A Simple Analytical Model of Evaporation in the Presence of Roots

Root systems can influence the dynamics of evapotranspiration of water out of a porous medium. The coupling of evapotranspiration remains a key aspect affecting overall root behavior. Predicting the evapotranspiration curve in the presence of roots helps keep track of the amount of water that remains in the porous medium. Using a controlled visual set-up of a 2D model soil system consisting of monodisperse glass beads, we first perform experiments on actual roots grown in partially saturated systems under different relative humidity conditions. We record parameters such as the total mass loss in the medium and the resulting position of the receding fronts and use these experimental results to develop a simple analytical model that predicts the position of the evaporating front as a function of time as well as the total amount of water that is lost from the medium due to the combined effects of evaporation and transpiration. The model is based on fundamental principles of evaporation flux and includes empirical assumptions on the quantity of stoma in the leaves and the transition time between regime 1 and regime 2. The model also underscores the importance of a much prolonged root life as long as the root is exposed to a partially saturated zone composed of a mixture of air and water. Comparison between the model and experimental results shows good prediction of the position of the evaporating front as well as the total mass loss from evapotranspiration in the presence of real root systems. These results provide additional understanding of both complex evaporation phenomenon and its influence on root mechanisms.Comment: 10 pages, 6 figure

Spatially heterogeneous dynamics in a thermosensitive soft suspension before and after the glass transition

The microscopic dynamics and aging of a soft thermosensitive suspension was investigated by looking at the thermal fluctuations of tracers in the suspension. Below and above the glass transition, the dense microgel particles suspension was found to develop an heterogeneous dynamics, featured by a non Gaussian Probability Distribution Function (PDF) of the probes' displacements, with an exponential tail. We show that non Gaussian shapes are a characteristic of the ensemble-averaged PDF, while local PDF remain Gaussian. This shows that the scenario behind the non Gaussian van Hove functions is a spatially heterogeneous dynamics, characterized by a spatial distribution of locally homogeneous dynamical environments through the sample, on the considered time scales. We characterize these statistical distributions of dynamical environments, in the liquid, supercooled, and glass states, and show that it can explain the observed exponential tail of the van Hove functions observed in the concentrated states. The intensity of spatial heterogeneities was found to amplify with increasing volume fraction. In the aging regime, it tends to increase as the glass gets more arrested.Comment: 19 pages, 10 figures, Soft Matter accepte

Convection in a vertical channel

International audienceThe flow generated by heat convection in a long, vertical channel is studied by means of particle imagery velocimetry techniques, with the help of the thermal measurements from a previous paper (Gibert et al 2009 Phys. Fluids 21 035109). We analyse the mean velocity profiles and the Reynolds stresses, and compare the present results with the previous ones obtained in a larger cell and at a larger Reynolds number.We calculate the horizontal temperature profile and the related horizontal heat flux. The pertinence of effective turbulent diffusivity and viscosity is confirmed by the low value of the associated mixing length. We study the one-point and two-point statistics of both velocity components. We show how the concept of turbulent viscosity explains the relations between the local probability density functions (pdf) of fluctuations for temperature, vertical and horizontal velocity components. Despite the low Reynolds number values explored, some conclusions can be drawn about the small scale velocity differences and the related energy cascade

Stability and Adsorption Properties of Electrostatic Complexes : Design of Hybrid Nanostructures for Coating Applications

We report the presence of a correlation between the bulk and interfacial properties of electrostatic coacervate complexes. Complexes were obtained by co-assembly between cationic-neutral diblocks and oppositely charged surfactant micelles or 7 nm cerium oxide nanoparticles. Light scattering and reflectometry measurements revealed that the hybrid nanoparticle aggregates were more stable both through dilution and rinsing (from either a polystyrene or a silica surfaces) than their surfactant counterparts. These findings were attributed to a marked difference in critical association concentration between the two systems and to the frozen state of the hybrid structures.Comment: 3 pages, 3 figures, 27 references, to appear in Langmuir Letter

