Unraveling dispersion and buoyancy dynamics around radial A + B → C reaction fronts: microgravity experiments and numerical simulations

Radial Reaction-Diffusion-Advection (RDA) fronts for A + B → C reactions find wide applications in many natural and technological processes. In liquid solutions, their dynamics can be perturbed by buoyancy-driven convection due to concentration gradients across the front. In this context, we conducted microgravity experiments aboard a sounding rocket, in order to disentangle dispersion and buoyancy effects in such fronts. We studied experimentally the dynamics due to the radial injection of A in B at a constant flow rate, in absence of gravity. We compared the obtained results with numerical simulations using either radial one- (1D) or two-dimensional (2D) models. We showed that gravitational acceleration significantly distorts the RDA dynamics on ground, even if the vertical dimension of the reactor and density gradients are small. We further quantified the importance of such buoyant phenomena. Finally, we showed that 1D numerical models with radial symmetry fail to predict the dynamics of RDA fronts in thicker geometries, while 2D radial models are necessary to accurately describe RDA dynamics where Taylor-Aris dispersion is significant.

Measurement of Capillary Forces Using Two Fibers Dynamically Withdrawn from a Liquid: Evidence for an Enhanced Cheerios Effect

We study the capillary attraction force between two fibers dynamically withdrawn from a bath. We propose an experimental method to measure this force and show that its magnitude strongly increases with the retraction speed by up to a factor of 10 compared to the static case. We show that this remarkable increase stems from the shape of the dynamical meniscus between the two fibers. We first study the dynamical meniscus around one fiber and obtain experimental and numerical scaling of its size increase with the capillary number, which is not captured by the classical Landau-Levich-Derjaguin theory. We then show that the shape of the deformed air-liquid interface around two fibers can be inferred from the linear superposition of the interface around a single fiber. These results yield an analytical expression for the capillary force which compares well with the experimental data. Our study reveals the critical role of the retraction speed to create stronger capillary interactions, with potential applications in industry or biology.

Peeling from a liquid

We establish the existence of a cusp in the curvature of a solid sheet at its contact with a liquid subphase. We study two configurations in floating sheets where the solid-vapor-liquid contact line is a straight line and a circle, respectively. In the former case, a rectangular sheet is lifted at its one edge, whereas in the latter a gas bubble is injected beneath a floating sheet. We show that in both geometries the derivative of the sheet's curvature is discontinuous. We demonstrate that the boundary condition at the contact is identical in these two geometries, even though the shape of the contact line and the stress distribution in the sheet are very different.

Honey bees switch mechanisms to drink deep nectar efficiently

The feeding mechanisms of animals constrain the spectrum of resources that they can exploit profitably. For floral nectar eaters, both corolla depth and nectar properties have marked influence on foraging choices. We report the multiple strategies used by honey bees to efficiently extract nectar at the range of sugar concentrations and corolla depths they face in nature. Honey bees can collect nectar by dipping their hairy tongues or capillary loading when lapping it, or they can attach the tongue to the wall of long corollas and directly suck the nectar along the tongue sides. The honey bee feeding apparatus is unveiled as a multifunctional tool that can switch between lapping and sucking nectar according to the instantaneous ingesting efficiency, which is determined by the interplay of nectar-mouth distance and sugar concentration. These versatile feeding mechanisms allow honey bees to extract nectar efficiently from a wider range of floral resources than previously appreciated and endow them with remarkable adaptability to diverse foraging environments.

Effect of external tension on the wetting of an elastic sheet

Recent studies of elastocapillary phenomena have triggered interest in a basic variant of the classical Young-Laplace-Dupré (YLD) problem: the capillary interaction between a liquid drop and a thin solid sheet of low bending stiffness. Here we consider a two-dimensional model where the sheet is subjected to an external tensile load and the drop is characterized by a well-defined Young's contact angle θ_{Y}. Using a combination of numerical, variational, and asymptotic techniques, we discuss wetting as a function of the applied tension. We find that, for wettable surfaces with 0<θ_{Y}<π/2, complete wetting is possible below a critical applied tension due to the deformation of the sheet in contrast with rigid substrates requiring θ_{Y}=0. Conversely, for very large applied tensions, the sheet becomes flat and the classical YLD situation of partial wetting is recovered. At intermediate tensions, a vesicle forms in the sheet, which encloses most of the fluid, and we provide an accurate asymptotic description of this wetting state in the limit of small bending stiffness. We show that bending stiffness, however small, affects the entire shape of the vesicle. Rich bifurcation diagrams involving partial wetting and "vesicle" solution are found. For moderately small bending stiffnesses, partial wetting can coexist with both the vesicle solution and complete wetting. Finally, we identify a tension-dependent bendocapillary length, λ_{BC}, and find that the shape of the drop is determined by the ratio A/λ_{BC}^{2}, where A is the area of the drop.

