Manipulations of silver nanowires in a droplet on a low-frequency ultrasonic stage

In this work, we report the use of a low-frequency circular ultrasonic stage to form a circular spot of silver nanowires (AgNWs) at the stage center and to radially align AgNWs on the stage surface. The manipulations are implemented within an AgNW suspension droplet at the center of the ultrasonic stage. The ultrasonic stage (50.8 mm diameter, 3.5 mm thick) operates at a flexural vibration mode symmetric about its center and has a vibration peak at the center. The AgNW suspension is formed of deionized water with AgNWs dispersed in it. The operating frequency of the ultrasonic stage is 21.3 kHz; the AgNWs have diameter of 100 nm and length of approximately 30 μm. When the ultrasonic stage vibrates properly, AgNWs on the substrate surface in the droplet may move to the stage center and form a spot or rotate to the radial direction and align radially. The spot diameter and thickness are several hundred micrometers and several micrometers, respectively. The rotation speed of a single AgNW can be up to 31°/min when the vibration velocity of the stage center is 42 mm/s (0-p) for a 40-μL droplet. After the droplet dries out by natural evaporation without ultrasound, the spot and radial alignment have little change in the size and pattern. Principle analyses show that the spot formation and radial alignment of AgNWs are caused by the acoustic streaming in the radial direction in the droplet.