Plasmonic Supercavitation Enables Nanoparticle Photo-Ejection Across Air/Water Interface

The ability to separate miniscule solid particles (e.g., nanoparticles) from liquid is important to a wide range of applications. However, directly moving nanoparticles out of liquid is difficult as the capillary force on the nanoparticle at the liquid interface is too large for common body forces to overcome. Here, we demonstrate the ability to eject metallic nanoparticles out of liquid with a laser excitation. The laser applies an optical force on the nanoparticles to drive them toward the liquid surface. In the meantime, it intensely heats the nanoparticle to form a nanobubble encapsulating the nanoparticle (i.e., supercavitation), which achieves the liquid-nanoparticle separation and thus eliminates the trapping force on the nanoparticle at the liquid/air interface. We show that such a mechanism can expel nanoparticles out of liquid using a transient scattering experiment, which is further confirmed by molecular dynamics simulations. We also demonstrate depositing the nanoparticles on a solid surface not in contact with the liquid. Leveraging such a feature, we show an example application where the deposited NPs are used as a template for 2D material nanotent fabrication. This study reveals an interesting fundamental mechanism to separate nanoparticles from liquid and could potentially benefit separation, nanomaterials, and biomedical applications.