Pure Diffusive Transport Research Articles

Colloidal transport of heavy metals in low‐advective‐velocity environmental systems: Reactive transport model on biogeochemical and hydrodynamic impacts

AbstractIn this study, the impact of colloid facilitated transport of heavy metals on the overall biogeochemical processes is demonstrated in example Lake Coeurd'Alene sediments. Release and transport of heavy metals (Pb and Zn) on initially sorbed colloidal Fe (hydr)oxide minerals are compared with immobile surfaces under various advective flow velocities. The reactive transport model integrates a coupled biotic reaction network with multiple terminal electron acceptors, including multicomponent diffusion and electrostatic double layer (EDL) treatment effects, illustrating the impact of colloidal transport under competing biogeochemical reaction dynamics for the first time to the authors’ knowledge. The model results illustrate the sensitivity of the results under low‐flow‐velocity conditions. Although enhanced Fe reduction prevails with immobile Fe (hydr)oxide mineral surfaces, the desorbed metal ions with aqueous sulfide complexes are rather “washed out” from the system along with advective transport of solutes, whereas the reductive dissolution of colloidal Fe (hydr)oxides from freshly coming colloidal surfaces results in the accumulation of metal and sulfide ions in the system. The results show that when the potential transport of sorbed contaminants with colloidal particles are ignored, the contaminant concentrations might be underestimated under low‐flow‐velocity conditions, especially around 10−8 or 10−9 m s−1, where the underestimation for the worst case scenario at the lowest bound of low‐flow‐velocity conditions may reach around 90% with depth. On the other hand, this impact may be less significant under cases of higher flow velocity, even around higher limits of low‐velocity environments around 10−7 m s−1, as well as in pure diffusive transport cases.

Modelling the transport of PFOS from single lined municipal solid waste landfill

The contaminant transport of perfluorooctane sulfonate (PFOS) through the base of a municipal solid waste landfill lined by a single composite liner comprised of a geomembrane over a geosynthetic clay liner (GCL) and a 3.75 m-thick attenuation layer underlain by an aquifer is examined. Both pure diffusive transport and advective transport through holed wrinkles are modelled. The peak concentrations of PFOS predicted for the aquifer beneath the landfill are compared to the maximum allowable PFOS concentration established by Australia, Canada, Europe, and various US states. The results show that (a) in most cases with zero leakage and pure diffusive transport, regulatory limits are met; however, (b) if there are holed wrinkles in the geomembrane, the impact is highly dependent on the length of holed wrinkle per hectare, the geomembrane-GCL interface transmissivity, and the GCL hydraulic conductivity. Factors affecting the length of holed wrinkles are discussed. In many of the cases examined, the peak concentrations of PFOS in the aquifer exceed the limits set by the different jurisdictions, indicating that a single composite liner may not be sufficient to contain PFOS acceptable level and recommends that realistic contaminant impact calculations be performed in each specific case.

Primary dendrite array morphology in Al-7 wt% Si alloy samples directionally solidified aboard the International Space Station

Under a NASA (National Aeronautics and Space Agency)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions), three Al-7 wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified aboard the International Space Station (ISS) to determine the effect of mitigating convection on the primary dendrite array morphology. MICAST-6 was processed with a step-increase in the growth speed from 5 to 50 µm s−1, MICAST-7 with a step-decrease from 20 to 10 µm s−1, and MICAST2-12 with constant speed of 40 µm s−1. Nearest-neighbor primary dendrite arm spacings and primary dendrite trunk diameters were measured on transverse sections taken at approximately 5 mm intervals along the length of the MICAST samples. In MICAST samples, especially in the steady-state growth regime, the observed primary dendrite nearest-neighbor spacings show a good agreement with predictions from the Hunt-Lu numerical model. The maximum to minimum (highest 10% to lowest 10%) nearest-neighbor spacing ratio in MICAST samples is 2.10 ± 0.27 in agreement with Hunt-Lu model, which assumes pure diffusive transport. Trunk diameters in MICAST samples also agree well with predictions from a coarsening based model. The terrestrial-grown equivalent samples, on the other hand, display smaller nearest-neighbor spacing, and larger primary dendrite trunk diameters than those predicted from the models. Thermosolutal convection appears to increase the dendrite tip radius, causing the trunk diameter to increase, while simultaneously reducing the primary dendrite spacing. This requires verification-experiments involving directional solidification followed by rapid quenching of the liquid-solid interface in metallic alloys in a low gravity environment. The International Space Station currently does not have on-board hardware to conduct such experiments.

