Controlled ultrasonic concentration of nano particles on a nonvibrating substrate

We show a method of concentrating nanoscale samples in a film of aqueous suspension on a stationary substrate (not in vibration), which employs the acoustic streaming generated by an ultrasonically vibrating needle parallel to and above the stationary substrate. The acoustic streaming eddy on the substrate surface brings nano particles (NPs) to the center of the eddy. Concentrated SiO2 nano particles with a diameter of 250 nm may form one circular spot on the stationary substrate, with a diameter up to 400 μm. The principles of the above phenomena are analyzed and verified by the finite element method.