Effect of radial advection on autocatalytic reaction-diffusion fronts

The reaction-diffusion-advection properties of autocatalytic fronts are studied both theoretically and numerically in the case where the autocatalytic species is injected radially into the reactant at a constant flow rate....

A snapshot of geriatric rehabilitation: one year experience

OBJECTIVE

Frailty is a common condition in older adults, characterized by multimorbidity, physical weakness and nutritional deficit. Frailty can be detected early and a prehabilitation treatment could reduce the incidence of disability.

PATIENTS AND METHODS

Two-hundred-fifteen elderly patients were admitted to the Rehabilitation and Physical Medicine Unit of Policlinico Gemelli for one year. Patients were clinically assessed by Charlson Comorbidity Index (CCI) and blood sample values. Numerical Pain Rating Scale (NRS) and Hand Grip Test were assessed before (T0) and after (T1) hospitalization. Number of drugs and number of infections were recorded.

RESULTS

Patients were originally hospitalized in orthopaedic, neurology and medical ward. Most patients (68%) after discharge return home. Negative correlations between albumin and CCI and between total protein and CCI were recorded. Positive correlation between CCI cognitive subscore and number of drugs and a negative correlation between that subscore and Vitamin D were detected. An improvement in NRS and in the handgrip strength was recorded. At discharge an increase in the number of drugs and the number of infections was noted.

CONCLUSIONS

The handgrip strength improvement increases quality of life. Pain management and NRS indicate a better recovery of activities of daily living. Malnutrition is a real problem; albumin is the principal negative acute-phase reactant and is related to a worse clinical condition and low vitamin D levels are associated with worse cognitive function. The goal of a Rehabilitation Unit is to create an effective multidisciplinary transitional care plan, involving the patient and caregivers, creating a continuity of care after discharge and a sustainable project.

Trade-off mechanism of honey bee sucking and lapping

Animals have developed various drinking strategies in capturing liquid to feed or to stay hydrated. In contrast with most animals, honey bees Apis mellifera that capture nectar with their tongue, can deliberately switch between sucking and lapping methods. They preferentially suck diluted nectar whereas they are prone to lap concentrated nectar. In vivo observations have shown that bees select the feeding method yielding the highest efficiency at a given sugar concentration. In this combined experimental and theoretical investigation, we propose two physical models for suction and lapping mode of capture that explain the transition between these two feeding strategy. The critical viscosity, μ*, at which the transition occurs, is derived from these models, and agrees well with in vivo measurements. The trade-off mechanism of honey bee sucking and lapping may further inspire microfluidics devices with higher capability of transporting liquids across a large range of viscosities.

Prehabilitation and heart failure: perspective in primary outcomes, a randomized controlled trial

OBJECTIVE

Prehabilitation, intended as a multidisciplinary approach where physical training is combined with educational and counselling training, in cardiology could optimizing care, and has been shown to be able to reduce morbidity and mortality in several diseases. The present study aims to assess the effectiveness of a prehabilitation program in elderly patients (over 65) with chronic heart failure and to evaluate functional and quality indices of life.

PATIENTS AND METHODS

This is randomized, single blind controlled trial. Fourteen older adult patients diagnosed with chronic heart failure were enrolled. Patients were randomly assigned into the study or the control group. Patients in the study group underwent physical training organized into 10 twice-weekly meetings, nutritional and lifestyle counseling.

RESULTS

In the Study Group, the quality of life improved significantly (EQoL-5D), and between the two groups there is a statistically significant difference in the motor dimension of SF-36.