Regulating lamellar eutectic trajectory through external perturbations.

The present understanding of asymmetric lamellar eutectics focuses on pure diffusive transport, and how the external perturbations cause asymmetric pattern transitions remains unclear. In this work, the effect of external perturbations is discussed in terms of both thermal and convective effects via phase-field modeling. The presence of thermal perturbation distorts eutectic lamellae, while the convective perturbation causes a tilt band. Both can adjust the eutectic trajectory to accommodate newly established thermodynamics by reconstructing the transport equilibrium. Furthermore, how to regulate the eutectic growth (eutectic colony, zigzag, and snakelike patterns) by altering external perturbations is investigated, which provides information on how to control eutectic evolution.

Impact of wind speed and soil permeability on aeration time in the upper vadose zone

In high permeability soils, gas flux through the Earth–atmosphere interface can be significantly greater than expected from only diffusion. One mechanism that can contribute to the overall flux increase is wind-induced transport (WIT). Here, we explored the magnitude of WIT as a function of the averaged wind speed and soil permeability. Five columns, each filled with homogeneous dry soil or soil-aggregates with different permeability were installed in a bare field. The permeabilities in the columns ranged from 3.87 × 10−10 m2 (sand) up to 2.67 × 10−6 m2 (large aggregates collected from a nearby agricultural field). CO2-enriched air was used to quantify air transport in each soil column. Measurements were carried out under natural wind conditions. Data collected included atmospheric (wind speed, air temperature, barometric pressure, etc.) and soil parameters inside the columns (temperatures and CO2 concentration at −0.2 m). Data of changing CO2 concentration over time were compared to (1) an analytical diffusion transport solution, and (2) a numerical advection–dispersion solution. Results show that for sand, air transport was governed by diffusion with a very small additional WIT effect, increasing total air transport by up to ∼25% for cases of high wind speed (>5 m/s). From the permeability of small gravel and above (≥1.02 × 10−8 m2), WIT dominated air transport and the effect of WIT was clear even under low wind speeds. The increase in total air transport in the large aggregates for high wind speed was up to one order of magnitude greater than pure diffusive transport.

Highly accurate, efficient, and automatic computation of reversible cyclic voltammograms, using double exponential formulas for numerical integration

Highly accurate reference values of theoretical cyclic voltammograms are indispensable for those who develop and examine simulation methods or other modelling approaches for electroanalytical chemistry. Reference values having relative errors close to ± 10−16 (which is the level of the representation errors of standard double precision variables) are desirable, but they cannot be obtained efficiently using conventional simulations. A class of reversible cyclic voltammograms can be expressed in terms of improper convolution integrals that have to be evaluated numerically. By employing the efficient automatic numerical integration package INTDE of Ooura, based on double exponential formulas, and written in “C”, it is shown that such voltammograms can be calculated with relative errors close to ± 10−16, in a time of ca. 1ms per single current function value, on a contemporary laptop computer. Furthermore, this method appears resistant to failures that may accompany highly accurate numerical integration by alternative methods and software. This is demonstrated for three kinetic examples under conditions of pure diffusion transport: reversible charge transfer at a planar electrode; reversible charge transfer at a spherical electrode; and ErevCirr’ mechanism at a planar electrode.

Water transport in gas diffusion layer of a polymer electrolyte fuel cell in the presence of a temperature gradient. Phase change effect

The gas diffusion layer (GDL) is a crucial component as regards the water management in proton exchange membrane fuel cells. The present work aims at discussing the mechanisms of water transport in GDL on the cathode side using pore network simulations. Various transport scenarios are considered from pure diffusive transport in gaseous phase to transport in liquid phase with or without liquid–vapor phase change. A somewhat novel aspect lies in the consideration of condensation and evaporation processes in the presence of a temperature gradient across the GDL. The effect of thermal gradient was overlooked in previous works based on pore network simulations. The temperature gradient notably leads to the possibility of condensation because of the existence of colder zones within the GDL. An algorithm is described to simulate the condensation process on a pore network.

