and T2D, utilizing strategies that target either senescent cells or DNA methylation. RESULTS: In this review, we summarize the current knowledge of the pathways that lead to AT dysfunction in the chronological ageing process as well as the pathophysiology of obesity and T2D, emphasizing the critical role of cellular senescence and DNA methylation. CONCLUSION: Finally, we highlight the need for further research focused on targeting these mechanisms.

Pattern formation induced by intraspecific interactions in a predator-prey system.

Differential diffusion is a source of instability in population dynamics systems when species diffuse with different rates. Predator-prey systems show this instability only under certain specific conditions, usually requiring one to involve Holling-type functionals. Here we study the effects of intraspecific cooperation and competition on diffusion-driven instability in a predator-prey system with a different structure. We conduct the analysis on a generalized population dynamics that bounds intraspecific and interspecific interactions with Verhulst-type saturation terms instead of Holling-type functionals. We find that instability occurs due to the intraspecific saturation or intraspecific interactions, both cooperative and competitive. We present numerical simulations and show spatial patterns due to diffusion.

Oscillatory instability in a reaction front separating fluids of different densities.

Reaction fronts described by the Kuramoto-Sivashinsky (KS) equation can exhibit complex behavior as they separate reacted from unreacted fluids. If the fluid of higher density is above a fluid of lower density, then the Rayleigh-Taylor instability can lead to fluid motion. In the reverse situation, where the lighter fluid is on top, gravitationally driven forces can stabilize a convectionless flat front inhibiting the complex front propagation described by the KS equation. In these cases, a critical density difference is required to provide stability to the flat front. A linear stability analysis shows that the transition from stable to unstable flat fronts can be oscillatory for viscous fluid motion. Once the transition takes place, the fronts exhibit oscillatory convection resulting in oscillations of the shape and speed of the front.

Marangoni flow traveling with reaction fronts: Eikonal approximation.

Chemical reaction fronts traveling in liquids generate gradients of surface tension leading to fluid motion. This surface tension driven flow, known as Marangoni flow, modifies the shape and the speed of the reaction front. We model the front propagation using the Eikonal relation between curvature and normal speed of the front, resulting in a front evolution equation that couples to the fluid velocity. The sharp discontinuity between the reactants and products leads to a surface tension gradient proportional to a delta function. The Stokes equations with the surface tension gradient as part of the boundary conditions provide the corresponding fluid velocity field. Considering stress free boundaries at the bottom of the liquid layer, we find an analytical solution for the fluid vorticity leading to the velocity field. Solving numerically the appropriate no-slip boundary condition, we gain insights into the role of the boundary condition at the bottom layer. We compare our results with results from two other models for front propagation: the deterministic Kardar-Parisi-Zhang equation and a reaction-diffusion equation with cubic autocatalysis, finding good agreement for small differences in surface tension.

Convection induced by thermal gradients on thin reaction fronts.

We present a thin front model for the propagation of chemical reaction fronts in liquids inside a Hele-Shaw cell or porous media. In this model we take into account density gradients due to thermal and compositional changes across a thin interface. The front separating reacted from unreacted fluids evolves following an eikonal relation between the normal speed and the curvature. We carry out a linear stability analysis of convectionless flat fronts confined in a two-dimensional rectangular domain. We find that all fronts are stable to perturbations of short wavelength, but they become unstable for some wavelengths depending on the values of compositional and thermal gradients. If the effects of these gradients oppose each other, we observe a range of wavelengths that make the flat front unstable. Numerical solutions of the nonlinear model show curved fronts of steady shape with convection propagating faster than flat fronts. Exothermic fronts increase the temperature of the fluid as they propagate through the domain. This increment in temperature decreases with increasing speed.

Epigenetics: spotlight on type 2 diabetes and obesity.

Type 2 diabetes (T2D) and obesity are the major public health problems. Substantial efforts have been made to define loci and variants contributing to the individual risk of these disorders. However, the overall risk explained by genetic variation is very modest. Epigenetics is one of the fastest growing research areas in biomedicine as changes in the epigenome are involved in many biological processes, impact on the risk for several complex diseases including diabetes and may explain susceptibility. In this review, we focus on the role of DNA methylation in contributing to the risk of T2D and obesity.