CONCLUSIONS

Because of our preliminary results, prehabilitation program should be included among the management strategies of in elderly patients with chronic heart failure to better manage their disease and to improve their Quality of Life.

Dynamics of A+B→C reaction fronts under radial advection in a Poiseuille flow

A+B→C reaction fronts describe a wide variety of natural and engineered dynamics, according to the specific nature of reactants and product. Recent works have shown that the properties of such reaction fronts depend on the system geometry, by focusing on one-dimensional plug flow radial injection. Here, we extend the theoretical formulation to radial deformation in two-dimensional systems. Specifically, we study the effect of a Poiseuille advective velocity profile on A+B→C fronts when A is injected radially into B at a constant flow rate in a confined axisymmetric system consisting of two parallel impermeable plates separated by a thin gap. We analyze the front dynamics by computing the temporal evolution of the average over the gap of the front position, the maximum production rate, and the front width. We further quantify the effects of the nonuniform flow on the total amount of product, as well as on its radial concentration profile. Through analytical and numerical analyses, we identify three distinct temporal regimes, namely (i) the early-time regime where the front dynamics is independent of the reaction, (ii) the transient regime where the front properties result from the interplay of reaction, diffusion that smooths the concentration gradients and advection, which stretches the spatial distribution of the chemicals, and (iii) the long-time regime where Taylor dispersion occurs and the system becomes equivalent to the one-dimensional plug flow case.

Oscillatory budding dynamics of a chemical garden within a co-flow of reactants

The oscillatory growth of chemical gardens is studied experimentally in the budding regime using a co-flow of two reactant solutions within a microfluidic reactor. The confined environment of the reactor tames the erratic budding growth and the oscillations leave their imprint with the formation of orderly spaced membranes on the precipitate surface. The average wavelength of the spacing between membranes, the growth velocity of the chemical garden and the oscillations period are measured as a function of the velocity of each reactant. By means of materials characterization techniques, the micro-morphology and the chemical composition of the precipitate are explored. A mathematical model is developed to explain the periodic rupture of droplets delimitated by a shell of precipitate and growing when one reactant is injected into the other. The predictions of this model are in good agreement with the experimental data.

Effects of radial injection and solution thickness on the dynamics of confined A + B → C chemical fronts

The reconstructed amount of product n_C as the volume V of KSCN injected radially into Fe(NO₃)₃ increases and comparison to theory.

Confined direct and reverse chemical gardens: Influence of local flow velocity on precipitation patterns

Various cobalt silicate precipitation patterns can be observed when an aqueous solution of cobalt ions gets into contact with a solution of silicate ions upon injection of one solution into the other in the confined geometry of a Hele-Shaw cell. The properties of these precipitation patterns are studied here as a function of the injection flow rate, densities and viscosities of the solutions, and the choice of which solution is injected into the other one. Our results show that the structure of the precipitation pattern depends on the local velocity as well as on the difference in viscosities between the injected and the displaced solutions. Specifically, decreasing the injection flow rate and/or decreasing the density jump while increasing the difference in viscosities between the reactant solutions results in more circular patterns. Moreover, we show that some structures are robustly observed in given ranges of the local flow velocity in the cell. Locally, precipitation can then transition from one type of pattern to another during injection, according to that preferred structure at the given local velocity. We also show that injection of the cobalt solution into the silicate solution results in the so-called direct patterns that are different from the reverse patterns obtained when the silicate solution is injected in the solution of cobalt ions. Our results help in understanding the production of precipitate structures under nonequilibrium flow conditions.

Dynamics of A+B → C reaction fronts under radial advection in three dimensions

The dynamics of A+B→C reaction fronts is studied both analytically and numerically in three-dimensional systems when A is injected radially into B at a constant flow rate. The front dynamics is characterized in terms of the temporal evolution of the reaction front position, r_{f}, of its width, w, of the maximum local production rate, R^{max}, and of the total amount of product generated by the reaction, n_{C}. We show that r_{f}, w, and R^{max} exhibit the same temporal scalings as observed in rectilinear and two-dimensional radial geometries both in the early-time limit controlled by diffusion, and in the longer time reaction-diffusion-advection regime. However, unlike the two-dimensional cases, the three-dimensional problem admits an asymptotic stationary solution for the reactant concentration profiles where n_{C} grows linearly in time. The timescales at which the transition between the regimes arise, as well as the properties of each regime, are determined in terms of the injection flow rate and reactant initial concentration ratio.

