# Let’s Talk About the Physics of Pancakes

Date:7 September 2019 Author: Tags:

Why does batter behave so weirdly when you mix it? Scientists solve the conundrum at last.

Whipping up the perfect fluffy pancakes means you first need a great batter. There are a few key ingredients—flour, baking powder, salt, sugar, eggs, butter and blueberries—plus some rigorous mixing action.

But by this point you may have realized something strange about the concoction: As you stir the batter clockwise, only a bit of the mixture moves in tandem with your whisk, while the rest swirls in the opposite direction, counterclockwise.

No, blueberries aren’t getting sucked into some strange breakfast food riptide, nor are your ingredients haunted. It’s actually about friction.

In a recent paper in the journal Physical Review E, which covers statistical, nonlinear, biological, and soft matter physics, Harvard researchers describe a pattern of movement in granular material, or a collection of macroscopic particles that compose a collective. (Consider how sand behaves: Each grain acts on its own and touches a few others, which shapes how the overall pile of sand will move).

Researcher Lisa Lee, a graduate student studying applied physics at Harvard’s School of Engineering and Applied Sciences, was surprised at what she and her team found: At low densities, the cluster of particles rotates in the same direction as the swirling motion of the container, and at high densities, the cluster will move in the opposite direction.

Their conclusion? The transition to counterrotation depends on friction. When there are higher particle counts, each is prevented from rolling and spinning, while fewer particles means each exercises more freedom of movement.

The researchers illustrated this point in a video posted to YouTube. In a small petri dish, a few beads are rolled clockwise in a container and continue to move that way. When far more beads are added to the dish, they begin to move counterclockwise, even though the dish is being rotated clockwise.

This behavior helped them to shed light on the transition of a material from liquid to solid, as macroscopic particles can move like the former or the latter, depending on conditions: Sand flows like water in an hourglass, but behaves like a solid when you stand on it at the beach. Scientists don’t yet fully understand how objects transition from liquid to solid.

The team found that smaller groups of beads will have lower friction than larger pairs of beads, creating the transition from liquid to solid. When one bead rolls in one direction, it experiences little friction, but if many particles that are in contact move in one direction, they experience high friction, which causes the group to solidify.

In our sand example, that’s the small amount of sand in the hourglass behaving as a liquid and the transition to acting like a solid when a large grouping of sand particles is on the beach.

If the same logic follows, then, wine swirled clockwise in a glass moves clockwise because it’s a liquid-like granular media in low friction, while pancake mix rotates counterclockwise when rotated clockwise because it acts like a granular material under high friction.

Using computer simulations, Lee and her coauthors showed that in the absence of friction, the particles never solidified, no matter the quantity in which they were present. And the rougher the particles were, the quicker the transition from liquid to solid.

Now who wants to meet us at IHOP?

Cover image: Pixabay

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