JOVIAN STRATOSPHERE AS A CHEMICAL TRANSPORT SYSTEM: BENCHMARK ANALYTICAL SOLUTIONS

We systematically investigated the solvable analytical benchmark cases in both one- and two-dimensional (1D and 2D) chemical-advective-diffusive systems. We use the stratosphere of Jupiter as an example but the results can be applied to other planetary atmospheres and exoplanetary atmospheres. In the 1D system, we show that CH_4 and C_2H_6 are mainly in diffusive equilibrium, and the C_2H_2 profile can be approximated by modified Bessel functions. In the 2D system in the meridional plane, analytical solutions for two typical circulation patterns are derived. Simple tracer transport modeling demonstrates that the distribution of a short-lived species (such as C_2H_2) is dominated by the local chemical sources and sinks, while that of a long-lived species (such as C_2H_6) is significantly influenced by the circulation pattern. We find that an equator-to-pole circulation could qualitatively explain the Cassini observations, but a pure diffusive transport process could not. For slowly rotating planets like the close-in extrasolar planets, the interaction between the advection by the zonal wind and chemistry might cause a phase lag between the final tracer distribution and the original source distribution. The numerical simulation results from the 2D Caltech/JPL chemistry-transport model agree well with the analytical solutions for various cases.

Value of the exponential current–time perturbation for achieving stationary polarisation curves at planar and spherical electrodes of any size

Stationary current–potential ( I– E) polarisation curves (SPCs) are of interest in electrochemistry, because the extraction of thermodynamic and kinetic information from such curves is easy. In order to obtain the SPCs under pure diffusion transport conditions, experiments are usually performed using spherical electrodes, at which the SPCs are rapidly attained. A purely diffusional steady state cannot be achieved at planar electrodes. However, the theoretical analysis presented in this work reveals that in the case of chronopotentiometry with exponential current–time perturbation, the Nernst diffusion layer reaches a constant thickness, for a simple charge transfer reaction. Consequently, the I– E curves obtained under these conditions are SPCs, both for the spherical and planar electrodes. This theoretical prediction is confirmed experimentally for the reversible oxidation of ferrocene in acetonitrile solution of lithium perchlorate, and irreversible reduction of hexaamminecobalt (III) nitrate in aqueous solution of sodium perchlorate.

A mathematical model and a numerical model for hyperbolic mass transport in compressible flows

A number of contributions have been made during the last decades to model pure-diffusive transport problems by using the so-called hyperbolic diffusion equations. These equations are used for both mass and heat transport. The hyperbolic diffusion equations are obtained by substituting the classic constitutive equation (Fick’s and Fourier’s law, respectively), by a more general differential equation, due to Cattaneo (C R Acad Sci Ser I Math 247:431–433, 1958). In some applications the use of a parabolic model for diffusive processes is assumed to be accurate enough in spite of predicting an infinite speed of propagation (Cattaneo, C R Acad Sci Ser I Math 247:431–433, 1958). However, the use of a wave-like equation that predicts a finite velocity of propagation is necessary in many other calculations. The studies of heat or mass transport with finite velocity of propagation have been traditionally limited to pure-diffusive situations. However, the authors have recently proposed a generalization of Cattaneo’s law that can also be used in convective-diffusive problems (Gomez, Technical Report (in Spanish), University of A Coruna, 2003; Gomez et al., in An alternative formulation for the advective-diffusive transport problem. 7th Congress on computational methods in engineering. Lisbon, Portugal, 2004a; Gomez et al., in On the intrinsic instability of the advection–diffusion equation. Proc. of the 4th European congress on computational methods in applied sciences and engineering (CDROM). Jyvaskyla, Finland, 2004b) (see also Christov and Jordan, Phys Rev Lett 94:4301–4304, 2005). This constitutive equation has been applied to engineering problems in the context of mass transport within an incompressible fluid (Gomez et al., Comput Methods Appl Mech Eng, doi:10.1016/j.cma.2006.09.016, 2006). In this paper we extend the model to compressible flow problems. A discontinuous Galerkin method is also proposed to numerically solve the equations. Finally, we present some examples to test out the performance of the numerical and the mathematical model.

