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The Secret of Cornstarch Physics 49

sciencehabit writes "Filling a small swimming pool with cornstarch and water has long been a physicist's party trick. Step onto it slowly and you'll sink, but run across quickly and the oozy mixture will support your weight — almost as though it has turned from liquid to solid. Several reasons have been offered for the phenomenon, but now researchers believe they have the real answer. The key to figuring things out: plunging a 370-gram aluminum rod from a slingshot at around 1 meter per second into a cornstarch suspension." One meter per second doesn't seem very fast for anything launched by a slingshot, but any speed is good as long as it advances important knowledge like this.
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The Secret of Cornstarch Physics

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  • by swb ( 14022 ) on Thursday July 12, 2012 @12:03PM (#40628777)

    Has anyone made the Sugru clone by mixing silicone I caulk and cornstarch (I think ratios are flexible, up to 1:3 with reasonable cure times)?

    As I understand it, the cornstarch absorbs the water and accelerates the silicone cure time, enabling a cure phase where it is hand-moldable like Sugru?

    I have a box of corn starch and two tubes of silicone waiting at home for me to try this. There's an instructable for this that focuses on a different project but a lot of the project is spent on making this hand-moldable silicone.

    I use Harvey plumber's epoxy (moldable, like clay, but gets rock hard) all the time and I've always wanted a hand-moldable product that would have the finished consistency of silicone. Silicone itself is too goopy and cures poorly if very thick.

    Sugru solves this, but its expensive. There are other two-part silicones that can be bought, but they are expensive, too, and this method seems pretty simple and inexpensive (I think I bought two full-size tubes of GE brand Silicone I and a box of cornstarch for about $6, certainly less than $10 total).

  • Why slingshot? (Score:5, Interesting)

    by Anonymous Coward on Thursday July 12, 2012 @12:13PM (#40628885)

    I really wonder about that slingshot... why not just drop the bar from 5cm above? 1m/s is the speed it reaches when dropped from that height.

    (actually, I think the journalists fucked up the numbers again)

  • by Anonymous Coward on Thursday July 12, 2012 @12:57PM (#40629411)

    Yes, I have. It works well, although you have to play with the ratios to get it to do what you want. It's also smelly and challenging to mix at first.

  • Simple behavior... (Score:5, Interesting)

    by Physics Dude ( 549061 ) on Thursday July 12, 2012 @01:27PM (#40629745) Homepage

    I thought this was well understood and have explained it to my nieces/nephews and many others.

    Corn starch as packaged for sale consists of VERY small grains (see []). When mixed with water in appropriate proportions, they exhibit the same behavior as sand at the beach's edge where it will apear to dry when stepped on and when the pressure is removed will return to being very wet.

    This has to do with dense packing behavior: When undisturbed, the particles naturally form a relatively dense packing due to it's low energy configuration. watter fills the space beween the particles. By disturbing the dense packing by applying an external force, the space beween particles increases alowing for more water to be stored in the inter-particle space. The natural dense packing will occur once the disruptive forces have disipated, so to remain in the more 'solid' state, the dense-packing arangement must constantly be disturbed.

    If this research is aimed at the reason for the natural dense-packing in the first place, I thought that was also well understood. Am I missing something?

  • by ebh ( 116526 ) <edhorch&gmail,com> on Thursday July 12, 2012 @01:29PM (#40629775) Journal

    If the compression forms a (near) solid column from the point of impact to the bottom of the container, that implies that as armor it would do exactly what you don't want it to do, which is transmit the impact through to the wearer over a relatively small area. Effective armor spreads the impact over a wide area, or better yet, turns it into a shock wave that spreads and dissipates throughout the armor.

  • Disposal (Score:4, Interesting)

    by Jason Levine ( 196982 ) on Thursday July 12, 2012 @01:34PM (#40629843) Homepage

    My first thought was "I should fill my kids' kiddie pool with corn starch and water to have some scientific fun with them!"

    My second thought was "How would I clean it up when we're done?"

    Somehow, I think dumping it all on the lawn would not be ideal.

  • by JWSmythe ( 446288 ) <> on Thursday July 12, 2012 @01:41PM (#40629929) Homepage Journal

        Actually, that would act more like vehicle armor. You want to stop the projectile, rather than it penetrating the vehicle.

        I just watched a few videos on the subject, where they were using pellet guns. The non-Newtonian fluid scattered, looking like foam under the same impact. When it recovered, it moved like a fluid again.

        I'd think, rather than supporting and spreading the force, it would just react like a weak solid. Sure, you can walk across a sheet of foam board without it collapsing. It doesn't make very good armor though.

        Something like layered Kevlar and non-Newtonian fluid may work well, but most likely the overall weight would prevent any sort of practical uses. Unless you're Chuck Norris, and we all know that he doesn't need armor at all. :)

No problem is so large it can't be fit in somewhere.