Nikhil Desai

Self-propulsion of chemically-driven isotropic 'swimmers' parallel to a rigid surface: comparisons between active particles and active drops

Chemically active drops release a chemical solute, gradients in whose concentration cause interfacial flows that advect the emitted solute. This non-linear coupling between fluid and solute transport causes the drop to swim spontaneously via an advective instability, if the system’s Peclet number Pe–ratio of advective-to-diffusive solute transport–is larger than a finite threshold. Theoretical analyses often model active drops as diffusioosmosis-driven active particles, primarily to simplify the implementation of boundary conditions at the drop surface. However, physical insights about drop motion that rely on this simplification risk having limited applicability to fluid drops, especially when the drop is interacting chemo-hydrodynamically with its environment.

In this talk, we analyse​ the translation of a chemically active viscous drop parallel to a rigid wall, where it is confined by an external force. We consider a propulsion mechanism where the drop swims due to interfacial Marangoni stresses and diffusioosmotic flows and compare our results to those of the “isotropic active particle” model, to examine the validity of the latter. We show that a less viscous drop swims closer to the wall than a more viscous drop. This results in faster swimming, due to stronger concentration gradients driving the drop’s motion. It is also shown that for moderate Pe, the swimming speed of the drop depends weakly on its mobility ratio (relative strength of diffusioosmotic and Marangoni flows). This weak dependence provides a quantitative justification for studying diffusioosmosis-driven active particles to infer the motion of Marangoni-stress-driven active drops, thus enabling mathematically simpler treatment of a broad class of active-drop problems.