A finite element formulation for a convection–diffusion equation based on Cattaneo’s law

A number of contributions have been made during the last decades to model pure-diffusive transport problems by using the so-called hyperbolic diffusion equations [M. Zakari, D. Jou, Equations of state and transport equations in viscous cosmological models, Phys. Rev. D 48 (1993) 1597–1601; T. Ruggeri, A. Muracchini, L. Seccia, Shock waves and second sound in a rigid heat conductor: a critical temperature for NaF and Bi, Phys. Rev. Lett. 64 (1990) 2640–2643]. These equations are used for both mass and heat transport. The hyperbolic diffusion equations are obtained by substituting the classic time-independent constitutive equation (Fick’s [A. Fick, Uber diffusion, Poggendorff’s Annal. Phys. Chem. 94 (1855) 59–86] and Fourier’s [J.B. Fourier, Théorie analytique de la chaleur, Jacques Gabay, 1822] law, respectively) by a more general time-dependent equation, due to Cattaneo [M.C. Cattaneo, Sur une forme de l’equation de la chaleur liminant le paradoxe d’une propagation instantane, Comptes Rendus de L’Academie des Sciences: Series I-Mathematics 247 (1958) 431–433]. In some applications the use of a parabolic model for diffusive processes is accurate enough in spite of predicting an infinite speed of propagation (Cattaneo, 1958). However, the use of a wave-like equation that predicts a finite velocity of propagation is necessary in many other calculations [A. Compte, The generalized Cattaneo equation for the description of anomalous transport processes, J. Phys. A: Math. General 30 (1997) 7277–7289]. The studies of heat or mass transport with finite velocity of propagation have been limited, so far, to pure-diffusive situations. This paper proposes a formulation for convection–diffusion problems based on a Cattaneo-type law. The finite element solution of the proposed equations is addressed. The performance of the algorithm is verified by solving some 2D test cases. Some interesting features of the proposed model can be observed from these examples and we conclude that the proposal is a feasible alternative to the parabolic model for real engineering simulations.

Extraction of Parameters and Their Error Distributions from Cyclic Voltammograms Using Bootstrap Resampling Enhanced by Solution Maps: Computational Study

The conventional determination of model parameter errors in least-squares regression of experimental cyclic voltammetric data assumes validity of local approximations (e.g., linearization) in the parameter space and normal distributions of the data and parameter errors. Such assumptions may not always be satisfied in practice. Bootstrap resampling techniques present a more universally applicable approach to error estimation, which until now has not been used in cyclic voltammetric studies, owing to the high costs of the required voltammogram simulations. We demonstrate that the burden of computing voltammograms can be significantly reduced by the use of high-dimensional model representation (HDMR) solution mapping techniques, thereby making it feasible to apply the bootstrap data analysis in cyclic voltammetry. We perform computational experiments with bootstrap resampling, enhanced by HDMR maps, for a typical cyclic voltammetric model (i.e., the Eqrev Cirr Eqrev reaction mechanism at a planar macroelectrode under semi-infinite, pure diffusion transport conditions). The experiments reveal that the bootstrap distributions of the estimated parameters provide a satisfactory quantification of the parameter errors and can also be used for detecting statistical correlations of the parameters.

High-Dimensional Model Representation of Cyclic Voltammograms

Digital simulation costs present an obstacle on the way to high-speed, real-time, on-line theoretical analysis of experimental data in cyclic voltammetry. To overcome this difficulty, we propose to use solution mapping based on a correlated, hierarchical expansion of multivariate functions, known as high-dimensional model representation (HDMR). The nonlinear dependencies of the simulated voltammograms on multiple model parameters are represented in the form of compact look-up tables, from which approximate voltammograms can be calculated rapidly by interpolation, for any model parameter combinations from a predefined domain. Most importantly, the HDMR does not suffer from the problem of the exponential growth of the look-up tables with the number of model parameters. The creation of a solution map requires a single effort of simulating many voltammograms. However, once the map is prepared, it can be stored and reused many times without the need to repeat costly simulations. HDMR maps are created and examined for five examples of cyclic voltammetry models at planar macroelectrodes in a one-dimensional spatial geometry under pure diffusion transport conditions. The usefulness of the maps for rapid visualization and exploration of the effects of the parameters on the voltammograms and for rapid simultaneous estimation of many parameters from cyclic voltammetric data is demonstrated through computational experiments.

Distributed modeling of diffusive solute transport in peritoneal dialysis.

