  Neutrinos in core-collapse supernovae and neutron-star mergers are
susceptible to flavor instabilities of three kinds: slow, fast, and
collisional. Prior work has established mappings of the first two onto abstract
mechanical systems in flavor space, respectively named the slow and fast flavor
pendula. Here we introduce and analyze the flavor pendulum associated with the
third class. We explain our results in terms of the recently developed theory
of neutrino quantum thermodynamics. Perhaps our most surprising finding is that
there exists a limit in which decoherent interactions drive perfectly coherent
flavor conversion.
