  Laser triggered and photothermally induced vapor bubbles have emerged as
promising approaches to facilitate optomechanical energy conversion for
numerous relevant applications in micro/nanofluidics. Here we report the
observation of a sub-megahertz spontaneous nucleation of explosive plasmonic
bubbles, triggered by a continuous wave laser. The periodic nucleation is found
to be a result of the competition of Kelvin impulsive forces and thermal
Marangoni forces applied on residual bubbles after collapse. The former
originates from asymmetric bubble collapse, resulting in the directed
locomotion of residual bubbles away from the laser spot. The latter arises in a
laser irradiation induced heat affected zone (HAZ). When the Kelvin impulses
dominates, residual bubbles move out of the HAZ and the periodic bubble
nucleation occurs, with terminated subsequent steadily growing phases. We
experimentally and numerically study the dependence of the nucleation frequency
f on laser power and laser spot size. Moreover, we show that strong fluid flows
over 10 mm/s in a millimeter range is steadily achievable by the periodically
nucleated bubbles. Overall, our observation highlights the opportunities of
remotely realizing strong localized flows, paving a way to achieve efficient
micro/nanofluidic operations.