Hoxa5 undergoes dynamic DNA methylation and transcriptional repression in the adipose tissue of mice exposed to high-fat diet.

BACKGROUND/OBJECTIVES: The genomic bases of the adipose tissue abnormalities induced by chronic positive calorie excess have been only partially elucidated. We adopted a genome-wide approach to directly test whether long-term high-fat diet (HFD) exposure affects the DNA methylation profile of the mouse adipose tissue and to identify the functional consequences of these changes. SUBJECTS/METHODS: We have used epididymal fat of mice fed either high-fat (HFD) or regular chow (STD) diet for 5 months and performed genome-wide DNA methylation analyses by methylated DNA immunoprecipitation sequencing (MeDIP-seq). Mouse Homeobox (Hox) Gene DNA Methylation PCR, RT-qPCR and bisulphite sequencing analyses were then performed. RESULTS: Mice fed the HFD progressively expanded their adipose mass accompanied by a significant decrease in glucose tolerance (P<0.001) and insulin sensitivity (P<0.05). MeDIP-seq data analysis revealed a uniform distribution of differentially methylated regions (DMR) through the entire adipocyte genome, with a higher number of hypermethylated regions in HFD mice (P<0.005). This different methylation profile was accompanied by increased expression of the Dnmt3a DNA methyltransferase (Dnmt; P<0.05) and the methyl-CpG-binding domain protein Mbd3 (P<0.05) genes in HFD mice. Gene ontology analysis revealed that, in the HFD-treated mice, the Hox family of development genes was highly enriched in differentially methylated genes (P=0.008). To validate this finding, Hoxa5, which is implicated in fat tissue differentiation and remodeling, has been selected and analyzed by bisulphite sequencing, confirming hypermethylation in the adipose tissue from the HFD mice. Hoxa5 hypermethylation was associated with downregulation of Hoxa5 mRNA and protein expression. Feeding animals previously exposed to the HFD with a standard chow diet for two further months improved the metabolic phenotype of the animals, accompanied by return of Hoxa5 methylation and expression levels (P<0.05) to values similar to those of the control mice maintained under standard chow. CONCLUSIONS: HFD induces adipose tissue abnormalities accompanied by epigenetic changes at the Hoxa5 adipose tissue remodeling gene.

Rayleigh-Taylor instability of steady fronts described by the Kuramoto-Sivashinsky equation.

We study steady thin reaction fronts described by the Kuramoto-Sivashinsky equation that separates fluids of different densities. This system may lead to hydrodynamic instabilities as buoyancy forces interact with the propagating fronts in a two-dimensional slab. We use Darcy's law to describe the fluid motion in this geometry. Steady front profiles can be flat, axisymmetric, or nonaxisymmetric, depending on the slab width, the density gradient, and fluid viscosity. Unstable flat fronts can be stabilized having a density gradient with the less dense fluid on top of a denser fluid. We find the steady front solutions from the nonlinear equations executing a linear stability analysis to determine their stability. We show regions of bistability where stable nonaxisymmetric and axisymmetric fronts can coexist. We also consider the stability of steady solutions in large domains, which can be constructed by dividing the domain into smaller parts or cells.

Pattern formation induced by a differential shear flow.

Fluid flow advecting one substance while others are immobilized can generate an instability in a homogeneous steady state of a reaction-diffusion-advection system. This differential-flow instability leads to the formation of steady spatial patterns in a moving reference frame. We study the effects of shear flow on this instability by considering two layers of fluid moving independently from each other, but allowing the substances to diffuse along and across the layers. We find that shear flow can generate instabilities even if the average flow velocity is zero for both substances. These instabilities are strongly dependent on which substance is advected by the shear flow. We explain these effects using the results of Taylor dispersion, where an effective diffusivity is enhanced by shear flow.

Convection in stable and unstable fronts.

