Perfectly round pearls (pictured) owe their spherical shape to spiral growth patterns of nacre, the iridescent material also known as mother of pearl. By contrast, non-spherical pearls such as drop pearls have longitudinal growth fronts positioned such that they work like teeth on a ratchet, spinning the gem as it grows in an oyster.
Julyan Cartwright of the University of Granada, Spain, and his team calculated the forces exerted by nacre particles sticking to and bouncing off the growth fronts of a developing pearl. The forces proved strong enough to rotate the pearl once every 20 days (the speed at which pearls have previously been found to rotate) and to influence its ultimate shape.
Microscopic control over macroscopic motion could be a useful design principle for building tiny machines, the researchers suggest.
Langmuir http://dx.doi.org/10.1021/la4014202 (2013)
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Ratchet action misshapes pearls. Nature 499, 8 (2013). https://doi.org/10.1038/499008c
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DOI: https://doi.org/10.1038/499008c
