Fluid motion produced by ultrasonic Lamb waves

Fluid motion induced by traveling flexural waves in 4-μm-thick membranes was observed using 2.5-μm-diameter polystyrene spheres in water to make their fluid motion visible. Visual observation of the spheres indicates that their velocity is in the direction of wave propagation and is proportional to the square of the acoustic amplitude. The maximum speed is 130 μm/s for an RF drive voltage of 7.1 V_rms at 3.5 MHz; the wavelength is 100 μm. Sphere motion has been observed in regions without acoustic waves, suggesting fluid recirculation. Standing Lamb waves, which can be seen visually with a phase-contrast microscope, are found to trap particles, including bacteria located in a drop of water in contact with the membrane. A first-order model of Lamb-wave micropumping is based on acoustic streaming theory. Possible applications include temperature redistribution in ICs and miniature chemical processing systems