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I work in a thin-film photovoltaics institute. We deposit 1 µm silicon on glass substrate. This silicon has tensile stress.
Then, by etching through a tiny hole in the silicon with HF, we underetch the thin layer. This way, a circular area of the silicon is unattached to the glass. The circle grows with time. We observe spirals of cracks at these positions. Sometimes, there's only one spiral, sometimes we see two nested spirals.