  Photothermal surface bubbles play important roles in a wide range of
applications like catalysis, microfluidics and biosensing, but their formation
on a transparent substrate immersed in a plasmonic nanoparticle (NP) suspension
has an unknown origin. Here, we show that NPs deposited on the substrate by
dispersive optical forces are responsible for the nucleation of such
photothermal surface bubbles. High-speed videography shows that the surface
bubble formation is always preceded by the optically driven NPs moving toward
and adhering to the surface. We observe that the thresholds of laser power
density to form a surface bubble drastically differ depending on if the surface
is forward- or backward-facing the light propagation direction, which cannot be
explained by a purely thermal process (e.g., volumetric heating by plasmonic
NPs). This can be attributed to different optical power densities needed to
enable optical pulling and pushing of NPs in the suspension. Optical pulling
requires higher light intensity to excite supercavitation around NPs to enable
proper optical configuration. Eventually, these optically deposited NPs work as
a surface photothermal heater, seeding the surface bubble nucleation. We also
find that there is a critical number density of deposited NPs to nucleate a
surface bubble at a given laser power density, and it is nearly the same for
both the forward- and the backward-facing surfaces. Our finding reveals
interesting physics leading to photothermal surface bubble generation in
plasmonic NP suspensions.
