Self-Propulsion of Pure Water Droplets by Spontaneous Marangoni-Stress-Driven Motion

TLDR

Theoretical work predicts spontaneous motion of isotropic particles at high Péclet numbers. Solubilization of water by reverse micelles creates a concentration gradient that drives Marangoni stresses. We observe spontaneous Marangoni‑driven motion of pure water droplets in an oil–surfactant medium, confirm the mechanism with salt and alternative solvents, and demonstrate that the droplets can transport colloids, salt crystals, and cells.

Abstract

We report spontaneous motion in a fully biocompatible system consisting of pure water droplets in an oil-surfactant medium of squalane and monoolein. Water from the droplet is solubilized by the reverse micellar solution, creating a concentration gradient of swollen reverse micelles around each droplet. The strong advection and weak diffusion conditions allow for the first experimental realization of spontaneous motion in a system of isotropic particles at sufficiently large P\'eclet number according to a straightforward generalization of a recently proposed mechanism [S. Michelin, E. Lauga, and D. Bartolo, Phys. Fluids 25, 061701 (2013); S. Michelin and E. Lauga, J. Fluid Mech. 747, 572 (2014)]. Experiments with a highly concentrated solution of salt instead of water, and tetradecane instead of squalane, confirm the above mechanism. The present swimming droplets are able to carry external bodies such as large colloids, salt crystals, and even cells.