|MadSci Network: Chemistry|
Playdough is what is called a viscoelastic solid, intermediate between a true liquid and a rigid solid. All the home-made playdoughs around seem to combine a solid (generally starch) and a liquid (invariably water). I expect the commercial producers of playdough use water too, since their products dry out so nicely in air, and something like starch, since you can survive eating them without too many serious side effects.
In a liquid, molecules can freely move past one another; in a solid, they canít. While this transition is very sharp for large collections of small molecules, like water or naphthalene, it becomes less defined as molecules become larger. And starch molecules are among the largest in nature (molecular weights up to 100 million Da are quoted).
Large molecules will slide past one another just like small molecules, but because they are so much larger it takes them more time to do it, and they may tangle up with one another and become difficult to move. You can visualise the starch molecules in playdough as strands of cooked spaghetti (though the main component of starch is highly branched and will tangle up even more). While strands of cooked spaghetti can move past each other, under gravity alone a heap of spaghetti will not spread out into a pool, but remain in a tangled mass. By applying a little force, you will be able to stretch out this mass, but it remains essentially solid in its behaviour.
At room temperature, starch molecules by themselves cannot slide past one another freely (imagine the same heap of spaghetti allowed to dry out for a few days), and neither can the starch molecules in dried up playdough. The water present in playdough acts as a lubricant on the microscopic scale, allowing the starch chains to move past one another rather than break when the playdough is squished.
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