Optically Driven Gold Nanoparticles Seed Surface Bubble Nucleation in Plasmonic Suspension

Abstract

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. Highspeed 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.

Surface Bubble Growth in Plasmonic Nanoparticle Suspension

“…Using high-speed videography with interferometry, we indeed observe that the front TPCL is pushed forward when the laser spot overlaps with the front contact line, which sequentially leads to the depinning of the trailing TPCL and eventually leads the bubble to slip forward within ~ 1 ms. This confirms that the TPCL de-pinning due to the plasmonic NPs heating is the main reason for the laser directed surface bubble movement, and the possibility of high-precision bubble manipulation has useful practical implications for a wide range of microfluidic applications [1][2][3][4][5][6][7][8][9][10] .…”

“…(3) where 𝜆 is the resonant laser wavelength of Au NPs in DI water, which is ~ 780 nm. 𝐴𝑑𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒(𝜆) is the absorbance amplitude of the NP concentration of 2 × 10 15 particles/m 3 in Figure 3b at the resonant wavelength. 𝑇(𝜆) is the transmission of laser at the resonant wavelength.…”

“…Plasmonic bubbles can be generated in noble metal plasmonic NP suspensions upon the irradiation of a pulsed laser due to the enhanced plasmonic resonance [1][2][3][4][5][6]. These micro-sized bubbles can play important roles in a wide range of applications, including biomedical imaging [7][8][9][10], healthcare diagnosis [11][12][13][14][15], and microfluidic bubble logics [16].…”

Plasmonic Nanobubbles–A Perspective

“…Using this feature, Zhang et al used light to deposit Au NPs onto the transparent surface by optical forces. The deposited Au NPs become the surface heater triggering the nucleation of a microsized plasmonic surface bubble (Figure b). , The deposited NPs, when heated by a laser, can also help to depin the front contact line of the plasmonic surface bubble, enabling light to guide the surface bubble to move (Figure c left panel) . As the surface bubble moves with the light, NPs can be deposited along the moving path.…”

“…As previously mentioned, the supercavitating NPs can be driven by light. Using this feature, Zhang et al 63 used light to deposit Au NPs onto the transparent surface by optical forces. The deposited Au NPs become the surface heater triggering the nucleation of a microsized plasmonic surface bubble (Figure 4b).…”