  Transforming a laser beam into a mass flow has been a challenge both
scientifically and technologically. Here we report the discovery of a new
optofluidics principle and demonstrate the generation of a steady-state water
flow by a pulsed laser beam through a glass window. In order to generate a flow
or stream in the same path as the refracted laser beam in pure water from an
arbitrary spot on the window, we first fill a glass cuvette with an aqueous
solution of Au nanoparticles. A flow will emerge from the focused laser spot on
the window after the laser is turned on for a few to tens of minutes, the flow
remains after the colloidal solution is completely replaced by pure water.
Microscopically, this transformation is made possible by an underlying
plasmonic nanoparticle-decorated cavity which is self-fabricated on the glass
by nanoparticle-assisted laser etching and exhibits size and shape uniquely
tailored to the incident beam profile. Hydrophone signals indicate that the
flow is driven via acoustic streaming by a long-lasting ultrasound wave that is
resonantly generated by the laser and the cavity through the photoacoustic
effect. The principle of this light-driven flow via ultrasound, i.e.
photoacoustic streaming by coupling photoacoustics to acoustic streaming, is
general and can be applied to any liquids, opening up new research and
applications in optofluidics as well as traditional photoacoustics and acoustic
streaming.
