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Weird Corn Starch trick

Phaedrus May 29, 2008 07:45 PM


Can someone explain this?

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  1. Sam Fujisaka RE: Phaedrus May 29, 2008 08:15 PM

    No idea, but my guess is that a slight compression of the saturated starch granules pushes them into a lattice much like a polymer, allowing the solution to have extremely high surface tension supported by the sheer mass of the starch mud below.

    1. m
      MysticYoYo RE: Phaedrus May 29, 2008 08:27 PM

      I can't explain it but it's sure pretty cool to watch!

      1. j
        jlafler RE: Phaedrus May 29, 2008 09:07 PM

        At science fiction conventions, it's called "ooblek" (name taken from a Dr. Seuss book). As in

        There's ooblek in the bathtub -- the con ain't over yet.
        It's pleasantly disgusting, and it's thick and white and wet.
        We know hotel security would say it's got to go,
        So there's ooblek in the bathtub, but no-one's s'posed to know.

        Google "ooblek" or "oobleck" and you'll find lots of explanations for how it works.

        4 Replies
        1. re: jlafler
          Phaedrus RE: jlafler May 30, 2008 05:19 AM

          Do you have a specific site? I Googled it and I got a lot of elementary science sites and a lot of YouTube sites. I now know that its a non-Newtonian liquid but what I have not been able to glean is what differentiates corn starch in water versus say flour in water.

          1. re: Phaedrus
            dolores RE: Phaedrus May 30, 2008 05:25 AM

            How cool.

            Would that help anyone stuck in quicksand?

            1. re: Phaedrus
              jlafler RE: Phaedrus May 30, 2008 11:30 AM

              I think the reason cornstarch acts differently from flour is that it's virtually pure starch, while flour has gluten and other proteins.

              "The most generally accepted explanation for the behavior of the cornstarch water mix is that when sitting still the granules of starch are surrounded by water. The surface tension of the water keeps it from completely flowing out of the spaces between the granules. The cushion of water provides quite a bit of lubrication and allows the granules to move freely. But, if the movement is abrupt, the water is squeezed out from between the granules and the friction between them increases rather dramatically." From http://www.seed.slb.com/en/scictr/not...

              There's a little more on that site, and also here: http://kaffee.50webs.com/Science/labs...

              Looking around, I find some disagreement about whether starch is a polymer. Hmph.

              1. re: jlafler
                Sam Fujisaka RE: jlafler May 30, 2008 12:50 PM

                The starch is not a polymer, but a tight lattice-like arrangement would have the stuff act like a polymer if the tiny granules transmit the force from the blow evenly and outwards, eliminating the water as the force wave proceeds.

          2. BobB RE: Phaedrus May 30, 2008 07:52 AM

            Never tried this with a bathtub full, but it was a standard "science for kids" experiment when I was young. It's somewhat related to Silly Putty (do they still make that?), which is soft when poked slowly but bounces like rubber when dropped.

            1. alliedawn_98 RE: Phaedrus May 30, 2008 08:24 AM

              My son did this at school in science a while back. He brought home a recipe for it but we haven't made it yet. I'm saving it for a rainy summer day when he's bored to tears.

              1. sparkalina RE: Phaedrus May 30, 2008 01:53 PM

                OK, had to post. I read this in the morning and at lunch a co-worker went to the store and bought a box.

                Needless to say it was the highlight of 20+/- adult employees for the rest of the afternoon. We punched it, we made balls, we got our hands stuck in it.

                This was the best day of work the whole week, and I highly recommend it young and old.

                1. porker RE: Phaedrus May 31, 2008 08:54 PM

                  I can only say its similar to a long chain polymer perhaps more like a long chain carbohydrate.

                  1. r
                    ricepad RE: Phaedrus Jun 1, 2008 07:28 AM

                    The guys on "Mythbusters" did this last year.

                    1. j
                      jayaymeye RE: Phaedrus Jun 2, 2008 07:57 AM

                      Here's what my cousin, the organic chemist said:

                      "For the science part, the mixture is called a non-Newtonian Fluid. This means that when a stress (change in pressure like a swift movement or temp) is applied, the viscosity changes. In this case, the mixture becomes instantaneously "solid" until the pressure can equalize and then returns to a goopy "liquid". In contrast, water is a Newtonian fluid, so nothing really changes when stressed, which is why you can mix water with a spoon without much resistance. I've even seen people make a whole tub full of the stuff and walk over it. Once you stand still, you will start to sink, I think wet sand is another example of the stuff.

                      Cornstarch doesn't actually dissolve in water, but rather forms a suspension where each granule is surrounded by water. When unagitated, the granules can move more or less freely and look like a liquid. When you compress it, the water is squeezed out and the granules come closer together and form a solid matrix. Interesting to note is that ketchup is the opposite. It is thicker when at rest and thins out when under stress. The natural tomato pulp forms a solid matrix and is disrupted when it's stirred or moved."

                      Cool, eh?

                      2 Replies
                      1. re: jayaymeye
                        Phaedrus RE: jayaymeye Jun 2, 2008 08:33 AM

                        Molto grazie. Now it makes sense.

                        1. re: jayaymeye
                          Ruth Lafler RE: jayaymeye Jun 2, 2008 03:04 PM

                          I had a college professor who had been a Princeton when Einstein was there. He remembers Einstein walking on the beach -- trailed by a bevy of grad students -- and being fascinated by the way that the degree of wetness of sand changes its properties.

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