Filiform corrosion as a pressure-driven delamination process

Filiform corrosion produces long and narrow trails on various coated metals through the detachment of the coating layer from the substrate. In this work, we present a combined experimental and theoretical analysis of this process with the aim to describe quantitatively the shape of the cross-section, perpendicular to the direction of propagation, of the filaments produced. For this purpose, we introduce a delamination model of filiform corrosion dynamics and show its compatibility with experimental data where the coating thickness has been varied systematically.

Filament dynamics in confined chemical gardens and in filiform corrosion

Once rescaled by the preferred wavenumber ω* of the curvature power spectrum, both filiform corrosion and chemical garden filaments display similar dynamics.

Flow Control of A+B→C Fronts by Radial Injection

The dynamics of A+B→C fronts is analyzed theoretically in the presence of passive advection when A is injected radially into B at a constant inlet flow rate Q. We compute the long-time evolution of the front position, r_{f}, of its width, w, and of the local production rate R of the product C at r_{f}. We show that, while advection does not change the well-known scaling exponents of the evolution of corresponding reaction-diffusion fronts, their dynamics is however significantly influenced by the injection. In particular, the total amount of product varies as Q^{-1/2} for a given volume of injected reactant and the front position as Q^{1/2} for a given time, paving the way to a flow control of the amount and spatial distribution of the reaction front product. This control strategy compares well with calcium carbonate precipitation experiments for which the amount of solid product generated in flow conditions at fixed concentrations of reactants and the front position can be tuned by varying the flow rate.

From Cylindrical to Stretching Ridges and Wrinkles in Twisted Ribbons

Twisted ribbons under tension exhibit a remarkably rich morphology, from smooth and wrinkled helicoids, to cylindrical or faceted patterns. This complexity emanates from the instability of the natural, helicoidal symmetry of the system, which generates both longitudinal and transverse stresses, thereby leading to buckling of the ribbon. Here, we focus on the tessellation patterns made of triangular facets. Our experimental observations are described within an "asymptotic isometry" approach that brings together geometry and elasticity. The geometry consists of parametrized families of surfaces, isometric to the undeformed ribbon in the singular limit of vanishing thickness and tensile load. The energy, whose minimization selects the favored structure among those families, is governed by the tensile work and bending cost of the pattern. This framework describes the coexistence lines in a morphological phase diagram, and determines the domain of existence of faceted structures.

Flow-driven control of calcium carbonate precipitation patterns in a confined geometry

Upon injection of an aqueous solution of carbonate into a solution of calcium ions in the confined geometry of a Hele-Shaw cell, various calcium carbonate precipitation patterns are observed.

Wrinkles and folds in a fluid-supported sheet of finite size

A laterally confined thin elastic sheet lying on a liquid substrate displays regular undulations, called wrinkles, characterized by a spatially extended energy distribution and a well-defined wavelength λ. As the confinement increases, the deformation energy is progressively localized into a single narrow fold. An exact solution for the deformation of an infinite sheet was previously found, indicating that wrinkles in an infinite sheet are unstable against localization for arbitrarily small confinement. We present an extension of the theory to sheets of finite length L, accounting for the experimentally observed wrinkle-to-fold transition. We derive an exact solution for the periodic deformation in the wrinkled state, and an approximate solution for the localized, folded state. We find that a second-order transition between these two states occurs at a critical confinement Δ(F)=λ(2)/L.

Silo collapse under granular discharge

We investigate, at a laboratory scale, the collapse of cylindrical shells of radius R and thickness t induced by a granular discharge. We measure the critical filling height for which the structure fails upon discharge. We observe that the silos sustain filling heights significantly above an estimation obtained by coupling standard shell-buckling and granular stress distribution theories. Two effects contribute to stabilize the structure: (i) below the critical filling height, a dynamical stabilization due to granular wall friction prevents the localized shell-buckling modes to grow irreversibly; (ii) above the critical filling height, collapse occurs before the downward sliding motion of the whole granular column sets in, such that only a partial friction mobilization is at play. However, we notice also that the critical filling height is reduced as the grain size d increases. The importance of grain size contribution is controlled by the ratio d/√[Rt]. We rationalize these antagonist effects with a novel fluid-structure theory both accounting for the actual status of granular friction at the wall and the inherent shell imperfections mediated by the grains. This theory yields new scaling predictions which are compared with the experimental results.

