Deserts may seem lifeless and inert, but they are very much alive. Sand dunes, in particular, grow and move – and according to a decades-long research project, they also breathe moist air.
The results show for the first time how water vapor penetrates powders and grains, and could have many applications far beyond the desert – in pharmaceutical research, agriculture and food processing, as well as planetary exploration.
The team diary, “Water Vapor Transport Through an Arid Sand Surface – Nonlinear Thermal Coupling, Wind-Driven Pore Advection, Ground Waves, and Exchange with the Atmospheric Boundary Layer», published on March 21 in the Journal of Geophysical Research-Earth Surface.
The project, led by lead author Michel Louge, a professor of mechanical and aerospace engineering at the College of Engineering, has not only traveled a lot of time, but also a variety of terrain. It all started almost 40 years ago, when Louge was studying the behavior of fluids, gases and solid particles.
Wanting to measure matter with greater sensitivity, he and his students developed a new form of instrumentation called capacitive probes, which use multiple sensors to record everything from solid concentration and velocity to water content, all with unprecedented spatial resolution.
When a colleague at the University of Utah suggested the technology could be useful for visualizing mountain snow layers and assessing the likelihood of avalanches, Louge went to his garage, grabbed some probes, and pulled them out. tested in a snowstorm. Very quickly, he joined forces with a company, Capacitec Inc, to combine their respective skills in geometry and electronics. The resulting probes have also proven useful in hydrology research.
In the early 2000s, Louge began collaborating with Ahmed Ould el-Moctar of the University of Nantes, France, to use the probes to study moisture content in sand dunes to better understand the process by which farmland is turning into desert – an interest that has become even more pressing with the rise of global climate change.
“The future of Earth, if we continue down this path, is a wasteland,” Louge said.
While other probes can measure large volumes of material, Louge’s probes go deep and are small, collecting millimeter-scale data to determine the exact amount of moisture in — and density — of the sand. However, to operate in a new environment, the probes had to be modified. And so began a decade-long process of trial and error, as Louge did periodic trips to the deserts in Qatar and Mauritania experimenting with different versions of the probe.
The probe eventually revealed how porous the sand is, with a tiny amount of air seeping through it. Previous research had suggested that this type of infiltration existed in sand dunes, but no one had been able to prove it until now.
“The wind blows over the dune and consequently creates imbalances in local pressure, which literally forces air in and out of the sand. So sand breathes, like an organism breathes,” Louge said.
This “breathing” is what allows microbes to persist deep inside hyper-arid sand dunes, despite the high temperature. For much of the past decade, Louge has collaborated with Anthony Hayassociate professor of microbiology in the College of Agriculture and Life Sciences, to study how microbes can help stabilize dunes and prevent them from encroaching on roads and infrastructure.
Louge and his team also determined that desert surfaces exchange less moisture with the atmosphere than expected, and that the evaporation of water from individual sand grains behaves like a slow chemical reaction.
The bulk of their data was collected in 2011, but it still took Louge and his collaborators a decade to make sense of some of the findings, such as identifying surface-level disturbances that force waves evanescent or non-linear moisture to propagate downward. through the dunes very quickly.
“We could have published the data 10 years ago to report on the accuracy of our approach,” Louge said. “But it wasn’t satisfying until we figured out what was going on. No one had really done anything like this before. This is the first time that such low humidity levels can be measured.
The researchers anticipate their probe will have a number of applications – from studying how soils soak up or drain water in agriculture, to calibrating satellite observations of deserts, to exploration of extraterrestrial environments that may contain traces of water. It wouldn’t be the first time Louge’s research had traveled to space.
But perhaps the most immediate application is the detection of moisture contamination in pharmaceuticals. Since 2018, Louge has collaborated with Merck to use the probes in continuous manufacturingwhich is considered a faster, more efficient and less expensive system than batch manufacturing.
“If you want to do continuous manufacturing, you need to have probes that will allow you, over time, and wherever it matters, to verify that you have the right behavior of your process,” Louge said.
Co-authors include Ould el-Moctar; Jin Xu, Ph.D. ’14; and Alexandre Valance and Patrick Chasle with the University of Rennes, France.
The research was supported by the Qatar Foundation.