Strange thing with corn flour and water?

Question

Have you ever done this? You mix corn flour and some water to make a thick liquid.... if you jab it quickly, it acts like a solid and you don't get any on you, but if you slowly put your finger in it, then ... it acts like a liquid.

Why is that?

Answer 1

Ah, oobleck.  Fun stuff.  As far as I know, scientists aren't completely sure why it does what it does.  There are a few good theories though, and most of them go something like this:

The corn flour molecules in oobleck, by comparison with water, are very large and very complex.  When the two substances are mixed together, they form a sort of suspension with large carbohydrate molecules held apart by a lubricating bath of water.  Surface tension ensures that the water will always keep adjacent starch molecules some distance apart, so if given enough time, the oobleck will settle into a state in which few of the starch molecules are touching each other.  When you push slowly, the oobleck has plenty of time to react, and the setup shifts around to accommodate your finger while still keeping everything nicely lubricated.  If you push quickly, though, the system doesn't have time to make adjustments, and some of the starch molecules end up ramming into each other.  Because of their size and complexity, they get tangled up and cause a lot of friction with each other, making them very difficult to move further until you remove the pressure and let some water back in between them.