Programmable Active Janus Droplets Driven by Water/Alcohol Phase Separation

TL;DR

This paper introduces self-propelled Janus droplets driven by water/ethanol phase separation, capable of programmable cargo delivery, with potential applications in targeted delivery systems.

Contribution

It demonstrates a novel active droplet system that self-assembles and propels via phase separation, enabling programmable cargo delivery.

Findings

Droplets evolve through three stages depending on ethanol concentration.

Flow field analysis shows transition from weak pusher to neutral swimmer.

Cargo delivery can be programmed by tuning phase separation timing.

Abstract

We report the existence of self-propelled Janus droplets driven by phase separation, which are able to deliver cargo in a programmable manner. The self-propelling droplets are initially formed by a water/ethanol mixture in a squalane/monoolein solution, and evolve in up to three stages depending on ethanol concentration. In the first stage, the droplet propulsion is generated by Marangoni flow originating from the solubilization of ethanol in the oily phase. During this process the droplets absorb surfactant molecules; in combination with the continuous loss of ethanol this leads to a phase separation of the water/ethanol/monoolein mixture and the formation of Janus droplets, i.e. a water-rich droplet connected to an ethanol-rich droplet that is able to deliver cargo. We characterize the different evolution stages of self-propulsion by the flow field around the droplet that evolves from a weak pusher, over a neutral swimmer, to a dimer of neutral swimmers. Finally, we utilize this active system to deliver DNA as a cargo. Tuning the delay time before phase separation, by varying the chemical composition of the droplets, several different cargo delivery processes can be programmed.