MRI evidence of nanoparticles migration in drying porous media

Due to the migration of elements they induce imbibition-drying cycles are known to play a major role in the colloid-facilitated transport in many industrials process, for instance for pollutants migration in soils or pores clogging in building materials. We study the drying of a colloidal suspension in a porous media. The critical physical phenomenon at work here is the displacement and redistribution of colloidal particles induced by evaporation of the liquid phase from the porous medium. This can be clearly seen by filling a bead packing with coffee. Indeed after full drying the sample has shaded tones with darker regions around the sample free surface and white regions almost free of particles around the bottom. The mechanisms are not yet fully understood and there is no straightforward observation and simple quantification of the spreading of the elements. To better understand the phenomenon, we perform the same experiment using with a clear scaling separation between the porous structure (glass beads diameter=200 μm) and nanoparticles in suspension in water (diameter = 20 nm). Using a new MRI technique to measure the distributions of water and particles we observe particles rising towards the free surface, as water remains homogenously distributed. The particles aggregation area is very large compare to their volume fraction in the pore volume. But we can quantify how the elements migrate towards the free surface of the sample and accumulate in the remaining liquid films. Our complete understanding of the process makes it possible to establish a simple model predicting the drying rate and the concentration distribution [7]. This opens the way to a control of salt or colloid migration and drying rate of soils and building materials

Evaporation from a capillary tube: Experiment and modelization

Drying is known to play a major role in soils and buildings materials. Better understanding the physics may help saving cost and energy. Thus control of the drying kinetics is a key factor. In permeable porous media, capillary forces lead to constant curvature of the air/water interface. The value of the curvature and the shape of the interface depend strongly on the pore geometry. Thus small change in their shape may lead to main change of the air/water interface as the medium desaturates. As the surface is supplied with water the drying rate remains at a constant value set by the area of air/water interface close to the surface. A capillary tube of rectangular cross section maintains water layers in its 4 corners and reproduce well the drying regimes of a porous medium. Here we show that a small variation in the shape of the cross section modify drastically the invasion of air due to equilibrium of capillary forces. Moreover not only the corners but a large part of the cross section remains wet in particular at the entrance of the tube allowing a high drying rate. Pore distribution and the opacity of samples make it difficult to locate water and estimate capillary forces with accuracy. Using a simple geometry, we can observe the water distribution and measure the shape of the air/water interface with good resolution in imaging and in time. We observe that the drying rate is constant during the main period of the desaturation even if the air/water interface increases by a factor 10. Using 2D finite element method (FEM), we show that the air inside a large portion of the capillary tube is saturate with water vapor ; thus only a small part of the interface close to the entrance participate to the evaporation flux. More generally we can infer that below one pore diameter air is saturated and the air/water interface does not contribute to drying. The three basic regimes of drying kinetics in porous media assumes that the drying rate will decrease as the capillary forces are no longer able to provide water to the evaporation surface. In our tube, as desaturation goes further, the drying rate decreases even if the capillary flow still supply water to the surface. Again using FEM, we show that as the wetting surface at the entrance decreases the drying rate will decrease even if no receding front progress. In this situation, the air/water interface inside the tube contributing to the drying increase progressively but this is not enough to maintain the initial high rate. Interpreting only the water mass loss as a function of time, we may lead to wrong conclusions considering basics drying regimes. In Porous media with the same porosity and a slight variation in pore shape drying rate may differ by order of magnitude. Our understanding of the drying kinetics of a simple geometry opens way to control the pore distribution to tune the drying rate of porous media in situation where capillary effects are dominant