  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.
