Making children walk across what appears to be a pool of liquid is one of the best known science demonstrations. Now researchers have discovered more about the way this material, dubbed oobleck, reacts to shocks — and the findings could improve car safety.
Oobleck, which takes its name from a Dr Seuss book, simply refers to a suspension of starch in liquid. It’s an example of a non-Newtonian fluid, meaning it doesn’t react to pressure in a “normal manner.”
Fill a pool with oobleck and you can demonstrate this by having children walk across it: if they walk confidently, the material will appear to solidify and will support their weight; if they hesitate, well, that’s why you normally get them to wear rubber boots. (Said demonstration is the reason around 12 kilograms of corn starch remains in my garden shed…)
Why this works has a relatively simple explanation: stepping on the oobleck creates impact on it, but the water is driven away faster than the starch. This means that as long as you only step on it briefly, the area supporting your foot will consist solely of the solid particles, which jam into place in a particularly sturdy matrix.
The principle is already used for a form of armor that is flexible enough for practical use, but can absorb sudden impact such as shooting or stabbing. It could one day be used in vehicles as a way to limit the effects of crashes.
Now a team based at Princeton University have examined what happens when you hit oobleck with so much force that it does break up. Before the experiment they expected that oobleck’s natural flexibility meant it would tear.
To test what happened, the researchers spread a layer of oobleck over a sheet of plexiglass (to allow filming from underneath) then dropped a tungsten carbide rod weighing 300 grams, repeating the process from different heights and with different thicknesses of oobleck.
The video shows that rather than tearing, the oobleck actually shattered in a similar way to glass — the difference being that the cracks quickly resealed. The results also showed that the height from which the rod was dropped (and thus the force of impact) made much less difference than the thickness of the oobleck.
Once the oobleck reached a certain thickness, it didn’t crack at all. That seems to be because enough of the bottom of the layer remained liquid to absorb the impact.