  By converting energy into mechanical work, engines play a central role in
most biological and technological processes. In particular, within the current
trend towards the development of nanoscience and nanotechnology, microscopic
engines have been attracting an ever-increasing interest. On the one hand,
there has been a quest to understand how biological molecular motors work. On
the other hand, several approaches have been proposed to realize artificial
microscopic engines, which have been powered by the transfer of light momentum,
by external magnetic fields, by in situ chemical reactions, or by the energy
flow between hot and cold heat reservoirs, in scaled-down versions of
macroscopic heat engines. Here, we experimentally demonstrate a microscopic
engine powered by the local reversible demixing of a critical mixture. We show
that, when an absorbing microsphere is optically trapped by a focused laser
beam in a sub-critical mixture, it is set into rotation around the optical axis
of the beam because of the emergence of diffusiophoretic propulsion; this
behavior can be controlled by adjusting the optical power, the temperature, and
the criticality of the mixture. Given its simplicity, this microscopic engine
provides a powerful tool to power micro- and nanodevices. Furthermore, since
many biological systems are tuned near criticality, this mechanism might
already be at work within living organisms, for example in proteins and in
cellular membranes.
