How Macroscopic Materials Transition From Liquid to Solid

Physicists used the pancake problem to explain how macroscopic materials transition from liquid to solid when the container is rotated.

But, before delving into the solution, we have to consider the big question — what is the pancake problem?

Well, imagine twirling a glass of wine in a clockwise direction, the wine inside rotates clockwise too, right?

Now, replace the wine glass with a pan of blueberry pancake and twirl in the same direction. Only this time, you’ll notice that the blueberry pancake in the can rotates in a counterclockwise direction.

The same thing happens when you swirl a glass of beads clockwise. The beads move in a counterclockwise direction.

A graduate student in Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), Lisa Lee said:

“It’s a shocking behavior because, unlike wine and pancakes, these are the exact same objects, in the exact same situation.”

So, Lee and colleagues set out to understand why a collection of particles behave this way. They decided to unravel the mystery of the pancake problem.

The Transition of Macroscopic Materials

A group of beads is a collection of macroscopic materials such as snow, sand, or a jar of nut. It belongs to a class of material called granular media.

Wine rotates clockwise, in the direction of the glass because it’s a liquid, and a granular media under low friction. Pancakes, on the other hand, are solid, and similar to granular media under high friction.

As the pancake swirls, its side catches the pan’s edge, which rotates the breakfast in an opposite direction.

Lee noted:

“Collections of macroscopic particles are very interesting because, depending on their conditions, they can behave like a liquid or solid. Sand in an hourglass, for example, flows like a liquid but sand on a beach behaves like a solid supporting your weight.”

For decades, researchers have debated how these objects transition back and forth, from liquid to solid. With the new study, Lee and the research team hope to put the debate to rest.

How Friction Affects Swirling Pancakes

The findings suggest that a small group of beads have low effective friction, compared with a more substantial group of beads. This results in a transition from liquid to solid.

In other words, while one rotating particle may experience little friction, the same is not true for many particles rolling in the same direction.

The large particles – all in contact with each other – often experience more friction, which causes the group to solidify. Like pancakes, the now solid group of swirling particle grabs their containers’ edges, and start rotating in the opposite direction.

Using a computer simulation, the team proved that the particles would never transition in the absence of friction. Meanwhile, the simulation also showed that particles transition faster from solid to liquid when the particles are rougher.

“This experiment is an interesting case of system-size behaviors emerging from the local interactions of individual elements,” said Shmuel Rubinstein, Associate Professor of Applied Physics at SEAS and senior author of the study.

“The emergence of coherent circulation is the subject of a lot of interest recently, for example, in the case of 2-D turbulence or active spinners. It’s cool that similar physics can also be obtained trivially with a dish and a handful of marbles.”