June 22-27, 2014
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 . This opens the way to a control of salt or colloid migration and drying rate of soils and building materials.
Emmanuel Keita, Pamela faure, Stephane Rodts, and Phillippe Coussot, "MRI evidence of nanoparticles migration in drying porous media" in "5th International Conference on Porous Media and Their Applications in Science, Engineering and Industry", Prof. Kambiz Vafai, University of California, Riverside; Prof. Adrian Bejan, Duke University; Prof. Akira Nakayama, Shizuoka University; Prof. Oronzio Manca, Seconda Università degli Studi Napoli Eds, ECI Symposium Series, (2014). http://dc.engconfintl.org/porous_media_V/30