Monday, August 06, 2012

The Secrets of Oobleck Revealed - Partially

I was surprised that there wasn't more of a splash in the general news about this research, but when the article is entitled Impact-activated soldification of dense suspension via dynamic jamming fronts ($), it's pretty tough for anyone to recognize this is an article about Oobleck, isn't it?

Oobleck is a term stolen from Dr. Seuss to describe dilatant/rheopectic materials, the most common of which is cornstarch/water suspensions. (Being a suspension, the cornstarch particles are not dissolved in the water, but are in it as solid particles with water in between them.) Although the suspensions are nominally of low viscosity (think interior house paint), when stressed rapidly, they become extremely stiff. Youtube has a number of videos of the stuff, with people walking on it, or bouncing it on speakers or other fun stuff. It's easy to make this stuff up and home with about a 50/50 w/w mix of cornstarch and water. My favorite trick is to roll a ball of it between my hand and then stop the rolling just to watch is dribble out.

Using a nicely instrumented tank with lasers and x-rays to look at the surface and interior of the fluid, University of Chicago Professor Heinrich Jaeger and his student Scott Waitukaitis looked at what was occurring when an aluminum rod impacted the Oobleck. They found that during the stress, the rod pushes the particles together and quickly displaces water, something they called the snowplow effect.
You may then think that this column of highly concentrated cornstarch beneath the rod contacts the bottom of the tank to provide the needed support, but that in fact is not the case. Certainly in shallow tanks it can't be avoided, but even in extremely deep tanks, the solid forms and resists the impact.

What is actually occurring is that the volume being displaced extends as a cone beyond the area of impact:
With such a large volume of mass being displaced, the solidity of the suspension now is a momentum transfer problem - you are transferring the momentum of the rod to this very large mass of suspension and it just isn't going to be going anywhere very quickly.

The title of this post states that the secrets of Oobleck are partially revealed and that is true. What still remains unknown is how this system relaxes. As you can see from the linked videos, the suspensions remains stiff, but only for a short period of time. For reason still unknown, it relaxes and returns to its fluid state, meaning that if you are walking on this stuff, you better keep walking. Standing still is not an option.


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