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The aim of this communication is to analyse the influence of water activity and total pressure on water evaporation. The system is composed of liquid and gas phases, separated by a plane surface, contained in a cylinder whose volume is regulated by a piston. Water activity is regulated by saturated salt solutions and pressure by the piston. The experimental device and procedures were defined to limit the temperature variation at the interface. A transient method is used. From a steady state, a volume increment is imposed; the resulting non-equilibrium leads to an increase in the partial pressure of water vapour to the equilibrium pressure imposed by the solution. Numerical calculation shows little variation in temperature in the gas-liquid interface under the experimental conditions. An evaporation model is adopted taking into account chemical potential discontinuity at the interface. The surface flux evaporation and chemical potential jump at the interface are deducted from the total pressure recording. In the neighbourhood of equilibrium, the surface flux of phase change is shown to be proportional to the chemical potential jump. The surface coefficient of evaporation increases with the total pressure of the gas phase and water activity.

The unsaturated soil hydraulic properties are key properties in the fields of soil science and civil engineering. Because of their strong dependence on water content, there are considerable experimental and numerical difficulties in their determination, specifically in the dry range. This situation is encountered regularly in arid and semi-arid regions. The models commonly used for predicting unsaturated hydraulic conductivity function rely on pore bundle concepts that account for capillary flow only and neglect film flow. Furthermore, the assumption of a local equilibrium between liquid water and its vapour is no longer valid at small water contents. Thus, with classical approaches, the experimental identification of hydraulic characteristics can fail at small water contents. To emphasize the weakness of capillary models, soil column experiments have been carried out with two sandy soils from Burkina Faso. Special care was taken to prevent any transport processes that are not directly related to liquid transport. Data from profiles of water content were introduced into an inverse numerical procedure to identify the coefficients of a new relative hydraulic conductivity function. Our results show that this simple approach is suitable for the analysis of flow processes at small water contents. It provides a simple, robust and inexpensive method to identify the properties of unsaturated conductivity function that account for capillary and film flows.

The aim of this paper is to propose a method to determine the chemical potential of two miscible components of a solution in porous media, particularly in the case of an unsaturated soil. It is limited to the capillary state; the equilibrium of the liquid phase in the soil is determined by gas–liquid interfaces whose geometry obeys Laplace's law. Deduction of the chemical potential of the two components from the chemical potential of pure water is based on the assumption that, for the same volume occupied by the fluid phase in the porous medium, the geometry of the fluid phase is independent of the composition of the solution. The expressions of the chemical potentials of the two components were established. They show the superposition of two effects: the effect of interfacial forces and the effect of the presence of the other component. A study of the water–alcohol solution in a clayey silty sand soil was then conducted to alcohol mole fractions between 0 and 0.15 and water content in the 5%–15% range. In this field, the chemical potential of the water was seen to be affected by the capillary effects below a water content of 10%. Above that, its variation as a function of the mole fraction of alcohol differed little from that of the potential of the water in a free solution. The chemical potential of the alcohol was little affected by the capillary effects.

In this paper, we analyze and model the mass transfer of trichloroethylene in the surface layer of a soil. Our study essentially focuses on arid soils taking into account the phase change liquid-vapor. We have then examined the validity of the assumption of local equilibrium by comparing the values of the instantaneous pressure of the trichloroethylene vapor and the equilibrium vapor pressure during the transfer process. It appears that the assumption of local equilibrium during the transfer of trichloroethylene cannot be admitted.

The sorption behaviour and water transport mechanisms inside Spirulina platensis samples were experimentally analysed during isothermal drying at 25°C and 50°C. Two different products grown in semi-industrial farms from Burkina Faso and France with initial water contents respectively of the range from 2.73 to 3.12 were characterized. A novel procedure has been developed to determine the water content profiles inside samples during isothermal drying. At both temperatures, experimental results underlined that the physical properties of Spirulina are not sensitive to the geographical origin, Burkina-Faso or France. To keep Spirulina at an water ac tivity below 0.6 in order to preserve it from micro-organisms development, sorption isotherm curves show that a sufficient requirement is to lower the water content until an upper limit of w=0.075. The evolution of water transport coefficient as a function of water content highlights a monotonous exponential dependence with a transport coefficient ranging from 1.70×10−10 to 94×10−10 m2/s. The contribution of solid phase shrinkage to the transport of water is negligible for the last drying steps.