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Blueberry pancake

Researchers explain the unique movement of Pancake when swirled

When a glass filled with wine is swirled clockwise, the wine will also spin in the clockwise direction, however, while making a blueberry pancake, if it is swirled in the clockwise direction, the pancake spins in an anticlockwise direction.

The same is the case with a glass of beads. 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.

Lisa Lee, a graduate student of Applied Physics at School of Engineering and Applied Sciences, Harvard said she was surprised at the behaviour of these exact same objects under the same situations.

The research team set about to understand the physics behind these actions and it turns out that friction is responsible for this. Beads are a part of a class of material called granular media, which means a collection of macroscopic particles such as sand or snow. The work appears in the Physical Review E journal.

Wine rotates clockwise when moved clockwise due to wine being a liquid-like granular media in low friction, while pancakes rotate in an anticlockwise direction when rotated clockwise which is similar to granular media under high friction.

Macroscopic particles are very interesting as they can move like a liquid or a solid depending on the conditions. Sand flows like a liquid in an hourglass but acts like a solid to support your weight on beaches. The object transition from liquid to solid has been an open question for decades.

Lee and the researchers found out that smaller groups of beads will have lower effective friction than larger pairs of beads which results in the transition from liquid to solid. When one particle rolls in one direction it experiences little friction however if many particles which are in contact with each other roll in the same direction, then they experience a large amount of friction which causes the group to solidify and thus change the behavior.

Using computer simulations, Lee and co-authors, John Paul Ryan and Miranda Holmes-Cerfon showed that in the absence of friction, the particles never solidified, no matter the quantity in which they were present. The rougher the particles were, the quicker was the transition from liquid to solid.

Shmuel Rubinstein, Associate Professor of Applied Physics at SEAS and senior author of the study said that this is an interesting case of system-size behaviors emerging from local interactions. The emergence of coherent circulations is an exciting subject for study like the case of 2D turbulence and active spinners. It is quite interesting that daily objects such as marbles and dishes can demonstrate similar physics.

Journal Reference: Physical Review E.