Watching things dry isn’t always the most enlightening project, but researchers publishing in the September Physical Review E have discovered some new twists to the drying process. They created three different simulated soils by mixing tiny glass beads with clay or gel and used magnetic resonance imaging (MRI) to watch as the water evaporated from the samples. The results point to the properties of the pasty material between the beads as the dominant factor determining the drying speed, an important soil property for farmers. The team also speculates that slower-drying material could be added to real soil to help it retain moisture.
Soil drying is a basic process in agriculture and may become even more important as a consequence of global climate change, but soil-drying research hasn’t advanced very far. Natural soils, which consist of a mix of smaller and larger particles, dry at a constant rate at first and then enter a phase of progressively slower drying. Researchers have interpreted this pattern as the result of a changing balance between the rate of evaporation and the rate of water moving up through the sample. The durations of the two phases vary strongly depending on the type of soil used, but researchers haven’t been able to tell which components of the soils are responsible for the rate differences, says Philippe Coussot of University of Paris–Est, in Champs sur Marne.
To tease apart the role of individual soil components, Coussot and his University of Paris colleague Paméla Faure simulated soil by mixing glass beads measuring 300 microns across with each of three different wet, paste-like materials. Two of the materials were clays. The first, called kaolin, is made of elongated particles measuring about one micron across. The second, called bentonite, consists of even more elongated particles of similar size that expand when wet. The third material was a French hair gel called Vivelle Dop, which consists of spherical blobs that also swell in water.
The researchers inserted each “soil” into its own vertically-oriented cylindrical container (measuring 34 millimeters in diameter) and then left it to dry at room temperature for several weeks, with no end cap on top. They periodically weighed each sample and used MRI to get a spatial map of the water content at successive points in the drying process.
The samples all dried at different speeds, the pair reports. The fastest drying sample was the kaolin, which dried three times faster than the slowest drying sample, the hair gel. Changing the size of the glass beads had no effect on the drying time, so Faure and Coussot concluded that it was the particles in the paste component of each sample that determined its drying properties. The MRI scans of the kaolin mixture showed that water was slowly evaporating throughout the entire sample, as opposed to drying from the top down. “The common idea is that you create a dry front progressing through the packing,” says Coussot. “This is not what occurs with sufficiently small particles.”
The researchers believe that kaolin dries evenly because, as the water evaporates, the particles don’t stick together, even though they pull closer to one another. Air is then drawn into the voids between particles, creating a series of bubbles throughout the sample that dry it quickly and evenly. In the hair gel sample, MRI revealed a drying front moving down the cylinder. In this case, as the gel particles dry out, the team believes they stick together, blocking air from penetrating the deeper regions of the sample. The bentonite particles appear to stick together but not as strongly as the gel particles, leading to an intermediate type of drying, between the two extremes.
“The experimental data are wonderful,” says Lei Xu of the Chinese University of Hong Kong, particularly the direct MRI measurement of water inside the samples. Examining soils at higher spatial resolution could give more insight into the drying mechanism, Xu says. Coussot says the results suggest a way to slow down the drying of real soil: simply inject a gel into the top layers. More generally, the team believes experiments like theirs could help engineers develop pasty materials with specific drying properties needed in mortar, paint, or concrete.