Rechargeable self-assembled droplet microswimmers driven by surface phase transitions

TL;DR

This paper introduces self-assembled microswimmers driven by surface phase transitions, which harness elastic tails and temperature oscillations to achieve autonomous, reversible propulsion without external forces, providing insights into symmetry breaking and biological propulsion.

Contribution

The study presents a novel class of microswimmers that self-assemble and propel via surface phase transitions, incorporating a detailed elastohydrodynamic model and highlighting molecular mechanisms involved.

Findings

Swimmers propel upon cooling due to elastic tail growth.

They recharge and reverse motion when heated, enabling multiple cycles.

Potential applications in studying biological propulsion and interactions.

Abstract

The design of artificial microswimmers is often inspired by the strategies of natural microorganisms. Many of these creatures exploit the fact that elasticity breaks the time-reversal symmetry of motion at low Reynolds numbers, but this principle has been notably absent from model systems of active, self-propelled microswimmers. Here we introduce a class of microswimmer that spontaneously self-assembles and swims without using external forces, driven instead by surface phase transitions induced by temperature variations. The swimmers are made from alkane droplets dispersed in aqueous surfactant solution, which start to self-propel upon cooling, pushed by rapidly growing thin elastic tails. When heated, the same droplets recharge by retracting their tails, swimming for up to tens of minutes in each cycle. Thermal oscillations of approximately 5 degrees Celsius induce the swimmers to harness heat from the environment and recharge multiple times. We develop a detailed elastohydrodynamic model of these processes and highlight the molecular mechanisms involved. The system offers a convenient platform for examining symmetry breaking in the motion of swimmers exploiting flagellar elasticity. The mild conditions and biocompatible media render these microswimmers potential probes for studying biological propulsion and interactions between artificial and biological swimmers.