A sand dune may look like a monolithic mass of sand, but there are multiple layers and structures within it.
- Nathalie Vriend
For Marco Polo, the desert could be a spooky place, filled with evil spirits. Writing in the 13th century, he described the famous singing sands, which “at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms.” But the low, loud rumbles coming from the dunes were not the work of spirits. They were the work of physics.
As grains of sand slide down the side of certain dunes, they create vibrations that can be heard for miles around. The sand avalanches trigger the dune’s natural resonance, but only when conditions are just right. It can’t be too humid, and the grains of sand need to be just the right size and contain silica. Only then will an avalanche cause the dunes to start singing.
An avalanche, whether it’s made of sand or snow, is an example of a granular flow, when solid particles flow like liquids, colliding, bouncing around, interacting, separating and coming back together again. Granular flow processes can be found everywhere from the world’s highest mountains to your morning bowl of cereal.
Dr Nathalie Vriend, a Royal Society Dorothy Hodgkin Research Fellow in the Department of Applied Mathematics and Theoretical Physics, is a specialist in granular flows. Her PhD research at the California Institute of Technology unravelled some of the physics at work in the same singing sands that mystified Marco Polo. At Cambridge, her research focuses both on sand dunes and on avalanches, and how to quantify their behaviour, which can have practical applications in industries including pharmaceuticals, oil and gas. Vriend’s work relies as much upon laboratory experiments and fieldwork as it does on mathematical models.
Listen to a sand dune and read the full story
Image: Sand dune
Credit: The District
Reproduced courtesy of the University of Cambridge
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