Sub-megahertz nucleation of plasmonic vapor microbubbles by asymmetric collapse

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.