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Published online by Cambridge University Press: 14 October 2024
We analyse the effect of drop-deformation-induced change in streamline topology on the scalar transport rate (the Nusselt number $Nu$) in an ambient planar linear flow. The drop-phase resistance is assumed dominant, and the drop deformation is characterised by the capillary number (
$Ca$). For a spherical drop (
$Ca = 0$) in an ambient planar extension, closed streamlines lead to
$Nu$ increasing with the Péclet number (
$Pe$), from
$Nu_0$, corresponding to purely diffusive transport, to
$4.1Nu_0$, corresponding to a large-
$Pe$ diffusion-limited plateau. For non-zero
$Ca$, we show that the flow field consists of spiralling streamlines densely wound around nested tori foliating the deformed drop interior. Now
$Nu$ increases beyond the aforementioned primary plateau, saturating in a secondary one that approaches
$22.3Nu_0$ for
$Ca \rightarrow 0$,
$Pe\,Ca \rightarrow \infty$. The enhancement appears independent of the drop-to-medium viscosity ratio. We further show that this singular dependence, of the transport rate on drop deformation, is generic across planar linear flows; chaotically wandering streamlines in some of these cases may even lead to a tertiary enhancement regime.