Why can a cornstarch suspension resist a quick change in motion?

A thick mixture of cornstarch and water can flow slowly yet resist a quick push. The tiny starch grains have not dissolved. They remain separate solid pieces surrounded by liquid water. A mixture with particles held in a liquid is called a suspension.

When one layer of the mixture slides past another, the grains must move past their neighbors. This sliding between layers is called shear. At a gentle push, the grains can move around one another more easily. A stronger push over the same area can press many grains into contact.

The touching grains rub against each other. Across a crowded mixture, many such contacts can resist motion together, like a connected crowd rather than one grain acting alone. This increase in resistance is called shear thickening. The picture enlarges the grains to reveal their contacts; the water remains between and around them.

When the force decreases, the contacts can loosen and the mixture can flow more easily again. It has not frozen or turned into a new substance. Its behavior comes from how its grains move and touch. A sudden impact can also jam grains together in more complicated ways, so saying that the mixture simply becomes solid whenever it moves fast misses what is happening.