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How To Swim Through Sand

Sand behaves like a fluid but also like a solid. So how can some lizards swim through it?

The sand skink, Plestiodon reynoldsi, is famous for its ability to swim through sand at depths of up to 10 centimetres. That’s strange because although sand sometimes act like a fluid, it also acts as a solid supporting large loads such as human footfall. So how do sand skinks do it?

Today, Takashi Shimada at the University of Tokyo in Japan and a few buddies reveal the secret. They say that the sand skinks’ swimming action exploits sand’s fluid-like nature AND its ability to act like a solid. And they’ve built a computer model to simulate how this works.

The model consists of two balls buried in sand and linked together by a spring. The balls can inflate and deflate to change their size. When they are small, they pass relatively easily through the sand. But when a ball is large, the grains jam against it becoming solid.

Shimada and co show that the pair of balls can move through the sand using a “push-me-pull-you” type motion. An inflated ball creates an anchor from which a deflated ball can push through the sand. So alternately inflating and deflating the balls while pushing and pulling allows the ablls to move through the sand. The team has posted several videos of this movement here.

It turns out that a sand skink’s movement is not dissimilar to this. It has the ability to expand its hindquarters and then thrust its wedge-shaped head into the sand ahead of it, thereby pushing through the sand.

(However, real sand skinks also have an undulatory motion that is still poorly understood.)

Nevertheless, Shimada and co have a cool simulation that may also turn out to be handy. They say the technique may lead to robots that can hunt for landmines beneath the desert floor. But we’ll believe that when we see it.

Ref: arxiv.org/abs/0911.1265: Swimming in Granular Media

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