Deserts may seem lifeless and inert, but they are very active. Sand dunes in particular grow and move — and they also breathe moist air, according to a decades-long research project.
The findings show for the first time how water vapor can penetrate powders and grains, and could have broad applications beyond deserts — in pharmaceutical research, agriculture and food processing, and planetary exploration.
The team’s paper, “Water Vapor Transport in Arid Sand Surfaces—Nonlinear Thermal Coupling, Wind-Driven Porous Convection, Subsurface Waves, and Exchange with the Atmospheric Boundary Layer,” was published March 21 in the Journal of Geophysical Research – Earth Surface .
Led by lead author Michel Louge, a professor of mechanical and aerospace engineering in the School of Engineering, the project spanned not only a great deal of time, but a variety of terrains. It started nearly 40 years ago when Louge was studying the behavior of fluids, gases and solid particles.
To measure substances with greater sensitivity, he and his students developed a new type of instrument called a capacitive probe, which uses multiple sensors to record everything from solids concentration to velocity to water content, all of which are with unprecedented spatial resolution.
When a colleague at the University of Utah suggested the technique might help in imaging mountain snow layers and assessing avalanche potential, Louge went to his garage, grabbed some probes and tested them in a snowstorm. Soon he formed a partnership with a company called Capacitec Inc to combine their respective skills in geometry and electronics. The resulting probes have also proven useful in hydrological research.
In the early 2000s, Louge began working with Ahmed Ould el-Moctar of the University of Nantes in France, using probes to study moisture levels in sand dunes to better understand the process by which agricultural land turns into desert – which will only change with global climate change become more urgent.
“If we continue like this, the future of the planet is a desert,” Lugar said.
While other probes can measure large quantities of matter, Louge’s probes are deep and small, collecting millimeter-level data to pinpoint the exact moisture content and density in the sand. However, to function in the new environment, the probe needs to be modified. So began a decade-long process of trial and error, with Lugar regularly traveling to the deserts of Qatar and Mauritania to experiment with different versions of the probe.
The detectors finally revealed how porous the sand was, with small amounts of air seeping into it. Previous research has hinted at such leaks in the dunes, but no one has been able to prove it until now.
“The wind flows over the dunes, and as a result, there is an imbalance in the local pressure, which actually forces air into the sand and out of the sand. So the sand is breathing, just as the organisms are breathing,” Louge said.
This “breathing” allows microbes to persist deep in the extremely arid dunes despite high temperatures. For much of the past decade, Louge has been working with Anthony Hay, associate professor of microbiology in the College of Agriculture and Life Sciences, to study how microbes help stabilize dunes and prevent them from encroaching on roads and infrastructure.
Louge and his team also determined that the desert surface was exchanging less water with the atmosphere than expected, and that the evaporation of water from individual sand grains manifested as a slow chemical reaction.
Most of their data was collected in 2011, but it still took Louge and his collaborators a decade to make sense of discoveries, such as identifying surface-level perturbations that force evanescent or nonlinear moisture waves. It travels down through the dunes very quickly.
“We could have published data 10 years ago reporting the accuracy of our method,” Louge said. “But until we understand what’s going on, it’s not satisfactory. No one has really done anything like this before. This is the first time it’s been possible to measure such low humidity.”
The researchers anticipate that their probe will have many applications — from studying the way soil absorbs or drains water in agriculture, to calibrating satellite observations over deserts, to exploring alien environments that may contain trace amounts of water. This isn’t the first time Louge’s research has gone into space.
But perhaps the most immediate application is the detection of water contamination in pharmaceuticals. Since 2018, Louge has been working with Merck to use probes for continuous manufacturing, which is seen as a faster, more efficient and lower cost system than batch manufacturing.
“If you want to do continuous manufacturing, you have to have probes that, as a function of time, wherever it matters, you can check that your process is behaving correctly,” Louge said.
Co-authors include Ould el-Moctar; Dr. Jin Xu ’14; Alexandre Valance and Patrick Chasle, University of Rennes, France.
The research was supported by the Qatar Foundation.