Genericity of confined chemical garden patterns with regard to changes in the reactants

Typical patterns emerging during the growth of chemical gardens in a confined geometry when the concentration of the reactants are changed. These patterns are robust to changes in the reactant ions.

Tearing of thin sheets: cracks interacting through an elastic ridge

We study the interaction between two cracks propagating quasistatically during the tearing of a thin brittle sheet. We show that the cracks attract each other following a path described by a power law resulting from the competition between elastic and fracture energies. The power law exponent (8/11) is in close agreement with experiments. We also show that a second (asymptotic) regime, with an exponent of 9/8, emerges for small distances between the two crack tips due to the finite transverse curvature of the elastic ridge joining them.

How geometry controls the tearing of adhesive thin films on curved surfaces

Flaps can be detached from a thin film glued on a solid substrate by tearing and peeling. For flat substrates, it has been shown that these flaps spontaneously narrow and collapse in pointy triangular shapes. Here we show that various shapes, triangular, elliptic, acuminate, or spatulate, can be observed for the tears by adjusting the curvature of the substrate. From combined experiments and theoretical models, we show that the flap morphology is governed by simple geometric rules.

Wrinkling hierarchy in constrained thin sheets from suspended graphene to curtains

We show that thin sheets under boundary confinement spontaneously generate a universal self-similar hierarchy of wrinkles. From simple geometry arguments and energy scalings, we develop a formalism based on wrinklons, the localized transition zone in the merging of two wrinkles, as building blocks of the global pattern. Contrary to the case of crumpled paper where elastic energy is focused, this transition is described as smooth in agreement with a recent numerical work [R. D. Schroll, E. Katifori, and B. Davidovitch, Phys. Rev. Lett. 106, 074301 (2011)]. This formalism is validated from hundreds of nanometers for graphene sheets to meters for ordinary curtains, which shows the universality of our description. We finally describe the effect of an external tension to the distribution of the wrinkles.

Neuroprotective and neuroproliferative activities of NeuroAid (MLC601, MLC901), a Chinese medicine, in vitro and in vivo

Although stroke remains a leading cause of death and adult disability, numerous recent failures in clinical stroke trials have led to some pessimism in the field. Interestingly, NeuroAid (MLC601), a traditional medicine, particularly used in China, South East Asia and Middle East has been reported to have beneficial effects in patients, particularly in post-stroke complications. Here, we demonstrate in a rodent model of focal ischemia that NeuroAid II (MLC901) pre- and post-treatments up to 3 h after stroke improve survival, protect the brain from the ischemic injury and drastically decrease functional deficits. MLC601 and MLC901 also prevent neuronal death in an in vitro model of excitotoxicity using primary cultures of cortical neurons exposed to glutamate. In addition, MLC601/MLC901 treatments were shown to induce neurogenesis in rodent and human cells, promote cell proliferation as well as neurite outgrowth and stimulate the development of a dense axonal and dendritic network. MLC601 and MLC901 clearly represent a very interesting strategy for stroke treatment at different stages of the disease.

Moving-boundary approximation for curved streamer ionization fronts: numerical tests

Recently a moving boundary approximation for the minimal model for negative streamer ionization fronts was extended with effects due to front curvature; this was done through a systematic solvability analysis. A central prediction of this analysis is the existence of a nonvanishing electric field in the streamer interior, whose value is proportional to the front curvature. In this paper we compare this result and other predictions of the solvability analysis with numerical simulations of the minimal model.

