Richard M. White
Elec. Eng. and Comput. Sci. Dept. and Berkeley Sensor and Actuator Ctr., Univ. of California at Berkeley, Berkeley, CA 94720
Using 1-mW, 5-MHz flexural plate waves in 3-(mu)m-thick membranes, mixing in liquids and transport of both liquids and of granular solids in air have been produced. In all cases, the plate-wave velocities are lower than the sound speeds in the adjacent fluids, so the elastic disturbances in the fluids are evanescent, with typical 1/e distances of only 16 (mu)m. Mixing has been studied with an electrochemical cell, while the transport of solids and liquids has been observed directly via optical microscopy. Acoustic streaming theory accounts quantitatively for the observed features of the liquid pumping---square-law dependence on wave amplitude, and liquid-pumping velocities which range up to 300 (mu)m/s. The approximate theory will be described, and the experimental observations will be presented and illustrated with a short video.