Charles E. Bradley
Richard M. White
Berkeley Sensor and Actuator Ctr., Dept. of Elect. Eng., 497 Cory Hall BSAC), University of California, Berkeley, CA 94720
The theory of nonlinear acoustic streaming is presented with emphasis on physical interpretation and general statements regarding the structure of the flow field. In addition, measurements are shown in which the flow is driven by the evanescent acoustic field near a micromachined thin plate with flexural waves traversing it. A fundamental acoustic field drives dc flow in four ways: (1) via a force field, (2) via an effective mass source distribution in the body of the fluid, (3) via a mass sink distribution at the acoustic projector, and (4), via Stokes pumping. The decomposition of the system of equations into irrotational and solenoidal components results in a number of findings. For instance, it is found that the irrotational flow that arises from the mass sources and sinks exactly cancels the irrotational component of the Stokes flow. Likewise, the solenoidal component of the force source distribution drives purely solenoidal flow while the irrotational component drives a dc pressure field. Several examples are presented to illustrate the findings. The application of the theory in the development of a micropump-on-a-chip is discussed.