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

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 Péclet 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.

Figure 1

(a) Pure water droplet motion. Trajectories of $\simeq 50$ water droplets in the observation room (diameter 1 cm) filled with $\mathrm{Sq}–25\mathrm{m}\mathit{M}$ MO, recorded during 500 s. The droplet trajectory is color coded with the time preceding its present location (see movie in Supplemental Material [23]). (b) Swimming droplets as microfluidic carriers. Transport of salt crystals (top left), Dami cells [22] (top right), and colloids (bottom).

Figure 2

Pure water droplet motion. (a) Velocity and (b) diameter versus time for a selection of eight trajectories. Insets: Linear-log plot for a selection of 35 trajectory parts.

Figure 3

Correlation between velocity and local radius of curvature of the trajectory. (a) A typical trajectory colored by the instantaneous tangential acceleration ($\mu \mathrm{m}{\mathrm{s}}^{-2}$). (b) Velocity as a function of the radius of curvature of the trajectory shown in (a). The color code indicates time ($\mathrm{s}$) from the beginning of the trajectory.

Figure 4

Comparison of three different systems. (a) Decay of the relative droplet radius versus time for three droplets under different conditions: water in Sq-MO (blue squares), water in Td-MO (green diamonds), water-26 wt % NaCl in Sq-MO (red circles). Inset: Velocity versus time for the same three droplets. (b) Parametric plot of the velocity versus $a\times \kappa$ for droplets under the same three different sets of conditions; each point represents a different droplet during a different period of time corresponding to a straight part of its trajectory.

Figure 5

Swimming mechanism behind the spontaneous autophoretic and Marangoni-driven motions of an isotropic droplet.