In this paper, we study the fundamental mechanism of surface bubble nucleation in nanoparticle (NP) suspension upon laser irradiation. Our experiments find that the bubble nucleation thresholds differ depending on if the surface is forward- or backward-facing the light propagation direction even if the laser power density is the same on the surface. High speed videography reveals that NP deposition on the surface is a pre-requisite for bubble nucleation, and it is the optical dispersive force that drives such deposition. Optical pulling force is needed to deposit NPs when the surface is backward-facing the laser propagation direction, and this is achieved only when the laser optical fluence is sufficiently high to generate a supercavitating nanobubble around the NP, which is needed to enable proper optical condition for optical pulling motion to happen. When the surface is forward-facing the laser propagation direction, optical pushing force, which exists on both bare or supercavitating NPs, makes the NP deposition easier and thus lowers the surface bubble nucleation threshold. Further experiments comparing the NPs with different surface plasmon resonance (SPR) frequencies show that optical pulling-induced nucleation is impossible if the laser is off the SPR peak of the NPs, since it cannot intensely excite the NP to form supercavitation. However, optical pushing-induced nucleation surface bubble nucleation always happens regardless if the NP SPR peak aligns with the laser wavelength. The results reveal interesting physics leading to photothermal surface bubble generation in NP suspensions.
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