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Pearls and the Puzzle of How They Form Perfect Spheres

Physicists have finally solved the problem of how pearls form almost-perfect spheres: they rotate as they grow.

“Pearls, the most flawless and highly prized of them, are perhaps the most perfectly spherical macroscopic bodies in the biological world. How are they so round?”

So begin Julyan Cartwright at the University of Granada in Spain and a few pals in a paper that gives an interesting answer to this question. Such a mechanism must not only explain the near spherical perfection but also drop-shaped pearls, which have rotational symmetry but are not spherical, and so-called baroque pearls that have no symmetry.

The answer turns out to be based on a relatively simple effect. Cartwright and co say the surface of a pearl has a ratchet-like texture. This generates a force that tends to turn the pearl as it grows in the presence of random jostling from the environment. “Pearl rotation is a self-organized phenomenon caused and sustained by physical forces from the growth fronts,” they say. “Rotating pearls are a—perhaps unique—example of a natural ratchet.”

In the absence of other forces, this rotational process causes the pearl to become spherical. However, small defects in the shape of the pearl can easily distort the process so that certain rotational symmetries end up being preferred. The result in that case is that the pearl becomes nonspherical but maintains a rotational symmetry to form a drop shap, for example.

Baroque pearls form when the defects in the pearl’s shape prevent the rotation from occurring at all. However the growth continues, leading to a shape that has no rotational symmetry.

That’s a interesting insight which Cartwright and co say could be useful for nanotechnology. “The understanding of the pearl as a natural ratchet should have interest for technological applications,” they say.

Cartwright and co aren’t clear about what they have in mind but that has never stopped readers of the Physics arXiv Blog from suggesting creative uses for new technologies. So if you have any ideas for pearl-inspired ratchets could be used, post them in the comments section, please.

Ref: arxiv.org/abs/1304.3704: Pearls Are Self-Organized Natural Ratchets

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