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Science

The Secret of Cornstarch Physics 49

Posted by timothy
from the never-invite-newton dept.
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 Kenja (541830) on Thursday July 12, 2012 @10:54AM (#40628673)
    Penn Jillette once almost drowned a midget while wrestling nude in a swimming pool filled with cornstarch. Oh and non-newtonien liquid bla, bla, etc.
  • ...is using a slingshot to shot Natalie Portman at 1 m/s into a vat of HOT GRITS!

    • by dpilot (134227)

      Darn... haven't seen a Natalie Portman reference here in a long time.

      Do you think the results would vary based on naked, petrified, both, or neither?

      • by Anonymous Coward

        Set up as a poll question and find out. Don't forget to include the Cowboy Neil option.

  • by Anonymous Coward

    is that you can make your own flesh light....

  • by swb (14022) on Thursday July 12, 2012 @11:03AM (#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).

    • Re: (Score:2, Interesting)

      by Anonymous Coward

      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.

      • by swb (14022)

        What ratios did you find worked best? I'm more interested in short working times (5 minutes or less) and putty-like malleability (which I think you might call very high viscosity).

        I think I've heard that 1:2 silicone:corn starch will accomplish this. I've also heard the mixing is a bitch, and I'm not entirely sure how you measure silicone from a caulk tube without a complete mess.

        I plan to try this this weekend, actually. I might even be able to grab some pigment to color it.

        • I'm not entirely sure how you measure silicone from a caulk tube without a complete mess.

          Attach a ruler to your caulk gun and measure how far you've squeezed the plunger... or just use the whole tube in a single batch.

          • by swb (14022)

            There's some logic to that, I guess. I could do the math on the interior of the tube and measure how far the rod on the caulk gun goes in.

            I was considering at least for the first batch using small dixie cups on a sheet of parchment paper, as I've read it doesn't stick well to that. The silicone would be squeezed out next to a dixie cup on the paper, with corn starch measured in the cups added to it and then mixed.

            I'm going to try 1:1 and 2:1 starch:silicone ratios first.

            • by sumdumass (711423)

              Or find the specific gravity of the caulk and starch then place your mixing container on a scale (zeroed out for the weight of the container of course).

              If you have disposable spoons, you can measure the weight of a level spoon full then and multiply that against the total amount to be mixed. OF course this is by volume which is what I think you are wanting to do. If a spoon of caulk is 1 oz minus the weight of the spoon, and the amount you squeeze out to your mixing container is 16 ounces, just adjust the w

    • by Inda (580031)
      Sugru is not something I've heard of but I've done a fair amount of plastic mixes.

      Go careful with accelerators. I stopped using them after the first fire. It's always better to show some patiences.
  • by k(wi)r(kipedia) (2648849) on Thursday July 12, 2012 @11:06AM (#40628803)
    ... the inanimate aluminum rod.
  • Why slingshot? (Score:5, Interesting)

    by Anonymous Coward on Thursday July 12, 2012 @11:13AM (#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

      Come on -- why couldn't you say "4.9cm"? What's a sigfig nazi to do when everyone gets it right?

  • by Anonymous Coward on Thursday July 12, 2012 @11:26AM (#40629025)

    This or it's a non-newtonian server and I clicked so fast I couldn't enter.

  • Mythbusters filled a small pool w/ oobleck and one or two of them ran across it successfully.
    The big deal here is not so much the metal projectile but the array of monitoring equipment used to analyze the change in structure of the goo.

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

    by Physics Dude (549061) on Thursday July 12, 2012 @12:27PM (#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 http://ars.els-cdn.com/content/image/1-s2.0-S0144861705005059-gr3.jpg [els-cdn.com]). 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?

    • I guess I should have read up on all the other ideas/theories first. Interesting, but it seems it's mostly addressed at a macroscopic level from what I have seen. Very interesting.
  • by ebh (116526) <ebh-slashdot.hyperreal@org> on Thursday July 12, 2012 @12:29PM (#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.

    •     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. :)

    • by SQLGuru (980662)

      Would it be possible to eliminate the effect by constructing some sort of disruptive surface below the rod? Something that causes the column to "collapse" as it were?

    • The point of armor is to prevent penetration, not impact. That's not to say that good armor doesn't also distribute the force, but I'd much rather get a bad bruise from a stopped bullet than shot by a bullet I felt no impact from.

      • by rahvin112 (446269)

        In general correct, but that depends entirely on what you are protecting against.

        A typical anti-armor projectile or shaped charge on impact will use a chemical reaction to create molten copper (from a copper ingot in the projectile) that is then explosively discharged into the armor at high speed on impact. This stream of liquid copper melts a hole though the armor and then sprays itself all over the crew compartment killing the occupants in a very gruesome manner while the projectile itself bounces harmles

        • by sjames (1099)

          Other (older) projectiles use the explosive to transmit a shockewave to the inside of the armor that causes fragments to fly off and kill the occupants. The armor has an extra layer inside to reflect that shockwave before it reaches the inside.

      • by sjames (1099)

        If the armor doesn't spread the force of impact at all, the impact can be just as deadly as the projectile itself without armor would be. The point of armor is to allow the user to survive contact with a weapon that would otherwise kill them.

  • Disposal (Score:4, Interesting)

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

    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 Anonymous Coward on Thursday July 12, 2012 @12:50PM (#40630051)

    > "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."

    This is how you chose to contribute to the submission? Really?

  • by guttentag (313541) on Thursday July 12, 2012 @06:24PM (#40633521) Journal
    Traditional Story
    In the Old Testament, Moses told the Israelites [wikipedia.org] not to leaven their bread, and subsequently led them safely across the Red Sea. When the Egyptians pursued them, they drowned.

    What Really Happened
    Obviously, Moses had someone go around and collect all the baking powder (which these days is made from sodium bicarbonate, tartaric acid and cornstarch [wikipedia.org]). Some kid started asking a bunch of questions that irritated Moses, such as, "Why is this night different from all other nights?" He explained that they were leaving Egypt. When the child asked, "Why are we eating unleavened bread?" Moses replied, "uh, because we're in a hurry, kid. Stop asking questions!" He then had the Israelites' baking powder dumped into the sea so his people could run across it [youtube.com]. However, since it wasn't pure cornstarch, it was unstable and collapsed by the time the Egyptians tried to cross.

    It all makes sense now. God I love mixing science and religion. It's a lot like mixing water and cornstarch. Anything holds up surprisingly well if you run through it fast enough.
    • by GrahamCox (741991)
      Anything holds up surprisingly well if you run through it fast enough

      You're right. I skim-read the Bible in about 2 minutes and it seemed fairly sound. Then I actually read it properly and realised it was all nonsense.

Vitamin C deficiency is apauling.

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