Oobleck, Cream, and Ketchup

Most liquids behave like we would expect them to. But there are liquids, known as Non-Newtonian Fluids, that behave strangely under stress.

Consider your ketchup bottle. Why is it that we hit the ketchup bottle to make the ketchup come out faster? Turns out it isn’t because the force knocks the ketchup lower down, but because the force actually makes the ketchup runnier. When a sudden force is applied to this type of fluid (called pseudoplastic) it becomes less viscous, as a result it runs out of the bottle.

Honey behaves in a similar way except that it is not the amount of force applied in a single instant, but the amount applied over time. Apparently (being allergic to honey I have no first hand experience of this one), if you leave honey in your cupboard for a long time it will thicken, or become more viscous. But if you stir the honey it will become runnier, or less viscous. This makes it a thixotropic fluid.

Cream also changes with sustained force, but it becomes thicker. This is why we can beat liquid cream and it will become thicker (more viscous) until we have the ever so delicious whip cream. It is a rheopectic fluid.

Finally, is my favourite, Oobleck. Oobleck is the name that has been given to a mixture of cornstarch and water. In the right proportions, these two materials produce a dilatant fluid that becomes more viscous when a sudden force is applied. The result, other than a really fun experiment/craft to do with kids (or adults), is a liquid that you can actually run across. You need to be careful to stomp as you go though, or the force may not be great enough to save you from a pudding like bath.

Institute of Mathematics and its Applications