Research On Acoustic Streaming In A Droplet On The Ultrasonic Stage

This work was supported by the National Natural Science Foundation of China(No. 91123020), Nanjing University of Aeronautics and Astronautics(S0896-013 and 56XZA12044), Innovation and Entrepreneurship Program of Jiangsu, State Key Lab of Mechanics and Control of Mechanical Structures(0314G01).In this paper, finite element model of a ultrasonic stage used to concentrate Ag NWs is analyzed by COMSOL Multiphysics. The acoustic streaming in the droplet at the center of the ultrasonic stage is researched. The main contents are shown as follows:The concentration experiment of Ag NWs is realized by the ultrasonic stage and the relationship between the acoustic streaming velocity and the center point’s vibration velocity of the ultrasonic stage is obtained. The finite element model of ultrasonic stage is established in the sound-piezoelectric coupling physical fields, and the mode analysis and harmonic response analysis are accomplished.After solving in the acoustic-piezoelectric coupling field, the vibration velocity and acoustic pressure distribution inside the water droplet are obtained. Those physical quantities are used to solve the acoustic streaming in the droplet. And the relationships between the acoustic streaming velocity in the droplets and the center point’s vibration velocity are calculated. The results are consistent with the experimental data well.The factors affecting the distribution of acoustic streaming in the droplet are researched. It is found that the gradient of Reynolds stress in the droplet plays a dominant role in the forming of acoustic streaming. In addition, relationships between the acoustic streaming velocity and size of ultrasonic stage are researched. It is concluded that the flow of acoustic streaming is concerned with the volume of the droplet. The acoustic streaming velocity increases as the silicon substrate’s thickness decreases. And the acoustic streaming velocity increases as the thickness of the piezoelectric ring increases. For fixed piezoelectric ring’s inner diameter or outer diameter, the acoustic streaming velocity reaches maximum at some corresponding outer diameter or inner diameter. The acoustic streaming velocity increases with a groove of suitable size in the silicon substrate.