Iso/ Thixotropy (What's That?)

Materials: Cornstarch (or 'Corn flour'), water, large bowls or something that can hold a few cups of water and still be able to reach your hands inside.


Place about a cup of cornstarch (aka corn flour) in a large bowl and add about a 1/4 cup of water to the cornstarch. Keep adding water until the mixture appears somewhat thicker than pancake batter. With your hands, take a handful and knead the mixture, like you would bread dough. As the mixture is agitated or squeezed, it will become firm as long as continuous kneading or pressure is exerted. As soon as the pressure stops, the 'batter' will revert to its original form and 'pour' through your fingers. Even though the cornstarch and water is a liquid, you can form the stuff into a ball if you can squeeze and knead it quickly enough. The sensation of this in the hands is unique and must be experienced to be believed. It feels great!
  What's going on?

This experiment demonstrates iso- and thixo-tropy properties of certain emulsions (fluid mixtures).

Isotropy is the property of a fluid to become firm when agitated. This is what you're experiencing with the cornstarch and water. You can also see this while walking on wet sand at a beach. The sand firms up below your feet as you first touch the sand and then becomes more fluid, as your feet sink into the sand, just a moment later. If you run over the sand, the sand will feel very firm. If you walk slowly, your feet will sink below the surface with every step.

Thixotropy is the opposite of isotropy in that the fluid mixture becomes more fluid (less firm) as it is agitated. An example of this is where you strike the end of a ketchup bottle to get ketchup to come out of the bottle. The striking force temporarily causes the ketchup become 'runny' and it flows more easily from the bottle (and on to your new clothes - white ones). Another example is the infamous "quicksand". If you thrash around in the quicksand, you will sink "quicker" because all that wiggling causes the sand to liquify.

  Mess Factor

Cornstarch is an edible, non-staining, harmless substance (probably one of the cheapest food stuffs you can buy), but this experiment tends to get cornstarch on clothing and the floor. The other thing about this experiment is that once you've put your hands into the cornstarch & water mixture (aka 'glop'), you won't want to take your hands out. It is quite a unique experience!

Things to Remember

Isotropic = cornstarch & water or beach sand
Thixotropic = ketchup or "quicksand" [think of 'thick ketchup' for Thixo]

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© 1998-2005 Randy Carr

Last Modified: July 19, 2005