When dealing with porous media, the liquid-gas phase-change is generally considered instantaneous, while a retardation time is observed in the case of hygroscopic soils. So far, little research has been done to characterize the non-equilibrium behaviour of water phase change. Therefore, we propose a macroscopic model of the liquid-gas phase-change rate in porous media, based on the difference of chemical potentials between the liquid and its vapour, which is taken as the driving force. It introduces a phenomenological coefficient that must be determined experimentally. An original experiment able to create a macroscopic non-equilibrium between the liquid and its vapour is described. Analysing the return to equilibrium leads to the determination of the phenomenological phase-change coefficient. Depending on the range of partial vapour pressure, two different behaviours are observed: a linear domain close to equilibrium and a nonlinear one far from equilibrium. The results emphasize the relation between water retention properties in hygroscopic porous media and these phase-change characteristics.

The phenomenological relation of non-equilibrium liquid-gas phase change in a porous medium is described at the macroscopic level using the difference in chemical potentials between the liquid and its vapor. The experiments conducted consisted in lowering the partial pressure of water vapor in the pores of a hygroscopic soil and analyzing the return to equilibrium by two measurements: the macroscopic temperature and the partial pressure of vapor. The central hypothesis of the study is that the characteristic time associated with thermal equilibrium is much lower than the characteristic time associated with mass transfers. From these measurements, it is possible to determine the relation that links phase change rate to the logarithm of the ratio of partial vapor pressure divided by the equilibrium pressure (RH). The representation of this relation according to RH reveals two regimes in the return to equilibrium. The characteristics of these regimes are analyzed according to water content, temperature, and total gas phase pressure.

The liquid/vapour phase change of water in soil is involved in many environmental geotechnical processes. In the case of hygroscopic soils, the liquid water is strongly adsorbed on the solid phase and this particular thermodynamic state can highly influence the phase change kinetics. Based on the linear Thermodynamic of Irreversible Processes ideas, the non-equilibrium phase change rate is written as a linear function of the water chemical potential difference between the liquid and vapour state. In this relation, the system is characterized by a phenomenological coefficient that depends on the state variables. Using an original experimental set-up able to analyze the response of a porous medium subjected to non-equilibrium conditions, the phase change coefficient is determined in various configurations. This paper focuses on the influence of the gas phase pressure and underlines that a low gas pressure decreases the phase change kinetics. Then, evaporation and condensation processes are compared showing an asymmetric behaviour. These experimental results are interpreted from a microscopic point of view by relying on recent works dealing with molecular dynamics numerical simulation of the liquid/gas interface.

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An experiment to calculate the heat transfer coefficient using a phase change material is described. The various tests use a naphthalene ball which changes from solid to gas. The technique is simple and inexpensive, and it is capable of yielding meaningful results. It will be seen that the heat transfer rises with the temperature and air flowrate. Also, students will be encouraged to analyse their own results and to find other variations to this experiment. This will considerably enhance the students' understanding of the processes at work.