Light-Generated Plasmonic Nano Bubble and Micro Surface Bubble in Gold Nanoparticle Suspension

Surface plasmonic heating effect is originated from the resonance oscillation of the free electrons in noble metal nanoparticles, i.e., plasmonic nanoparticles, when they are irradiated by the incident laser with resonance wavelength. The resonance wavelength of plasmonic nanoparticle can be tuned by engineering the materials, geometry and size of the nanoparticle. When surface plasmonic heating effect occurs in nanoparticle suspension, a large amount of heat is produced locally around the irradiated nanoparticles, which can vaporize the surrounding liquid and generate plasmonic bubbles. There are two major types of plasmonic bubbles, i.e., supercavitating nanobubble and surface microbubble. In this work, the coreshell gold/SiO₂ nanoparticle (120 nm diameter) and a near-infrared femtosecond pulsed laser (800 nm) are used to study the novel opto-thermo-fluidic behaviors and related applications of these plasmonic bubbles.

Supercavitating nanobubble is a vapor bubble generated to encapsulate a single nanoparticle, i.e., supercavitating nanoparticle, which has a stabilized diameter of a few hundreds of nanometers. Supercavitating nanobubble can be detected by visualizing the magnification in the far-field cross section scattered probe light in pump-probe optical scattering imaging. The nanoparticles in a suspension can be driven by scattering optical force originated from the momentum exchange between incident photons and the nanoparticles. The photon stream in the laser beam usually exerts an optical pushing force that drives the nanoparticles to move in the light propagating direction. However, when a nanoparticle is encapsulated by a nanobubble, the moving speed of the nanoparticle can be ~2 orders of magnitude larger than a bare nanoparticle, since the nanoparticle moves in the vapor medium, that has a much lower viscosity, instead of liquid. Moreover, this supercavitating nanobubble can optically couple to the encapsulated nanoparticle to trigger the “negative” scattering optical forces on the nanoparticle, leading to an optical pulling force, depending on the position of a nanoparticle inside the nanobubble. Supercavitating nanobubble does not only influence the motions of suspended nanoparticles in bulk liquid, but also can disturb the capillary trapping force at the liquid/air interface and let supercavitating nanoparticles to move across the interface when driven by laser.

Using the optical pulling or pushing force, we can directly deposit plasmonic nanoparticles onto optically transparent substrates when they are immersed in nanoparticle suspensions. Once the nanoparticles deposited reach the critical number at a given laser power density, the surface heating effect can allow the substrate to reach a threshold temperature for the nucleation of surface microbubbles. This method eliminates the complicated surface plasmonic nanostructures pre-fabrication process in conventional surface microbubble generation methodology. Moreover, it is interesting that we observed much faster surface bubble growth rates in nanoparticle suspension compared to those in pure water with pre-fabricated nanostructures. Our analyses show that the volumetric heating effect around the surface bubble due to the existence of nanoparticles in the suspension is the key to explain this difference. Such volumetric heating increases the temperature around the surface bubble more efficiently compared to sorely surface heating which enhances the expelling of dissolved gas. In addition, the volumetric heating can also bring some hot nanoparticles to depin and extend the front three-phase contact line of the surface bubble enabling precise spatiotemporal light-controlled surface bubble movement. With this technique, we demonstrate that surface bubbles on a solid surface are directed by a laser to move at high speeds (>1.8 mm/s). Our findings are beneficial to a wide range of applications like combinatorial material development, microfluidic logic, catalysis, micropatterning, cell-level therapy and imaging, controlled drug delivery, and photothermal energy conversion.