Density gradients across a reaction front can lead to convective fluid motion. Stable fronts require a heavier fluid on top of a lighter one to generate convective fluid motion. On the other hand, unstable fronts can be stabilized with an opposing density gradient, where the lighter fluid is on top. In this case, we can have a stable flat front without convection or a steady convective front of a given wavelength near the onset of convection. The fronts are described with the Kuramoto-Sivashinsky equation coupled to hydrodynamics governed by Darcy's law. We obtain a dispersion relation between growth rates and perturbation wave numbers in the presence of a density discontinuity accross the front. We also analyze the effects of this density change in the transition to chaos.

Stability of fronts in the Kuramoto-Sivashinsky equation advected by a Poiseuille flow.

We study reaction fronts described by the Kuramoto-Sivashinsky equation subject to a Poiseuille flow. The fronts propagate with or against the flow located inside a two-dimensional slab. Steady front profiles can be flat, axisymmetric, or nonaxisymmetric, depending on the gap between the plates and the average flow speed. We first obtain the steady front solutions, later executing a linear stability analysis to determine the stability of the fronts. Applying fluid flow can turn initially unstable fronts into stable fronts. Stable steady fronts propagating in the adverse direction of the Poiseuille flow are axisymmetric for slow fluid flows. However, for higher speeds an adverse flow can lead to stable nonaxisymmetric fronts. We also show regions of bistability where stable nonaxisymmetric and axisymmetric fronts can coexist.

Fijación quirúrgica de las fracturas costales con placas de osteosíntesis. Experiencia inicial / Surgical fixatiion of fractures of the rib with osteosynthesis plates

Las fracturas costales son muy frecuentes en los traumatismos torácicos cerrados. La mayoría de estos pacientes tienen un dolor importante con los movimientos y con la tos. Los pacientes politraumatizados con tórax inestable presentan una alta morbimortalidad, el principal enfoque del tratamiento se centra en la terapia del dolor y la ventilación con presión positiva. Actualmente existen estudios que demuestran la mejoría en el tratamiento al utilizar la estabilización quirúrgica. Material y métodos: en el período comprendido entre diciembre de 2010 y diciembre de 2011, en nuestro hospital y en la práctica privada, se realizó la comparación prospectiva, de la evolución de 5 pacientes en los que se fijaron las fracturas costales con osteosíntesis con placa "U", con 8 pacientes a los que no se les realizó fijación quirúrgica de las fracturas costales. La edad promedio de cada grupo fue similar (34 y 39 años respectivamente). Solo se tuvieron en cuenta a aquellos pacientes que sufrieron al menos 2 fracturas en costillas diferentes. La placa en U posee entre 4-6 tornillos, orificios con rosca en la cara profunda de la placa y un puente que separa ambos segmentos estabilizadores. Para la valoración del dolor se utilizó la escala lineal analógica subjetiva asociada a la demanda de analgesia endovenosa en las primeras 24 horas y oral posteriormente. Esquema protocolizado (Diclofenac EV. 2 mg/kg, ibuprofeno 5,33 mg/kg VO., paracetamo 6,33 mg/kg VO). Analgesia adicional: primeras 24 hs se utilizó D-propoxifenos 1,52 mg/kg EV y luego de las 24 hs D-propoxifeno 2,6 mg/kg VO. Resultados: sobre el grupo control se observó que con el tratamiento farmacológico se logró controlar el dolor en el 37,5 % de los casos (3 pacientes), mientras que el 62,5 % restante requirió analgesia adicional. En el grupo con fijación quirúrgica solo un paciente (20%) requirió analgésicos sobre el grupo control. Todos los pacientes con dolor o inestabilidad mostraron mejoría subjetiva ...

Trauma to the chest wall frequently results in fracture of the ribs which provides its stability. As an injury it ranges from one which causes little but discomfort to one which is immediately life threatening. Most of these patients show important pain during movements and with cough. Politraumatized patients with unstable thorax present a high morbimortality, and the mainstay approach of treatment focuses in pain management and ventilation. Fixation of fractured ribs may be indicated if the injured chest wall requires a chest wall fixation with osteosynthesis plates. An initial experience presented by the authors, is discussed

Stability of convective patterns in reaction fronts: a comparison of three models.