Moving boundary approximation for curved streamer ionization fronts: solvability analysis

The minimal density model for negative streamer ionization fronts is investigated. An earlier moving boundary approximation for this model consisted of a "kinetic undercooling" type boundary condition in a Laplacian growth problem of Hele-Shaw type. Here we derive a curvature correction to the moving boundary approximation that resembles surface tension. The calculation is based on solvability analysis with unconventional features, namely, there are three relevant zero modes of the adjoint operator, one of them diverging; furthermore, the inner-outer matching ahead of the front must be performed on a line rather than on an extended region; and the whole calculation can be performed analytically. The analysis reveals a relation between the fields ahead and behind a slowly evolving curved front, the curvature and the generated conductivity. This relation forces us to give up the ideal conductivity approximation, and we suggest to replace it by a charge neutrality approximation. This implies that the electric potential in the streamer interior is no longer constant but solves a Laplace equation; this leads to a Muskat-type problem.

Saffman-Taylor streamers: mutual finger interaction in spark formation

Bunches of streamers form the early stages of sparks and lightning but theory presently concentrates on single streamers or on coarse approximations of whole breakdown trees. Here a periodic array of interacting streamer discharges in a strong homogeneous electric field is studied in density or fluid approximation in two dimensions. If the period of the streamer array is small enough, the streamers do not branch, but approach uniform translation. When the streamers are close to the branching regime, the enhanced field at the tip of the streamer is close to 2Einfinity, where Einfinity is the homogeneous field applied between the electrodes. We discuss a moving boundary approximation to the density model. This moving boundary model turns out to be essentially the same as the one for two-fluid Hele-Shaw flows. In two dimensions, this model possesses a known analytical solution. The shape of the two-dimensional interacting streamers in uniform motion obtained from the PDE simulations is actually well fitted by the analytically known "selected Saffman-Taylor finger." This finding helps to understand streamer interactions and raises new questions on the general theory of finger selection in moving boundary problems.

Construction and test of a moving boundary model for negative streamer discharges

Starting from the minimal model for the electrically interacting densities of electrons and ions in negative streamer discharges, we derive a moving boundary approximation for the ionization fronts. Solutions of the moving boundary model have already been discussed, but the derivation of the model was postponed to the present paper. The key ingredient of the model is the boundary condition on the moving front. It is found to be of kinetic undercooling type, and the relation to other moving boundary models is discussed. Furthermore, the model is compared to two-dimensional simulations of the underlying density model. The results suggest that our moving boundary approximation adequately represents the essential dynamics of negative streamer fronts.

Pentaquarks uudds with one color sextet diquark

The masses of pentaquarks uudds are calculated within the framework of a semirelativistic effective QCD Hamiltonian using a diquark picture. This approximation allows a correct treatment of the confinement, assumed here to be similar to a Y junction. With only color antitriplet diquarks, the mass of the pentaquark candidate Theta with positive parity is found around 2.2 GeV. It is shown that, if a color sextet diquark is present, the lowest uudds pentaquark is characterized by a much smaller mass with a negative parity. A mass below 1.7 GeV is computed if the masses of the color antitriplet and color sextet diquarks are taken similar.

Barrier-wave-internal-wave interference and airy minima in 16O16+O elastic scattering

Taking 16O+16O elastic scattering at 124 MeV as an example, we show that a barrier-wave-internal-wave decomposition of the elastic scattering amplitude provides valuable information on the light heavy-ion interaction and complements the more conventional nearside-farside decomposition. In particular, we show that the Airy minima present in the angular distributions are due to a barrier-wave-internal-wave interference mechanism, which sheds additional light on the exceptional transparency displayed by some light heavy-ion scattering systems. Extension of these ideas to other fields, like atomic and molecular collision physics, could prove rewarding.

Firefly luciferase generates two low-molecular-weight light-emitting species

A bioluminescent D-luciferin-luciferase mixture is separated by gel filtration during the time course of the reaction. A simultaneous analysis with an UV-visible diode array detector and an on-line luminometer gives nonsuperimposable chromatograms. Luminescence recordings display three peaks, one associated with the enzyme (light-emitting species 1: LES(1)), and two other species free from the luciferase: LES(2), with a luciferyl-adenylate-like spectrum and LES(3). Production of these two species is nucleotide (ATP or 2'-dATP)- and pH-dependent. The chromatographic data presented here could lead to reconsideration of the generally assumed emission mechanism, which involves one emitter only. It could also suggest that each free emitting species is related to a colour of emission corresponding to the two defined wavelengths previously described ( approximately 575 and approximately 620 nm).