We provide a brief review of thermohaline physics and why it is a candidate extra mixing mechanism during the red giant branch (RGB). We discuss how thermohaline mixing (also called $delta$ $mu$ mixing) during the RGB due to helium-3 burning, is more complicated than the operation of thermohaline mixing in other stellar contexts (such as following accretion from a binary companion). We try to use observations of carbon depletion in globular clusters to help constrain the formalism and the diffusion coefficient or mixing velocity that should be used in stellar models. We are able to match the spread of carbon depletion for metal poor field giants but are unable to do so for cluster giants, which may show evidence of mixing prior to even the first dredge-up event.
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