Controlled concentration and transportation of nanoparticles at the interface between a plain substrate and droplet

Controlled concentration of nanoscale materials on the surface of a smooth substrate without vibration excitation mechanism and micro channels (termed as plain substrate), and transportation of the concentrated nano material on the surface, have large potential applications in the fabrication of nano sensors and electrodes, decoration and assembly of nano materials, etc. However, implementation of these two nano manipulation functions by one single device has been a big challenge. Here we report a strategy to concentrate nanoparticles at an arbitrary location at the interface between a plain substrate and water droplet, and to transportation the concentrated nano material freely at the interface. It employs the acoustic streaming, which is generated by a micro manipulating probe (MMP) vibrating linearly above the substrate. 500 nm-diameter silicon nanoparticles (SiNPs) can be concentrated under the MMP at a desired location, forming a round spot of nano materials with a diameter up to 230 μm. The concentrated nano material can be transported through an arbitrary path at the interface by shifting the device, and has little change in size and shape during the transportation. The dependency of acoustic streaming field around the MMP on device parameters are clarified by numerical computation and verified by experiments.