We have observed numerous kinetic effects using ultrasonic flexural plate waves (EPWs) in 4-um-thick composite plates of low-stress silicon nitride, piezoelectric zinc oxide and aluminum. The wavelength is typically 100 um, and the area 3 x 8 mm^2. A successful new surface micromachining fabrication process is presented here for the first time. FPWs have been used to move liquids and gasses with motion typically indicated by polysilicon blocks in air and polystyrene spheres in water; the velocity in air is 4.5 mm/s (with a zero-to-peak input of 3 V), and in water it is 100 um/s (with an input of 7.8 V). Other observations include pumping of a liquid dye, and mixing near the FTW surface.
All quantitative observations demonstrate that the kinetic effects of FPWs are proportional to the square of the wave amplitude. The amplitude for a typical device is 250 A at 9 V input; the power in a typical FPW is about 2 mW. The amplitude can be accurately measured using a laser diffraction technique. Experimental error is about +/-l0%, and many of the results agree well with a simple theory to predict the FPW amplitude; extensions of the theory model the fluid loading of EPW devices, but experiment and theory disagree by about 15%.
Pumping by flexural plate waves is an example of the phenomenon known as acoustic streaming. A common solution approach is the method of successive approximations, where the nonlinear equations are first linearized and solved. This "first-order" solution is then used to determine the inhomogeneous source terms in the linearized, "second-order" equations of motion. Theoretical predictions of streaming theory are in excellent agreement with experiment in the case where the FPW device contacts a half-space of fluid; predictions for flow in small channels encourage the development of integrated micropumps.
Applications for microflow include thermal redistribution in integrated circuits and liquid movement in analytical instruments-particularly where a small dead volume is required. Capabilities of this technology and further applications are discussed. Microflow systems that integrate transport of fluids and solids with sensing, mixing and other useful tasks may become a new market-leading application for the sensor and actuator field.
May 31, 1995
Moroney, R. M. (1995). Ultrasonic Microtransport. United States: University of California, Berkeley.