# Sustained interfacial powering through self-generated mantle and siphon of a gelling droplet

## TL;DR

A gelling droplet creates a self-powered propulsion system that lasts much longer than traditional methods by using a mantle-siphon mechanism.

## Contribution

A novel self-generated mantle-siphon mechanism in gelling droplets significantly extends the lifetime of Marangoni motors.

## Key findings

- Droplets with a hydrogel shell and active molecules achieve propulsion lifetimes 300-1000 times longer.
- The mantle-siphon mechanism prolongs adsorption saturation and improves motor efficiency.
- This strategy offers potential for microscale release control in robotics and biomolecule transport.

## Abstract

Autonomous motion in a persistent manner such as spinning of Euler’s disk is long-sought-after by natural or artificial microsystems due to their limited energy loading and is particularly challenging for Marangoni motors as inhomogeneity of active molecules is difficult to sustain. Here we show that by releasing a droplet containing hydrogel precursor and non-small active molecules on a diluted crosslinking-agent solution, the droplet self-propels with a lifetime 300-to-1000-fold longer. It is found that continuously crosslinking hydrogel shell cuts rapid surfactant diffusion and accompanying volumetric contraction perforates the shell and generates a vent through which active molecules are unidirectionally released. The mechanism echoes squid’s jet propulsion wherein water is expelled out of a siphon by contracting mantle. Such self-generated contracting mantle-siphon configuration of a gelling droplet maximizes the localized concentration inhomogeneity and protracts adsorption saturation on water surface, improving the efficiency and lifetime of Marangoni motors for sustained powering of interfacial machines. The unfolded strategy potentially provides solutions for microscale release control which will be of interest to microrobots, materials assembly, and biomolecules transport.

The development of autonomous microsystems capable of sustained motion is crucial for advancing applications in robotics and materials science. This study demonstrates an approach to prolonging the lifetime of Marangoni motors by utilizing a self-generated contracting mantle-siphon mechanism in a gelling droplet, achieving propulsion lifetimes 300 to 1000 times longer than conventional methods.