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A fluid instability at the origin of salt polygons
Dry salt lakes form spectacular landscapes in arid regions. Strong evaporation and weak precipitation (but also human intervention) can progressively deplete lakes of water, leaving surfaces that appear dry while the water table remains shallow. In such environments, evaporation continues to play an important role, driving dissolved minerals upward and leading to the growth of a salt crust at the surface. These crusts are known to form polygonal patterns with a characteristic wavelength of the order of one meter, bounded by ridges approximately 10cm tall. Remarkably, these patterns show little variability on Earth.
Understanding the formation of these patterns is a fascinating problem in itself, but it is also important for controlling dust emission. In this talk, I will describe the main physical processes driving the fluid dynamics beneath the surface of these lakes. The system is modeled by a three-dimensional porous medium subject to a vertical through-flow induced by evaporation and to buoyancy forces : the presence of a salt crust at the surface yields an unstable stratification and competes with the evaporative drive. I will describe how this instability develops and how it ultimately leads to the emergence of polygonal structures at the surface.