Forward, reverse, and no motion of Marangoni surfers under confinement

Document Type

Article

Publication Date

8-12-2020

Department

Department of Mechanical Engineering-Engineering Mechanics

Abstract

We examine the mobility of a chemically active particle straddling the interface between a liquid layer of finite depth and a semi-infinite layer of gas. A surface-active agent is released asymmetrically from the particle that locally lowers the interfacial surface tension. It is commonly presumed that the uneven distribution of the surface tension and the associated Marangoni flow lead to the propulsion of the active surfer opposite to the release direction, where the surface tension is higher. This is considered forward motion. However, our recent theoretical analysis—in the limits of negligible inertia and diffusion-dominated transport of the active agent—has shown that this trend may be reversed for certain shapes of the surfer and shallow enough liquid layers. Advancing beyond the Stokes regime, here, we study the Marangoni-driven motion of thin cylindrical disks and oblate spheroids for a wide range of release rates and diffusivity of the exuded chemical species, that control the effective Reynolds and Péclet numbers. We consider various degrees of confinement represented by the thickness of the liquid film, and show that indeed the surfers can undergo a forward, a backward, or an arrested motion. We also identify the links between these modes of mobility and the forces acting on the surfers as well as the flow structure in their vicinity. Rather unexpectedly, we discover that negative pressure is the primary contributor to the fluid force experienced by the surfer and that this suction force is mainly responsible for the reverse Marangoni propulsion. The reported results are based on closely corroborating numerical calculations and experimental measurements.

Publication Title

Physical Review Fluids

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