  Accurate models of cooling white dwarfs must treat the energy released as
their cores crystallize. This phase transition slows the cooling by releasing
latent heat and also gravitational energy, which results from phase separation:
liquid C is released from the solid C/O core, driving an outward carbon flux.
The Gaia color-magnitude diagram provides striking confirmation of this theory
by revealing a mass-dependent overdensity of white dwarfs, indicating slowed
cooling at the expected location. However, the observed overdensity is enhanced
relative to the models. Additionally, it is associated with increased
magnetism, suggesting a link between crystallization and magnetic field
generation. Recent works aimed at explaining an enhanced cooling delay and
magnetic field generation employ a uniform mixing prescription that assumes
large-scale turbulent motions; we show here that these calculations are not
self-consistent. We also show that thermohaline mixing is most likely efficient
enough to provide the required chemical redistribution during C/O phase
separation, and that the resulting velocities and mixing lengths are much
smaller than previous estimates. These reduced fluid motions cannot generate
measurable magnetic fields, suggesting any link with crystallization needs to
invoke a separate mechanism. Finally, this mixing alters the chemical profiles
which in turn affects the frequencies of the pulsation modes.
