Self-propulsion of pure water droplets by spontaneous Marangoni stress driven motion

TL;DR

This study demonstrates that pure water droplets can spontaneously move in an oil-surfactant medium due to Marangoni stress caused by solubilization, enabling potential applications in targeted delivery and microrobotics.

Contribution

First experimental realization of spontaneous Marangoni-driven motion in isotropic water droplets within a biocompatible oil-surfactant system at high Péclet number.

Findings

Water droplets exhibit spontaneous motion driven by solubilization gradients.

The mechanism is confirmed with salt solutions and tetradecane as alternative media.

Droplets can carry external objects like colloids, crystals, and cells.

Abstract

We report spontaneous motion in a fully bio-compatible 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. 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.