Autocatalytic reaction fronts generate density gradients that may lead to convection. Fronts propagating in vertical tubes can be flat, axisymmetric, or nonaxisymmetric, depending on the diameter of the tube. In this paper, we study the transitions to convection as well as the stability of different types of fronts. We analyze the stability of the convective reaction fronts using three different models for front propagation. We use a model based on a reaction-diffusion-advection equation coupled to the Navier-Stokes equations to account for fluid flow. A second model replaces the reaction-diffusion equation with a thin front approximation where the front speed depends on the front curvature. We also introduce a new low-dimensional model based on a finite mode truncation. This model allows a complete analysis of all stable and unstable fronts.

Chemical pattern formation induced by a shear flow in a two-layer model.

We study chemical patterns arising from instabilities in reaction-diffusion-advection systems under the influence of shear flow. Turing pattern formation without shear flow can occur in an activator-inhibitor system as long as the diffusivity of the inhibitor is larger than the diffusivity of the activator. In the presence of shear flow, a homogeneous steady state can become unstable even if this condition is not satisfied. Chemical patterns arise as a result of this instability. We study this instability in a simple system consisting of two layers moving relative to each other. We carry out a linear stability analysis showing the onset of the instability as a function of the relative speed between the layers. We solve numerically the nonlinear reaction-diffusion-advection equations to obtain these patterns. We find stationary, oscillatory, and drifting patterns extending along each layer. We also find regions of bistability that allow the formation of localized structures. The instability is analyzed in terms of Taylor dispersion.

Convective chemical fronts in a Poiseuille flow.

Autocatalytic reaction fronts propagating in a Poiseuille flow present a change of speed and curvature depending on the strength of the flow and on the direction of front propagation. These chemical fronts separate reacted and unreacted fluids of different densities, consequently convection will always be present due to the horizontal density gradient of the curved front. In this paper, we find the change of speed caused by gravity for fronts propagating in vertical tubes under a Poiseuille flow. For small density differences, we find axisymmetric fronts. Our theory predicts a transition to nonaxisymmetric fronts as the distance between the walls is increased. The transition depends on the average speed of the Poiseuille flow.

Chemical instability induced by a shear flow.

We predict a new type of instability induced by shear flow in chemical systems. A homogeneous steady state solution of a reaction-diffusion system loses stability in a Poiseuille flow. The instability appears as the speed of the flow increases beyond a certain threshold. This results in a steady pattern moving with the average fluid velocity. The chemical reaction consists of two species (activator and inhibitor) moving with identical velocities. Contrary to Turing's instability, the pattern arises when the activator has a higher diffusivity than the inhibitor.

Dispersion relations for the convective instability of an acidity front in Hele-Shaw cells.

Autocatalytic chemical fronts of the chlorite-tetrathionate (CT) reaction become buoyantly unstable when they travel downwards in the gravity field because they imply an unfavorable density stratification of heavier products on top of lighter reactants. When such a density fingering instability occurs in extended Hele-Shaw cells, several fingers appear at onset which can be characterized by dispersion relations giving the growth rate of the perturbations as a function of their wave number. We analyze here theoretically such dispersion curves comparing the results for various models obtained by coupling Darcy's law or Brinkman's equation to either a one-variable reaction-diffusion model for the CT reaction or an eikonal equation. Our theoretical results are compared to recent experimental data.

Convection in chemical fronts with quadratic and cubic autocatalysis.

Convection in chemical fronts enhances the speed and determines the curvature of the front. Convection is due to density gradients across the front. Fronts propagating in narrow vertical tubes do not exhibit convection, while convection develops in tubes of larger diameter. The transition to convection is determined not only by the tube diameter, but also by the type of chemical reaction. We determine the transition to convection for chemical fronts with quadratic and cubic autocatalysis. We show that quadratic fronts are more stable to convection than cubic fronts. We compare these results to a thin front approximation based on an eikonal relation. In contrast to the thin front approximation, reaction-diffusion models show a transition to convection that depends on the ratio between the kinematic viscosity and the molecular diffusivity. (c) 2002 American Institute of Physics.