Scalar transport from deformed drops: the singular role of streamline topology

Abstract

We examine scalar transport from a neutrally buoyant drop, in an ambient planar extensional flow, in the limit of a dominant drop phase resistance. For this interior problem, we consider the effect of drop-deformation-induced change in streamline topology on the transport rate (the Nusselt number $Nu$). The importance of drop deformation is characterized by the Capillary number ($Ca$). For a spherical drop ($Ca = 0$), closed streamlines lead to the ratio $Nu/Nu_0$ increasing with the Peclet number($Pe$), from unity to a diffusion-limited plateau value ($\approx 4.1$); $Nu_0$ here denotes the purely diffusive rate of transport. For any finite $Ca$, the flow field consists of spiralling streamlines that densely wind around nested tori foliating the deformed drop interior. $Nu$ now increases beyond the aforementioned primary plateau, saturating in a secondary plateau that approaches $23.3$ for $Ca \rightarrow 0$, $Pe Ca \rightarrow \infty$, and appears independent of the drop-to-medium viscosity ratio. $Nu/Nu_0$ exhibits an analogous variation for other planar linear flows, although chaotically wandering streamlines in these cases are expected to lead to a tertiary enhancement regime.