Analyses of acoustofluidic field in ultrasonic needle–liquid–substrate system for micro-/nanoscale material concentration

The ultrasonic needle–liquid–substrate system, in which an ultrasonically vibrating steel needle is inserted into an aqueous suspension film of micro-/nanoscale materials on a nonvibration silicon substrate, has large potential applications in micro-/nanoconcentration. However, research on its detailed concentration mechanism and the structural parameters’ effect on concentration characteristics has been scarce. In this work, the acoustic streaming field and acoustic radiation force in an ultrasonic needle–liquid–substrate system, which are generated by a vibrating needle parallel to the substrate, are numerically investigated by the finite element method. The computational results show that the ultrasonic needle’s vibration can generate the acoustic streaming field capable of concentrating micro-/nanoscale materials, and the acoustic radiation force has little contribution to the concentration. The computation results well explain the experimental phenomena that the micro-/nanoscale materials can be concentrated at some conditions and cannot at others. The computational results clarify the effects of the distance between the needle center and substrate surface, the needle’s radius, the water film’s height and radius and the shape of the needle’s cross section on the acoustic streaming field and concentration capability.