Heat convection in a vertical channel : Plumes versus turbulent diffusion

11 pagesInternational audienceFollowing a previous study [Gibert , Phys. Rev. Lett. 96, 084501 (2006)], convective heat transfer in a vertical channel of moderate dimensions follows purely inertial laws. It would be therefore a good model for convective flows of stars and ocean. Here we report new measurements on this system. We use an intrinsic length in the definition of the characteristic Rayleigh and Reynolds numbers. We explicit the relation between this intrinsic length and the thermal correlation length. Using particle imaging velocimetry, we show that the flow undergoes irregular reversals. We measure the average velocity profiles and the Reynolds stress tensor components. The momentum flux toward the vertical walls seems negligible compared to the shear turbulent stress. A mixing length theory seems adequate to describe the horizontal turbulent heat and momentum fluxes, but fails for the vertical ones. We propose a naive model for vertical heat transport inspired by the Knudsen regime in gases

Redispersible Hybrid Nanopowders: Cerium Oxide Nanoparticle Complexes with Phosphonated-PEG Oligomers

Rare earth cerium oxide (ceria) nanoparticles are stabilized using end-functional phosphonated-PEG oligomers. The complexation process and structure of the resulting hybrid core-shell singlet nanocolloids are described, characterized and modeled using light and neutron scattering data. The adsorption mechanism is non-stoichiometric, yielding the number of adsorbed chains per particle Nads = 270 at saturation. Adsorption isotherms show a high affinity of the phosphonate head for the ceria surface (adsorption energy ~ -16 kT) suggesting an electrostatic driving force for the complexation. The ease, efficiency and integrity of the complexation is highlighted by the formation of nanometric sized cerium oxide particles covered with a well anchored PEG layer, maintaining the characteristics of the original sol. This solvating brush-like layer is sufficient to solubilize the particles and greatly expand the stability range of the original sol up to pH = 9. We underscore two key attributes of the tailored sol: i) strong UV absorption capability after functionalization and ii) ability to re-disperse after freeze-drying as powder in aqueous or organic solvents in varying concentrations as singlet nanocolloids. This robust platform enables translation of intrinsic properties of mineral oxide nanoparticles to critical end use.Comment: 10 figures, 12 pages, accepted at ACSNano March 0

Comparison between rough and smooth plates within the same Rayleigh-Bénard cell

International audienceIn a Rayleigh-Bénard cell at high Rayleigh number, the bulk temperature is nearly uniform. The mean temperature gradient differs from zero only in the thin boundary layers close to the plates. Measuring this bulk temperature allows to separately determine the thermal impedance of each plate. In this work, the bottom plate is rough and the top plate is smooth; both interact with the same bulk flow. We compare them and address in particular the question whether the influence of roughness goes through a modification of the bulk flow

Thermal boundary layer near roughnesses in turbulent Rayleigh-Bénard convection: flow structure and multistability

We present global heat-transfer and local temperature measurements, in an asymmetric parallelepiped Rayleigh-B ́enard cell, in which controlled square-studs roughnesses have been added. A global heat transfer enhancement arises when the thickness of the boundary layer matches the height of the roughnesses. The enhanced regime exhibits an increase of the heat transfer scaling. Local temperature measurements have been carried out in the range of parameters where the enhancement of the global heat transfer is observed. They show that the boundary layer at the top of the square-stub roughness is thinner than the boundary layer of a smooth plate, which accounts for most of the heat-transfer enhancement. We also report multistability at long time scales between two enhanced heat-transfer regimes. The flow structure of both regimes is imaged with background-oriented synthetic Schlieren and reveals intermittent bursts of coherent plumes

Modelling and dynamic simulation of solar heat integration into a manufacturing process in Réunion Island

International audienceAccording to the International Energy Agency, solar thermal energy is rarely used in industrial sector, despite its capacity to generate heat with the existing technologies. Moreover, a large part of the firms of Réunion Island are in the food-processing industry, which is the most heat consuming sector according to these studies. Among them, the Sorélait-Danone company (yoghurt production) has decided to collaborate so as to analyze the potential impact of solar heat integration into its actual heat generation system. The company has a fuel boiler which generates 7 bar saturated vapor (corresponding to a temperature of 165°C) at a nominal flow rate of 2,5 tons per hour. This boiler is supplied by a tank filled with water. In the latter, condensate coming from the process unit is mixed with the district's cold water, in order to compensate the vapor lost in the process. In this work, possibilities in terms of solar heat integration into the system have been investigated. 4 different cases have been simulated (direct or indirect integration, into the mixing tank or the boiler tank) and the corresponding saved fuel quantities have been estimated. The results of simulation show that in the best case, a fuel saving of 25 % can be expected with a total collector area of 600 m² for this company