Why's it hard for Ketchup to flow?

Why is it so tough to get ketchup out of the bottle?

When you overturn a sauce bottle that has been left untouched for some time, chances are, either the sauce will not come out at all or a gigantic blob will plop down on your plate. Getting ketchup to ‘flow’ out of the bottle can be quite an ordeal.

However, if you shake the bottle before overturning it, the thick gooey ketchup will flow out more freely.

Why does this happen?

This strange property of changing from a gel like thickness (half liquid-half solid, like your toothpaste) to a free flowing liquid and back again is called thixotropy (pronounced thick-saw-tro-pea).

The word has been derived from the Greek thixis, which means “the act of handling” and trope, meaning “change”. So a ketchup is a semi-liquid substance which changes on being handled.

This happens because of the shape of the ketchup molecules. They are in the form of long chains. When the ketchup bottle is left untouched for some time, the molecule chains coil up together in a jumble.

When the bottle is shaken, the molecules break apart and become loose. Hence, the ketchup starts flowing.

You can even make your own goo that behaves in a strange, opposite way. For that, you need one cup of cold water and one and a half cups of cornstarch. Pour the water into a shallow dish. Add the cornstarch to the water slowly, one teaspoon of at a time, stirring all the while. Continue till you have exhausted all the cornstarch.

Strike the surface of the water-cornstarch mixture with the back of a spoon. The mixture will not splatter like any other liquid — instead it turns firm the instant you hit it. Notice that this is the opposite of ketchup: ketchup grows runnier when you shake it, while this goo grows harder the more you push it.

If you drop a small hard object, such as a tennis ball, on its surface, the mixture becomes firm and does not splash. This is because cornstarch and water molecules have fairly fixed positions. They cannot move freely under pressure and behave more like a solid. However, if there is no pressure, the mixture behaves like a liquid.

Quicksand works like the cornstarch goo too: the more you struggle and press, the harder it grips you. The best way out of quicksand is to move very slowly.