# Solving the pancake problem

If you swirl a glass of wine clockwise, the wine inside will also rotate clockwise. But, if you're making a blueberry pancake and you swirl the pan clockwise, the pancake will rotate counterclockwise. Don't believe us? Go try it.

The same thing happens with a glass of . A few beads will rotate clockwise when the glass is swirled clockwise. However, a lot of beads in a glass when swirled clockwise will rotate counterclockwise.

"It's a really surprising behavior because, unlike wine and pancakes, these are the exact same objects, in the exact same situation," said Lisa Lee, a in Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS).

Lee and the rest of the research team set about to understand physically why collections of particles behave like this. As it turns out, it's all about friction.

The research was published in Physical Review E.

A group of beads is part of a class of materials known as , a collection of macroscopic particles, like sand, snow or a jar of nuts.

The reason that wine rotates clockwise when swirled clockwise while pancakes will rotate in the opposite direction is because wine is a liquid, similar to granular media under low friction, while pancakes are a solid, similar to granular media under high friction. When a pan is swirled, the edges of the pancake will catch the edges of the pan and rotate the delicious breakfast food in the opposite direction.

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

How these objects transition from liquid to has been an open question for decades.

Lee and the research team found that small groups of beads had lower effective friction than larger groups of beads, resulting in the transition from liquid to solid.

"One particle rolling in one direction encounters very little friction," said Lee. "But many particles, rolling in the same direction, all in contact with each other, experience a lot of friction, causing the group to solidify and change behavior."

Like pancakes, this solid group of swirling particles grabs a hold of the edges of their container and start rotating in the opposite direction.

Using a computer simulation, Lee, along with co-authors John Paul Ryan and Miranda Holmes-Cerfon, demonstrated that when all was removed, the particles never solidified, no matter how many there were. If the particles were rougher, they transitioned faster from liquid to solid.

"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."

Explore further

How birds fly in flocks

More information: Lisa M. Lee et al. Geometric frustration induces the transition between rotation and counterrotation in swirled granular media, Physical Review E (2019). DOI: 10.1103/PhysRevE.100.012903
Journal information: Physical Review E

Provided by Harvard University

Feedback to editors

Aug 30, 2019
"Sand in an hourglass, for example, flows like a liquid but sand on a beach behaves like a solid supporting your weight."
This is like saying: A rug placed over a hole in the floor will not support your weight, while a rug placed over a solid floor will.
Of Course sand will flow, through an hour glass. There is a hole in the bottom.
There is no hole in the bottom of the beach.
What a Dumb analogy.

Aug 30, 2019
What a Dumb analogy.

It's not a dumb analogy, it seems that you've missed the point of the article. The sentence preceding the one you quoted is important:
"Collections of macroscopic particles are very interesting because, depending on their conditions, they can behave like a liquid or solid,"

In one scenario the sand flows like a liquid, and in the other it does not. Just because there isn't a hole under your feet when you're walking along the beach doesn't mean the sand (were the conditions to be different) couldn't flow to the sides, and give under your weight. What actually happens is that it doesn't move much, and in fact supports you because of the friction between the individual particles.

"One particle rolling in one direction encounters very little friction," said Lee. "But many particles, rolling in the same direction, all in contact with each other, experience a lot of friction, causing the group to solidify and change behavior."

Aug 30, 2019
I think the article should cleave to the simplest mechanism that suits the obvious case of a solid disk in a pan with a wider radius: The disk is essentially rolling around the periphery, the point of side contact always suffering sufficient centrifugal force normal to the surface to effect static friction. In fact, even if the disk isn't contacting the side of the pan, its static or dynamic friction at the rotating area of maximum excursion is always going to be greater than at that of the antipode.
In the case of beads, then the mechanism of the article serves to explain the lock of the particles into a solid-like disk, and contact with the side becomes essential.

Aug 30, 2019
Mmmm.... pancakes.
What's the problem, again?

Aug 30, 2019
they forgot the side of bacon?

my first thought as to why the pancake moves in reverse to the motion of the frying pan?

was Newton's rule of clumsy thumbs "for every action, there is an equal & opposite action"

did i quote that correctly?

frankly & ernestly, i wonder if instead of "opposite action"
it should be "opposite re-action"?

Aug 30, 2019
This is a great example of showing physical properties via a very simple experiment. I congratulate and thank the researchers for discovering and sharing it.

Aug 30, 2019
@pntaylor

The sand grains in an hourglass flows downward and is subjected to the pull of Gravity. After a long while of turning the hourglass over and over, the coarse sand becomes rounder and smoother and loses a bit of friction from contact with other grains. Due to the changes in the geometry of the grains, the hourglass may not reflect a duration of an hour any longer, and may reflect a bit less than a full hour. Thus rendering the hourglass inaccurate as a true timepiece unless the sand grains placed in the bottom of the hourglass were already smooth and round.

Aug 30, 2019
"If you swirl a glass of wine clockwise, the wine inside will also rotate clockwise. But, if you're making a blueberry pancake and you swirl the pan clockwise, the pancake will rotate counterclockwise."

They don't mention the 'TILTING' of the pan. If the pan is tilted backward, the pancake mix will flow backward even if the pan itself is rotated clockwise.

In the first half of the video, the plastic dish with fewer round balls is being rotated counterclockwise at a slower pace. I counted 15 rotations. But in the second half of the video the dish is rotated counterclockwise, but at a faster pace/movement of the hand. I counted 18 rotations. Again, the tilting of the dish backward would cause the balls to move in that direction.

Aug 31, 2019
The matter can have properties of solid and also exhibit wave-like properties, this is what Louis de Broglie told us. It depends on the environmental conditions. Matter is after all energy and vice versa, Einstein told us. I liked the curiosity of experimenters.

Sep 06, 2019
I think the article should cleave to the simplest mechanism that suits the obvious case of a solid disk in a pan with a wider radius: The disk is essentially rolling around the periphery, the point of side contact always suffering sufficient centrifugal force normal to the surface to effect static friction. In fact, even if the disk isn't contacting the side of the pan, its static or dynamic friction at the rotating area of maximum excursion is always going to be greater than at that of the antipode.
In the case of beads, then the mechanism of the article serves to explain the lock of the particles into a solid-like disk, and contact with the side becomes essential.

@danR
So, in plain english this time. How does this add to what was already said? Thanks.