Liquid Armor – It’s not the Terminator, it’s rheology

I’m stealing a slashdot link here, but I thought this story was so cool I just had to post it. Besides, even if I posted a comment on slashdot, it would only be drowned and unnoticed among the flood of inane comments every post gets on slashdot. This way, my comments will instead be drowned among the thousands of inane personal blogs, like this one.

This new kind of armor uses an interesting property that chemical and mechanical (and perhaps civil) engineers learn when they study fluid mechanics. These kinds of fluids are called rheopectic, which is defined as fluids whose viscosity increases according to the rate of shear stress being applied. In other words, it will flow slowly, but if you try and make it flow quickly it will become thicker and more difficult to flow. A good practical example of this is the corn-starch-in-water that many of us played with as children. If you just hold it in your hands it will ooze through your fingers, but if you try and work it like clay or play-doh, it will get very hard, even to the point where it will crumble or even break in your hands.

In rheopectic fluids, the harder you try and disturb it, the harder the fluid will resist that flow. So in this liquid armor, when it is hit with an extremelly hard shear stress (i.e. a bullet, a knife, etc.) the harder it is hit, the harder it will become in turn. This liquid armor is quite an ingenious innovation, I hope it works as well as they are predicting.

The opposite of rheopectic fluids also exist, they are called thixotropic. A good example of this is latex paint: it gets thick as you leave it set out, but if you stir it for a few minutes it gets thin again. You also may be wondering what in the world rheology means. It basically means the study of the deformation and flow of matter. Pretty much the same thing as fluid mechanics, for all practical purposes. And in case you care, rheology in Japanese is 流体力学.

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