The diffusive transport between blood and an ex-tissue medium (dialysis fluid) is evaluated using a mathematical model that takes into account the (quasicontinuous) distribution of capillaries within the tissue at various distances from the tissue surface, and includes diffusive-convective transport through the capillary wall and lymphatic absorption from the tissue. General formulas for solute penetration depth, lambda, and for the diffusive mass transport coefficient for the transport between blood and dialysis fluid, K(BD), are provided in terms of local transport coefficients for capillary wall, tissue, and lymphatic absorption. For pure diffusive transport between blood and dialysis fluid and thick tissue layers (i.e., if the solute penetration depth is much lower than the tissue thickness) these formulas yield previously known expressions. It is shown that apparent tissue layers, with widths lambdaTBL and lambdaT, respectively, may be defined according to the values of local transport parameters in such a way that K(BD) is equal to the solute clearance K(TBL) from the tissue by blood and lymph for a layer with width lambdaTBL or to the solute clearance K(T) from blood to dialysate by diffusion through the tissue layer with width lambdaT. For tissue layers with width much higher than the penetration depth: lambdaT approximately = lambdaTBL approximately = lambda. These characteristic width lengths depend on the transport parameters (and thus on the size) of solutes. Effective blood flow, which may be related to the exchange of the solute between blood and dialysate, is defined using an analogy to the extraction/absorption coefficients for blood-tissue exchange. Various approximations for the distributed model formula for diffusive mass transport coefficient (K(BD)) are possible. The appropriate range for their application is obtained from the general formula.

Reaction kernel structure and stabilizing mechanisms of jet diffusion flames in microgravity

The structure of axisymmetric and two-dimensional, laminar, methane jet diffusion flames in normal earth gravity (1g) and zero gravity (Og) has been simulated numerically. Computations of the time-dependent full Navier-Stokes equations with buoyancy were performed using an implicit, third-order accurate numerical scheme and a detailed C 2 -chemistry model. An optically thin media radiation model for heat losses from CO 2 , H 2 O, CH 4 , and CO is included. Observations of the flames were also made at the NASA Glenn 2.2-s drop tower. The parametric computational results are as follows: (1) In a quasi-quiescent oxidizing environment in 1g, buoyancy induced the entrainment of the surrounding oxidant into the flame base and enhanced the convective-diffusive oxygen influx and key radical reactions, thereby forming a peak reactivity spot, that is, reaction kernel , responsible for flame stabilization. (2) In a quasi-quiescent, Og environment, diffusion-dominated transport processes shifted the flame zone outwardly and reduced the peak reactivity (heat-release rate, oxygen consumption rate, etc.) an order of magnitude smaller. (3) In a coflowing oxidant stream in 0g, the flame structure became similar to that in the 1g environment as a result of a blowing effect. (4) The two-dimensional (quasi-cylindrical) flames were generally larger than the axisymmetric (quasi-spherical) flames because of geometrically limited transport processes in two-dimensional directions. (5) The flames in an oxygen-enriched (30% O 2 /70% N 2 ) environment were generally smaller than those in air as a result of higher reaction rates and stoichiometry. This paper extends the previously proposed reaction-kernel reactivity-velocity correlations and the reaction-kernel hypothesis for diffusion flame stabilization to the near-zero velocity conditions in microgravity. Furthermore, the correlations are extrapolated to a null velocity; that is, the pure diffusive transport regime, in which the flame base is formed simply by quenching and stabilization is no longer required.

Similarity properties of demineralization and degradation of cracked porous materials

Abstract The paper analyze the similarity properties of demineralization processes in cracked porous materials, coupled with diffusive and advective mass transfer through fractures. The aim is to find out, whether, when and how fractures accelerate or magnify the overall chemical degradation process. A dimensional analysis of a simplified dissolution process around a fracture channel reveals different self-similar properties for diffusion and advection dominated mass transport in the fracture. For a pure diffusive transport, the fracture degradation length develops with the quadratic root of time; while it evolves with the square root of time when advection dominates. These asymptotic behaviors are confirmed by model-based simulations of `real' calcium leaching in cracked cement-based materials. It is shown, that a diffusion dominated mass transport in a fracture does not significantly accelerate the overall chemical degradation, since the one-dimensional-degradation through the porous material catches up with the initially faster diffusion through the fracture. In turn, advection dominated mass transfer in fractures can significantly accelerate the overall calcium depletion for `high' fluid velocities in the fracture. Finally, diffusive dominated mass transport in a crack network may also accelerate the calcium leaching process for small values of crack spacing factors. A rough analytical solution for estimating this effect of a crack network is derived and validated through model-based simulations.

