International audienceThe evaporation of droplets on a substrate that is wetting to the liquid is studied. The radius $R(t)$ of the droplet is followed in time until it reaches zero. If the evaporation is purely diffusive, $R \propto \sqrt{t_0\,{-}\,t}$ is expected, where $t_0$ is the time at which the droplet vanishes; this is found for organic liquids, but water has a different exponent. We show here that the difference is likely to be due to the fact that water vapour is lighter than air, and the vapour of other liquids more dense. If we carefully confine the water so that a diffusive boundary layer may develop, we retrieve $R(t) \propto \sqrt{t_0\,{-}\,t}$. On the other hand, if we force convection for an organic liquid, we retrieve the anomalous exponent for water

The influence of the wetting properties of a model porous medium on the evaporation rate of water contained in the sample is studied experimentally. For a hydrophilic porous medium, drying is mainly controlled by the liquid film covering the solid grains and capillary rise inside the pores, leading to a constant drying rate and a homogeneous desaturation of the whole sample in time. For a hydrophobic porous medium, a drying front penetrates into the sample in the early stages of evaporation and the drying rate is found to strongly depend on the boundary conditions and wetting heterogeneities. In the presence of an air flow along the free surface of the sample, the drying rate varies as the square root of time, indicating a diffusive transport mechanism. Without air flow, a power law behaviour for the drying rate as a function of time is observed with an exponent of 0.75 ± 0.03. This is likely to be due to competition between diffusion through the vapour phase and local capillary rise of the liquid due to wetting heterogeneities. A surprising consequence is that for the late stages of drying, the total evaporated mass may become larger without air flow than with air flow.

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A model for penetrative convection in which a stably stratified layer of fluid is bounded by two unstable layers is considered. This configuration is obtained in a horizontal water layer around its density extremum when a quadratic temperature profile is maintained by internal heating. A linear stability analysis shows that either stationary or oscillatory modes set in at the onset of instability depending on the values of the control parameter. Moreover, two types of stationary modes have been identified that differ by the value of their critical wave number and the number of vertical cells. These results are discussed in the light of recent studies on related topics.