quicksand -- 3/8/23

Today's selection -- from Eureka!, introduced by Jim Al-Khalili. Remember the movies where some hapless soul is the victim of quicksand? What is quicksand?
"Thick slurries of beach sand are often dilatant. Dilatant liquids are utterly perplexing because the harder you stir them, the more solid they become. Stop stirring and you have a liquid again. These positively mind-bending materials lend themselves to that old favourite of film directors: the victim trapped in quicksand. As the person struggles the sand takes a firmer grip. But using just cornflour and water, you'll now know exactly how to defy Hollywood legend.

"Mix 300g corn flour and 250ml water in a medium-sized metal bowl until it becomes too hard to move the spoon. Stop stirring and tip the bowl. You'll notice the mixture becomes a liquid again and flows. Stir again vigorously and it will thicken once more. Try dipping a finger or the spoon into the mixture very slowly -- if you are extremely gentle it will remain liquid, but pull out your finger or the spoon in a hurry and it will solidify. Drag your fingers quickly through the mixture and you'll be able to lift out a putty-like ball that you can work in your hands. But if you open your fingers and stop roiling it, it will quickly become a liquid again, so keep your hands over the bowl. Now try striking it with a hammer (this explains why you need a metal bowl). If you time the blow just right, the mixture will shatter. Best of all, the broken pieces will re-liquefy and pool together like the shape-shifting creatures in Terminator 2. You can even throw the material against an outside wall and watch it shatter (this bit can get quite messy).

"The mixture you have created is a dilatant material, or a shear-thickening fluid (STF). In these materials viscosity increases as the force, or shear, on them is increased. So, as you have already discovered, the more pressure you apply, the more resistant to deform­ation they become. This is because the application of force to the material causes it to adopt a more ordered structure. Under normal conditions the particulates in the liquid are only loosely arranged, but the shock of any impact or pressure changes their alignment, locking them into place. When the stress dissipates the material relaxes again.

"Research into STFs has led to the development of 'smart materials', ones that respond to changes in their surrounding environment. For example, military researchers are attempting to treat fabrics with STFs so that when a bullet strikes an STF-treated uniform, the uniform becomes rigid at the point of impact and the bullet fails to penetrate it. Under normal conditions, however, the fabric would be as flexible as normal clothing.

"So if you ever find yourself in quicksand, remember that gentle, sedate movements (like a slow breaststroke), not kicking legs and arms, provide the route to safety."

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