Ultrasonically induced microtransport

Fluid motion induced by traveling flexural waves in 4 mu m thick membranes was observed using 2.5 mu m diameter polystyrene spheres in water to make the fluid motion visible. Visual observation of the spheres indicates that they move in the direction of wave propagation with a speed proportional to the square of the acoustic amplitude. The maximum speed is 130 mu m/s for an RF drive voltage of 7.1 Vrms at 3.5 MHz; the wavelength is 100 mu m. 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 that contacts the membrane. A first order model of Lamb-wave micropumping is presented, based on acoustic streaming theory. Important parameters for device design and operation are discussed, along with options for integrating the device into microflow systems. Possible applications include temperature redistribution in ICs and miniature chemical processing systems.<>