Cornstarch solution

  1. I was playing around with cornstarch solution as I saw on T.V. that it had some really funny properties. It becomes hard when you hit it, but its soft if you go slow on it.
    I was preparing some at home in two containers. In one container however, I added too much water. Having finished the cornstarch, I couldn't add more. So to get rid of this extra water, I tried to heat the solution, but instead of returning to normal, globules formed in the solution as it became thicker. I'm finding it very difficult to make it normal.
    Why does this happen? What does adding extra water and then heating do to the mix?
     
  2. jcsd
  3. Anyone?
     
  4. Sounds to me like you simply cooked the cornstarch. If you want to remove water, just let the (uncooked) mixture sit, undisturbed, and the cornstarch will eventually settle, then just pour off some of the water.

    Google "non-newtonion fluids" for more information, recipes and experiments.
     
  5. Andy Resnick

    Andy Resnick 5,780
    Science Advisor

    Cornstarch in solution is an example of a dilatant (shear thickening) fluid:

    http://en.wikipedia.org/wiki/Dilatant

    The phenomenon is partially understood at best- hand-waving arguments about tangled polymers are usually invoked. It's a nonlinear effect, and part of a more general phenomena:

    http://en.wikipedia.org/wiki/Non-Newtonian_fluid

    Unfortunately, when you heated the solution, you most likely made some irreversible changes to the starch itself, causing the behavior you experienced. best to just start over- next time, mix the material much more slowly. It's a nonlinear effect, so there's a narrow range of concentration that will produce the dramatic effect:

    http://en.wikipedia.org/wiki/Oobleck
     
  6. I see. So not many people have done research on this stuff yet? Is it an open topic?
     
  7. Andy Resnick

    Andy Resnick 5,780
    Science Advisor

    In as much as the nonlinear dynamics of materials is an open topic, yes.
     
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