Tuesday January 15, 2019 Q&A

Why is that sometimes you don’t sink in the sand?


1 You’re not heavy enough.

2 The grains of sand are packed too tightly together.

3 The sand doesn’t have the right temperature or level of humidit

Correct answer: 2 - The grains of sand are packed too tightly together.

Image © J. J. S. Jerome, N. Vandenberghe, Y. Forterre

At some point you’ve probably walked along the beach and wondered why you aren’t swallowed up by the sand, as if it were quicksand. And while jogging on the beach is hard (especially on your calves!), your feet are unlikely to sink very deep in the sand.

Sand is made up of tiny grains that are either dry or surrounded by liquid. When the grains are surrounded by liquid – such as grains of sand on a beach – an object will either sink through them or be stopped cold, as if it hit a floor.

Which one happens depends on how tightly packed the sand grains are. When an object is placed in sand, the resulting pressure on the grains causes the liquid between them to move. If the sand is compact, the grains can move only by spreading further apart, which results in aspiration of the liquid. The grains then become squeezed between each other and the liquid – meaning the sand loses flexibility and responds to shocks as a solid. However, if the sand is even just 4% less compact, the grains will be able to move more freely and push out the liquid. In this case, an object will sink through the sand.

In other words, it’s the movement of fluid in the sand that determines whether the sand behaves as a solid or a liquid. The less the fluid is able to circulate between grains, the more the sand will act as a solid and resist objects. And the easier it is for us to take a stroll on the beach!

Learn more:

J. John Soundar Jerome, Nicolas Vandenberghe and Yoël Forterre, “Unifying Impacts in Granular Matter from Quicksand to Cornstarch,” Phys. Rev. Lett. 117, 098003.

Pourquoi les solides ne s’enfoncent-ils pas toujours dans les grains en suspension?, CNRS.

Comment faire un sable mouvant?, Zeste de Science – CNRS original series.