Fluctuations in the number of irreversibly adsorbed particles

Fluctuations in the number of colloid particles adsorbed irreversibly under pure diffusion transport conditions were determined as a function of surface density and ionic strength of the suspension. The experiments were carried out for monodisperse polystyrene latex particles of micrometer size range adsorbing irreversibly at mica surface. The surface density of adsorbed particles at various areas was determined using the direct microscope observation method. A new experimental cell was used enabling in situ observations of particles adsorption under conditions of negligible gravity effects. It was found that the particle density fluctuations for high ionic strength were in a good agreement with the theoretical results derived from the random sequential adsorption (RSA) model. Also, the theoretical results stemming from the equilibrium scaled particle theory reflected the experimental data satisfactorily. For lower ionic strength a deviation from the hard sphere behavior was experimentally demonstrated. This effect due to the repulsive electrostatic interactions was interpreted in terms of the effective hard particle concept. The universal dependence of variance on particle density obtained in this way was found in a good agreement with the RSA model for all ionic strength. These results proved that fluctuations in particle density of monolayer formed under diffusional conditions differ fundamentally from these obtained under ballistic transport conditions.

Model Calculations of Porosity Reduction Resulting From Cement-Tuff Diffusive Interaction

ABSTRACTTo determine the potential effects of alkaline plume migration on the near-field environment of the proposed high-level radioactive waste geologic repository at Yucca Mountain, Nevada, calculations are conducted simulating interactions between cement and tuff with pure diffusive transport of solute species. The calculations used the reactive transport submodule GEM of the computer code MULTIFLO [6]. The results suggest that strong alteration of the tuff host rock and of cement in contact with the tuff could result from these interactions. Porosity reduction within the tuff could isolate the matrix from fracture pore water. The model calculations predict calcification of the cement as would be expected. In simulations involving counter-diffusive transport across the cement-tuff contact, calcification is more pronounced in a partially-saturated environment compared to a fully-saturated one.

A comparative study of thermal and thermosolutal convective effects in vertical Bridgman crystal growth

A comparative experimental study of thermal and thermosolutal convection is carried out by solidifying a concentrated Ge-Si binary system and a dilute Ge-Ga system in a new Bridgman furnace where radial thermal gradients have been minimized and well characterized. Very different solutal boundary layer extents (δ GeSi > 3 cm, δ Gega ≦ 2 mm) are found for the two systems solidified under the same thermal conditions. Simple analytical hydrodynamic models are then used to explain these results. It is demonstrated that the larger boundary layer extent observed for Ge-Si and the corresponding pure diffusive transport regime are the result of the stabilizing effect of the longitudinal solutal gradient in this system. It is also shown that Hart's analytical model [J. Fluid Mech, 49 (1971) 279] can be adapted to describe the thermal/solutal coupling of the fluid flow in crystal growth configurations and therefore to predict, in a first approximation, the segregation behaviour in the crystals through a boundary layer model [Favier, Acta Met. 29 (1981) 197, 205].

Thermal and thermosolutal convective effects in bridgman solidification: Effect of orientation relative to gravity

A concentrated binary system (Ge-Si) and a dilute one (Ge-Ga) have been solidified at 1 g, in a new vertical Bridgman furnace where radial thermal gradients have been minimized. Very different solutal boundary layer extents ( δ Ge- Si = 3 cm, δ Ge- Ga = 2.5 mm) obtained for both systems solidified in the same thermal conditions are explained by simple analytical hydrodynamic models. It is demonstrated that : 1. The convective transport of Ga in the Ge-Ga system is due to the unavoidable residual horizontal thermal gradients associated with discontinuity of thermal properties at the solid-liquid interface. 2. The larger boundary layer extent observed for Ge-Si and the corresponding pure diffusive transport is the result of the stabilizing effect of the longitudinal solutal gradient in this system. Thus, in the case of dilute systems, convective effects can only be cancelled through a reduction of the gravity level. On the opposite, pure diffusive solute transport can be achieved in the vertical configuration a 1 g in the case of concentrated systems where a stabilizing solutal effect is operating.