solely by diffusion. However, if ${\dot{\epsilon}s_0^2}/{K}\gg 1$, equation \[eq:t12\_Ranz\] shows that $t_{h}$ depends predominantly on the stretching rate $\dot{\epsilon}$, and only weakly (logarithmically) on the tracer diffusivity $K$. Stretching enhanced diffusion is a kinematic theory, and makes no assumption on the nature of the flow responsible for the stretching. Though in deep Earth geodynamics this formalism has been used in the context of low-Reynolds, laminar flows (mixing of compositional heterogeneities in the mantle) [*e.g.* @coltice_2006; @kellogg_1987; @olson_1984], it has been developed with turbulent flows in mind [@ranz_1979], and has been used for example to characterize mixing in turbulent jets [*e.g.* @duplat_2008; @villermaux_2004].

Our goal is to test whether stretching enhanced diffusion can be an efficient mean of equilibrating (thermally and chemically) the metal phase of the impactors’ cores with the immiscible molten silicates of the magma ocean, and to generalize its formalism (and equation \[eq:t12\_Ranz\] in particular) to heat and mass transfer between two stirred immiscible liquid phases. After presenting the governing equations and introducing a change of variable allowing to consider heat and mass transfer with the same set of equations (section \[sec:equations\]), we build a regime diagram for the deformation of an initially round drop falling under its 